Sound level meters: 1928 to 2012 level meters 1928 to 2012.pdf · Sound level meters: 1928 to 2012 . This article reviews the development of sound level meters, their principal

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Text & Figs Sound level meters AERC

Sound level meters: 1928 to 2012

This article reviews the development of sound level meters, their principal electroacoustical performance characteristics, and the role of International

Standards in the development of the instruments

Prepared by Alan Marsh for publication in the Japanese Research Journal on Aviation Environment

Translated from English to Japanese by Ichiro Yamada and Chitose Nakamura

2012 March

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Contents Introduction ................................................................................................................................................... 6

1928-1929: The first sound level meter (“noise” meter) ....................................................................... 6

Loudness .................................................................................................................................................... 6

Noise survey of 1928-1929..................................................................................................................... 7

Noise Survey Apparatus.......................................................................................................................... 7

Acoustical Society of America .............................................................................................................. 8

1930s............................................................................................................................................................... 8

World’s first standard for the performance of a sound level meter, Z24.3-1936 .......................... 8

Design characteristics.............................................................................................................................. 8

1940s to 1950s .............................................................................................................................................. 9

Z24.3-1944 ................................................................................................................................................ 9

Chicago Noise Survey of 1947 .............................................................................................................. 9

General Radio instruments ..................................................................................................................... 9

Rion sound meter ................................................................................................................................... 10

IEC and IEC Technical Committee 29 ............................................................................................... 10

1960s to 1990s ............................................................................................................................................ 10

IEC Publicat ion 123............................................................................................................................... 10

Instruments of the 1960s ....................................................................................................................... 11

Standards of the 1970s .......................................................................................................................... 11


What do we mean by time-weighted sound level?............................................................................ 12


What do we mean by time-averaged sound level?............................................................................ 13

An integrating-averaging and integrating sound level meter of the 1980s ................................... 14

What do we mean by sound exposure level? ..................................................................................... 14

A mult i-function sound level meter of the 1990s ............................................................................. 15

2000s to present .......................................................................................................................................... 15

IEC 61672, International Standard for sound level meters ............................................................. 15

Current versions of mult i-function sound level meters conforming to IEC 61672-1.................. 15

What are key electroacoustical performance requirements in IEC 61672-1? .............................. 16

Concluding remarks ................................................................................................................................... 16

Acknowledgments ...................................................................................................................................... 17

References ................................................................................................................................................... 17

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Figures Figure 1 — Loudness “thermometer” scale with rankings for common sound sources. Scale, in “sensation units”, runs from the threshold of audibil ity at 0 units at the bottom of the scale to 108 units at the threshold of feeling. The rustle of leaves in a gentle breeze is shown at 10 units. Noise in an airplane is at about 96 units. Il lustration is from an article by Wallace Waterfall in the “Enginee ring News Re cord” of 1929 Janu ary 10. ..........................................................18

Figure 2 — Truck and equipment used for survey of “noise” in New York City in 1929. Sign on side of truck reads: “Official Measuring Apparatus; Noise Abatement Commission.” Weight of the “n oi se mete r” wa s in exce ss o f 25 kg, including the bat terie s. .........................................................18

Figure 3 — Cover page of first Standard with design goals and tolerances for the electroacoustical performance of sound level meters. The Standard was sponsored for the American Standards Association by the Acoustical Society of America (founded in 1929). The organizing meeting occurred in 1932 May for the committee that was established to draft a Standard. A draft Standard was discussed in 1934 May, revised, and approved by the American Standards Association in 1936 February 17. The Z24.3-1936 Standard was published in the Jou rnal of the Acou stical So ciety of Ame rica in 1936. ..................................................................19

Figure 4 — Sound level meter, octave-band fi lter set, and magnetic recorder as used for 1947 noise su rvey o f Chicago; in st ruments we re moun ted in the back of a station wagon. ...........................20

Figure 5 — Portable hand-held sound level meter introduced by the General Radio Company (USA) a s model 1551 A in the earl y 1950 s. Scale inte rval s a re in de cibel step s. ..................................20

Figure 6 — Illustration of recommended use of GR 1551A sound level meter. Instrument was suppo rted b y a st rap h eld by the right hand with cont rol s ope rated by the left ha nd. ............................21

Figure 7 — GR model 1555A sound survey meter introduced in the mid 1950s. Logarithmically spaced markings on indicator dial for display of frequency-weighted sound level in decibels, but with variable re solution. ............................................................................................................................21

Figure 8 — Rion N1101 sound meter introduced in the 1950s. Display indicates frequency-weighted sound level in phons because of requirements as given in a German National Standa rd of 1942. ......................................................................................................................................21

Figure 9 — Cover page for first international document issued in 1961 with recommendations for design goals and tolerances for the electroacoustical performance of general purpose sound level meters measuring frequency-weighted and time-weighted sound levels, International Recommendation IEC 123:196 1. ..............................................................................................................22

Figure 10 — Model S.S.T.1 sound level meter from LEA in France introduced in the 1960s. Instrument was held by the handle and pointed at the source of sound with the display toward the observer; microphone was in the case opposite the display of sound levels in decibels. Logarithmically spa ced ma rking s on di splay. ...........................................................................................23

Figure 11 — Design was by Peekel of Holland, but packaged and sold in the 1960s by Advance in England as model SPM1. No frequency weightings provided; instrument only measured octave-band sound pressure levels. Microphone was mounted on a short extension rod and was flush with the case when not in use. Pushing the button above the word “microphone” near the top left of the case released a spring that extended the microphone. Logarithmically spaced marking s shown on the di splay. ...............................................................................................................23

Figure 12 — Rion NA-07 sound level meter (Japan) from the 1960s for measuring frequency-weighted sound level in phons; later modified to indicate sound levels in decibels. Toggle switch abo ve di spla y de vice to select F o r S time weighting. ..................................................................24

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Figure 13 — B&K model 2203 sound level meter (Denmark); weight about 3 kg (1960s). 25-mm-diameter capacitor microphone. Logarithmically spaced markings shown on the display. Time weighting s F and S we re p rovided . .................................................................................................24

Figure 14 — Cover page for International Standard issued in 1979 with design goals and tolerances for the electroacoustical performance of three classes of sound level meters for measurement of frequency-weighted and time-weighted sound levels, International Standard IEC 651:1979 . ...........................................................................................................................................25

Figure 15 — Class 2 sound level meters from the 1970s: Cirrus 2.21 (England) and Rion NA-09 (Japan). Both indicated frequency-weighted sound level in decibels on a scale with logarithmically spaced markings. F or S time weightings were provided. The Cirrus 2.21 had a cavity on the left side of the upper case where the microphone on the gooseneck could be inserted. The cavity had a loudspeaker to produce a sound-pressu re signal that could be used to che ck acou stical sen sitivit y be cau se many cu stome rs re fu sed to pu rcha se a sound calibrato r. ......26

Figure 16 — Integrating-averaging sound level meters from late 1980s. Left: Cesva SC-10 (Spain); right: Cirrus 2.22 (England). SC-10 only measured A-weighted sound levels. 2.22 measured A-weighted, C-weighted, or flat-weighted sound levels as time-averaged sound levels or as sound exposure levels. SC-10 has digital indicator scale; display for 2.22 has equally-spaced markings in decibels. Thickness of instrument set by diameter of AA batteries. Microphon e and p reamplifier relati vely clo se to tape red ca se of in strument. .........................................26

Figure 17 — Cover page for International Standard issued in 1985 with design goals and tolerances for the electroacoustical performance of three classes of sound level meters for measurement of frequency-weighted and time-averaged sound levels or frequency-weighted sound e xpo sure level s, Inte rnational S tanda rd IEC 804:1985 . ...............................................................27

Figure 18 — Multi-function B&K model 2231 class 1 sound level meter of the 1980s. 13-mm-diameter microphone and preamplifier mounted away from tapered case of instrument. Digital display for level of selected quantity. Instrument measures time-averaged sound levels; F or S time-weighted sound levels, maximum time-weighted sound level in a time period, and sound expo su re level s, all with choice of f requen cy weighting s A, C, o r flat. ...................................................28

Figure 19 — Multi-function B&K model 2260 class 1 sound level meter introduced in the 1990s. Long tapered case minimizes effect of sound reflected from case to microphone and helps to conform to directional response requirements of international performance standards. 13-mm-diameter capacitor microphone and preamplifier. Screen can display a spectrum of one-third-octave-b and sound p re ssu re le vel s with o r without fre quency weighting. ..............................................28

Figure 20 — Cover page for International Standard IEC 61672-1 issued by the International Electrotechnical Commission in 2002 May with electroacoustical performance specifications for class 1 and class 2 sound level meters measuring frequency-weighted and time-weighted sound levels, frequency-weighted and time-averaged sound levels, and frequency-weighted sound expo su re level s. ........................................................................................................................................29

Figure 21 — Modern designs for multi-function class 1 sound level meters–Larson Davis model 831 (Provo, Utah, USA on left); Quest Technologies/3M model SoundPro (Oconomowoc, Wi scon sin, USA on right ). .........................................................................................................................30

Figure 22 — Modern designs for multi-function class 1 sound level meters: Cirrus Research model 171B Optimus (North Yorkshire, England on left); Svantek model Svan 979 (Warsa w, Poland on right). ........................................................................................................................................30

Figure 23 — Modern designs for multi-function class 1 sound level meters: Cesva model SC 310 (Barcelona, Spain on left ); Rion mo del NL -62 (To kyo , Jap an on right ). ..........................................31

Figure 24 — Modern-designs for multi-function class 1 sound level meters: Norsonic model Nor 140 (Oslo, No rwa y on left ); B&K m odel 2270 (Nae rum, Denma rk on right ). ..........................................31

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Figure 25 — Design-goal response for frequency weightings A, C, and Z as specified in IEC 61672-1 fo r sound level meters. ...............................................................................................................32

Figure 26 — Example of l inearity of indicated level of output signal levels in response to a range of input electrical signal levels. In region 1, the design goal for the output signal level is a l inear response to changes in input signal levels; the extent of the linear response is the linear operating range. In region 2, the self-generated noise within a sound level meter l imits the level of the indicated output signal. In region 3, elements within a sound level meter (including those that cause overload conditions) l imit the indicated level of the output signal. According to IEC 61672-1 , the exten t of the linear ope rating range a t 1 kHz i s re quired to be a t lea st 60 dB . ................33

Figure 27 — Design-goal response of a sound level meter to bursts of 4-kHz sinusoidal electrical input signals of various burst durations. A burst with a duration of 0.25 ms contains one complete cycle of a 4-kHz signal from the initial zero crossing to the final zero crossing. Toneburst response is defined as the difference between an indicated frequency-weighted sound exposure level of a toneburst and the time-averaged frequency-weighted sound level of the co rre sponding steady signal f rom which the to nebu rst i s e xt racte d. ................................................34

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Sound level meters: 1928 to 2012

Introduction

Sound level meters are used for a large range of applications to determine objective measures of sounds, outdoors as well as indoors. Applications include: product development where the level of the sound from a device is either a contractual matter or a sales feature; monitoring the sound of a machine, road traffic, or airplane overflights; or assessing compliance with community noise ordinances or regulations. Sound level meters are also used to evaluate the acoustical performance of structural elements of a building or office or its heating-ventilating and air-conditioning system.

This article traces the evolution of the design of sound level meters from the in itial concept to the multi-function instruments that were available in 2012. Accompanying the evolution of the design for the instruments was a series of national (init ially) and then international standards for the design characteristics to be incorporated in a sound level meter.

The effort to develop performance standards for sound level meters began in the USA in the early 1930s. Later, many countries developed their own national standards for sound level meters. This article primarily describes the international standards that were initially developed in the late 1950s by a Technical Committee of the International Electrotechnical Commission (IEC). The first IEC Standard for a sound level meter was published in 1961.

This article includes a selection of photographs of sound level meters that were developed over an 80-year period. The photographs that were included to illustrate the development were selected to show the variety of designs and capabilit ies consistent with the requirements of national and international standards as the standards evolved to reflect design and manufacturing capabilities.

1928-1929: The first sound level meter (“noise” meter)

Loudness

In the 1920s, research on perception of a sound studied ways to characterize the loudness of sounds. Audiometers were developed to assist in the study of human hearing and the response to sounds. These audiometers utilized the capabilit ies of the newly developed “vacuum tubes” to control the level of sounds presented to listeners. Much of this research was conducted at the Bell Telephone Laboratory, then located in New York City. [In some countries, vacuum tubes were known as “valves”.]

Sounds were “measured” by the “deafening effect” or “ear balance” method where the goal was for an observer to listen to a sound from some source and then to adjust the level of the sound from a mechanical source (for example, a buzzer) until the sound of the mechanical source just masked (or “deafened”) the sound from the source of interest.

The sensation of the loudness of a sound was known to vary with the amplitude of the sound as well as its frequency content. Because the sensation varied over a very wide range of sound pressure, the then-preferred unit of loudness sensation was logarithmic and not linear with changes in the amplitude of the sound.

Figure 1 shows the sensation unit scale for several common sound sources. The range of loudness sensation units, as published in a 1929 January art icle by Wallace Waterfall [1]1, was in the form of a thermometer scale where 0 sensation unit was at the average threshold of audibility 1 Numbers enclosed within bracket s represent entries in the list of references.

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(as determined by tests of the hearing of many subjects). The upper end of the range was at 108 sensation units at the average threshold of feeling. Noise surv ey of 1928-1929

In the period from about 1920 up to about mid-1929 (known in the USA as the Roaring Twenties), the general level of the sound in New York City from construction, automotive traffic, elevated subway trains, and many other sources led to complaints to the New York Department of Health about the near-constant noise and its effect on the quality of life in the City. In 1928, the Department established a Noise Abatement Commission. The Commission established a series of Committees, one of which, the Committee on Noise Measurement and Survey, was charged with the task of measuring the sound in the City over several contiguous 24-h periods at several indoor and outdoor locations over a period of a year.

The City engaged the services of engineers from the Bell Telephone Laboratory and from Johns-Manville2 to carry out the work of the Committee.

But what aspect of a sound should be measured and with what instrument? In 1928, there was no general agreement on what quantity to measure to characterize what is

now called environmental sound. There was no commercially -availab le instrument designed to provide objective measurements of the physical characteristics of sounds. There was no accepted standard with design goals for the electroacoustical performance of such an instrument.

There was no loudness meter then and there still is none because the loudness of a sound depends on the amplitude and frequency content of a sound as well as human judgments.

The Noise Measurement Committee made the following key decisions about the quantity that should be measured for the noise survey.

• The quantity to be measured would be the level of the frequency-weighted sound pressure signal where the frequency weighting was the equal-loudness frequency contour at 30 sensation units for normal hearing of pure-tone sounds. [Figure 1 indicates that 35 sensation units corresponds to the loudness of soft radio music in an apartment.]

• Although the basic unit of the level of a frequency-weighted sound pressure would be the “bel” (named in honor of Alexander Graham Bell, inventor of the telephone), the results of the noise survey would be determined and reported in “decibels” in order to provide an easier-to-understand resolution of indicated sound levels.

• Differences in indicated noise levels (in decibels) were supposed to match differences in judgments of loudness.

Noise Surv ey Apparatus

Figure 2 is a photograph of the truck and equipment used for the noise survey of New York City in 1928-1929. The sign on the side of the truck reads: “Official Measuring Apparatus; Noise Abatement Commission.”

One operator stood near a rear corner and listened to the simultaneous presentation of the sound of a buzzer and the general sound at the location of the microphone; the level of the buzzer sound was adjusted to just mask (deafen) the noise. Another operator inside the truck recorded the signals from the microphone.

The noise meter was “calibrated” at Bell Labs by comparisons with judgments of the loudness of street noise, room noise, and warbled tones at 55 dB above the threshold of hearing for the

2 Johns-Manvill e was, and is, a manufacturer of products for the construction industry, including acoustical products used for noise control.

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subjects. In this way, the level indicated by the noise meter corresponded to the same sensation as the judgment of loudness, within 3 dB.

The sounds of street noise and room noise were recorded on a phonograph and then reproduced in the laboratory. Microphones were those developed for radio broadcasting.

The weight of the “noise meter” was in excess of 25 kg, including the batteries. Acoustical Society of America

On 1928 December 27, approximately 40 scientists and engineers interested in acoustics met at the Bell Telephone Laboratories in New York City to consider forming a scientific society for acoustics. The Acoustical Society of America (ASA) was chartered in 1929; its first meeting was held in New York on 1929 May 10-11, with approximately 450 charter members in attendance.

The three major objectives of the new society were to: (1) hold meetings where papers would be presented in various fields of acoustics, (2) publish an archival journal of technical papers and other information, and (3) sponsor the development of standards in acoustics. Volume 1, Number 1 of the Journal of the Acoustical Society of America was issued in 1929 October. For the third purpose, a Committee on Acoustical Standardization was formed in 1929 December.

1930s

World’s first standard for the performance of a sound lev el meter, Z24.3-1936

In 1930, the Acoustical Society of America petit ioned the American Standards Association for permission to establish a Sectional Committee to prepare definitions, standards, and specifications for acoustical work.

With the approval of the American Standards Association,3 Sectional Committee Z24 was appointed with V.O. Knudsen as Chairman; the organizing meeting was held in May 1932. The Sectional Committee established four subcommittees: acoustical terminology, noise measurement, measurement of fundamental acoustical quantities, and measurement of sound absorption and sound insulation.

The Technical Committee on Sound Levels and Sound Level Meters was formed under the Noise Measurement Subcommittee. A standard for the performance of sound level meters was drafted in 1932-1933 and considered at a meeting of Sectional Committee Z24 in May 1934

Copies of the draft were d istributed widely in mimeographed form for comment and criticism. The draft was revised by the Technical Committee, re-considered by the Z24 Sectional Committee, approved for publication by the American Standards Association, and issued on 1936 February 17. A copy of the Standard was also published in the 1936 October issue of the Journal of the Acoustical Society of America.

Figure 3 shows the cover page from the Z24.3-1936 standard [2] that provided design goals for crit ical characteristics of sound level meters. The remarkable speed of less than 3 years between initial drafting and publication contrasts with much longer durations that are often required now. Design characteristics

Important precedents established in the Z24.3 1936 standard for sound level meters included the following.

• A sound level meter shall have a scale marked in decibels.

3 The American Standards Association evolved into the American National Standards Institute (ANSI).

http://en.wikipedia.org/wiki/Bell_Labs

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• The quantity measured by a sound level meter shall be referred to as sound level; for example as a “sound level of 60 decibels.” [Not loudness level, not sensation level, not noise level, not sound pressure level, but sound level.] [In a test report for measurements of sound levels, the choice of frequency weighting was to be stated explicitly.]

• Two frequency weightings were specified: A and B; frequency weighting A was mandatory for a sound level meter. [A “flat” frequency response was permitted which later evolved to frequency weighting C.]

• The reference value for sound level was 0.0002 dynes/cm2 for a free progressive sound wave having a sound intensity of 10-16 W/cm2 at 1000 Hz. [The choice of an objective value for the reference permitted consistent measurements of sound levels.] [The choice for the reference value from the 1936 standard corresponds, in SI units, to 20 micropascals (20 µPa) where 1 Pa = 1 N/m2 = 0.1 dynes/cm2.]

The response of frequency-weighting A in the 1936 Standard was similar to that of the frequency weighting used for the 1929 noise survey of New York City, but with significant modifications. Frequency-weighting A was from the 40-dB equal-loudness contour, modified “by the difference between random and normal free-field thresholds” as derived from Z24.2-1936 [3]. The response of frequency-weighting B was between that of frequency-weighting A and a flat frequency response.

The intent of the 1936 Standard was that frequency-weighting A would be used to measure low-to-moderate sound levels, frequency-weighting B would be used to measure moderate-to-high sound levels, and the optional flat response to measure high sound levels.

1940s to 1950s

Z24.3-1944

Throughout the early 1940s, the Acoustical Society of America continued work on acoustical standards including an update of the 1936 edition of Z24.3 for sound level meters.

The 1944 issue of Z24.3 [4] incorporated small revisions to the A and B frequency weightings and added the design goal for the C frequency weighting. Chicago Noise Surv ey of 1947

In the late 1930s, cit izens of the City of Chicago in the state of Illinois complained about the general level of noise in the City. A survey of noise levels was authorized in 1941, but postponed until 1946 when it was re-started under the direction of the Armour Research Foundation of the Illinois Institute of Technology.

Figure 4 shows the instruments used for the 1947 noise survey as installed in the back of a station wagon with access provided across the lowered tailgate. The instruments were big, heavy, and required large batteries to provide the electrical power.

The octave-band filter set shown in Figure 4 was intended to be used to measure the spectrum of essentially steady sounds. The filter set provided measurements of octave-band sound pressure levels, one octave band at a time. Sound at a location was recorded on a magnetic wire recorder for subsequent analysis in a laboratory. The intent was to calculate loudness levels from the octave-band sound pressure levels. General Radio instruments

In 1947, the General Radio Company, located in Cambridge, Massachusetts, USA, decided to develop portable acoustical instruments – specifically a sound level meter and an acoustical calibrator that could be used to check and adjust the sensitivity of the microphone.

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Figure 5 shows the GR model 1551-A sound level meter that was introduced in the early 1950s. The GR 1551-A incorporated significant advances over the sound level meters illustrated in Figures 2 and 4. It probably was the world’s first portable sound level meter designed to conform to the requirements of a performance standard. Weight with batteries was about 5 kg.

The microphone was a Shure model 98-98 that used a crystal of Rochelle salt as the sensing element. Sensitivity of the microphone changed dramatically with variations in temperature and humidity.

The knob on the left below the sound-level d isplay was used to select FAST or SLOW exponential t ime weightings as well as checks of the internal batteries that provided power for the “filaments” of the vacuum tubes used for the microphone preamplifier and for amplify ing the level of the signal from the microphone output. The knob on the right below the large central knob selected the A, B, or C frequency weighting, a wideband frequency response, or a “calibrat ion” setting that used an internal electrical signal. Level range control was achieved by the large knob in right center.

The indicator scale of the display device was marked with linear increments of sound level in decibels, giving the same resolution for the indications of sound level across the span of the scale.

Figure 6 shows the manufacturer’s recommendation for using the sound level meter where the end of the instrument was placed against the body of an observer while the weight was supported by the right hand through the strap on the right side of the body of the instrument. The left hand operated the controls while the operator observed the average tendency of the fluctuating pointer as it moved over the scale of the display device.

Figure 7 shows the less-expensive GR model 1555-A sound survey meter. Note that the linear display of the GR 1551-A in Figure 6 was replaced by a non-linear scale where the resolution of the displayed sound level was excellent at the right end of the scale but poor at the left end. The non-linear scale with markings that were logarithmically spaced took the place of the equally spaced markings of the linear scale and reduced the manufacturing cost by eliminating the need to take the logarithm of the frequency-weighted sound-pressure signal before displaying the level. Rion sound meter

The Japanese manufacturer Rion introduced a compact model N1101 sound meter in the 1950s; see Figure 8. The scale displayed sound levels in phons because the designers relied on a German National Standard of 1942 for determining loudness levels in phons. IEC and IEC Technical Committee 29

The International Electrotechnical Commission (IEC) was founded in 1905 and is headquartered in Geneva, Switzerland. In response to petitions from the National Committees of various countries, IEC agreed to establish a Technical Committee to prepare international standards for acoustical instruments and devices. Technical Committee 29, Electroacoustics, was established and held its first meeting in 1953 in Paris. One of the Working Groups of TC 29 was charged with draft ing IEC Standards for acoustical instruments, specifically sound level meters.

1960s to 1990s

IEC Publication 123

The first priority of the Working Group of IEC/TC 29 for sound level meters was to prepare a document with recommendations (not a standard) for the characteristics of a general-purpose sound level meter. The recommendations were given in IEC Publication 123 [5] that was issued

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in 1961 with design goals for measurements of frequency-weighted sound levels. Large allowances for deviations from the design goals were consistent with the requirements for sound surveys.

Figure 9 shows the cover page from IEC Publication 123:1961. Instruments of the 1960s

Figure 10 shows an interesting concept for a hand-held sound level meter from LEA (Laboratoire Electro-Acoustique), a manufacturer in France. The instrument was pointed at a sound source and the display indicated the level of frequency-weighted and time-weighted sound pressure. The scale on the display was a non-linear logarithmic scale with variable resolution.

Figure 11 shows a different concept for a hand-held instrument to measure octave-band sound pressure levels, one band at a time. The design came from Peekel in The Netherlands, but was packaged and sold by Advance in the U.K. The microphone was mounted on a rod that was extended from the case of the instrument by pushing a button to release a spring.

Figure 12 shows the Rion model NA-07 from the 1960s for a general-purpose sound level meter measuring frequency-weighted and time-weighted sound levels in decibels (initially intended for measuring loudness levels in phons but later modified). The scale on the d isplay was another non-linear logarithmic scale with variable resolution across the range of the display.

Figure 13 shows the Brüel & Kjær (B&K) model 2203 sound level meter introduced in 1960. It was about 30 cm long and weighed about 3 kg with D-cell batteries. The display was a non-linear logarithmic scale with variab le resolution. The instrument was designed to conform to the requirements that were under discussion at the time in the Working Group of IEC/TC 29 for a “precision” sound level meter [6] to measure frequency-weighted and time-weighted sound levels.

Special features of the 2203 included:

• Transistors (solid-state devices) instead of vacuum tubes for the amplifier section. • A special miniature vacuum tube was used as a cathode follower for the microphone

preamplifier; the vacuum tube was supported by springs to minimize the effects of microphonics which caused the elements in the vacuum tube to vibrate and hence produce equivalent variations in the output signal.

• A tapered shape of the instrument case below the microphone was used to min imize the effects of sounds reflected from the instrument case to the microphone; later a flexib le “gooseneck” adapter was provided to extend the large (25-mm d iameter) capacitor microphone away from the case.

• In addition to the usual frequency weightings, the 2203 provided an electrical response from the preamplifier input to the electrical output that was flat (nominal 0 dB) from 2 Hz to 20 kHz.

Standards of the 1970s

The 1965 IEC standard for “precision” sound level meters was updated in a document issued in 1973 with tighter tolerances around the design goals for the frequency weightings in the range from 10 Hz to 80 Hz [7].

Throughout the 1970s, the sound-level-meter Working Group of IEC/TC 29 worked on a consolidated revision that incorporated the general-purpose instruments specified in IEC 123 and the precision instruments specified in IEC 179. Th is consolidated revision was issued as IEC 651:1979 [8] with specifications for three performance classes [then called performance “types”].

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Figure 14 shows the cover page of IEC 651:1979. Sound level meters that conformed to the specificat ions of IEC 651:1979 provided measurements of frequency-weighted and time-weighted sound levels. Instruments of the 1970s

A variety of sound level meters was designed in the 1970s to conform to the requirements of the international standard for instruments to measure frequency-weighted and time-weighted sound levels.

Figure 15 shows two designs for class 2 sound level meters that were introduced in the 1970s—the Cirrus model 2.21 and the Rion model NA-09. Both instruments used a scale with logarithmically spaced markings as the means to display sound level in decibels. Note the tapered case on the Rion NA-09.

The Cirrus 2.21 had an interesting feature with a cavity on the top of the instrument’s case into which the microphone (mounted on a gooseneck extension) could be inserted. A small loudspeaker at the bottom of the cavity provided an acoustic signal to indicate that the microphone was functioning. This approach was adopted in lieu of applying a signal from a sound calibrator that many customers did not want to purchase. What do we mean by time-weighted sound lev el?

Until the advent of digital technology, it was not practical for a sound level meter to form and display the level of a t ime-averaged sound pressure signal. The display device was often a form of a galvanometer where a thin pointer moved over the extent of the display in response to fluctuations in the sound-pressure signal from the microphone.

Without some damping of the signal applied to the pointer, the fluctuations often were so rapid that a meaningful central tendency could not be reliably obtained from observations. It was necessary to introduce a damping or exponential time weighting for the instantaneous frequency-weighted signal. The rate at which the indicated sound level fluctuates depends on the choice of the time constant for the exponential weighting function.

The running-average of the time-weighted and frequency-weighted squared sound pressure signal, in pascal squared, can be represented by the following expression where frequency-weighting A was assumed for convenience.

( )2 2 ( )/A A( ) 1/ ( ) e d

t tp t p ξ ττ ξ ξ− −

−∞⟨ ⟩ = ∫ (1)

The left-hand side of Equation (1) represents the result of carrying out the operations noted by the expression on the right-hand side. The angle brackets indicate a running average that reflects the continuous accumulation of contributions to the integral from some time in the past, as symbolized by the -∞ at the start of the integration, to the time of observation t.

At each instant of time throughout the integration period, the square of the frequency-weighted instantaneous sound-pressure signal is modified (or “weighted”) by the exponential function. In Equation (1), symbol ξ represents a dummy variable for t ime integration. Symbol τ represents the exponential time constant.

Two values for the exponential time constant are standardized: 125 ms and 1000 ms. The value of 125 ms was known originally as FAST while the 1000 ms time constant was called SLOW. Later, for international standardization, the names were reduced to letters F and S to avoid the use of English-language words.

With the F time constant, sound-pressure signals that occur earlier than about 2 s before the time of observation are essentially neglected. With the S time constant, signals that occur earlier

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than about 5 s are essentially neglected. The t ime constants slow the speed of the fluctuating pointer for the display device.

Time-weighted and A-frequency-weighted sound level LAτ(t) is formed according to:

2A

A 20

( )( ) 10 lg p tL tpτ

⟨ ⟩=

(2)

In Equation (2), the numerator of the argument of the base-10 logarithm (represented by symbol lg) is from Equation (1). The denominator of the rat io is the square of the internationally standardized reference value of 20 µPa (twenty micropascals). The logarithm of the ratio is mult iplied by 10 to yield sound levels in decibels. It is necessary to state the choice for the frequency weighting. Instruments of the 1980s

Digital technology, in contrast to the analog technology of previous years, was rapidly adopted by manufacturers of sound level meters.

Figure 16 shows two instruments that were introduced in the 1980s; both are integrating-averaging sound level meters that form and display the level of time-averaged frequency-weighted sound pressure. [This quantity is also known as an equivalent-continuous frequency-weighted sound level where the meaning of “equivalent-continuous” was that the indicated sound level of a t ime-vary ing sound pressure was equivalent to the level of a corresponding steady sound.]

Each instrument in Figure 16 has a tapered shape for the case below the microphone to help reduce the influence of sounds reflected from the case to the microphone. The shape of the instrument case is quite thin because of the use of a printed-circuit board with the elements needed to convert the analog signals to digital signals, perform the computations, and deliver the signals to the display device, The use of AA battery cells instead of the D cells, which were required for designs that required more current, also helped minimize the thickness of the case.

Note also that the diameter of the microphone is less than the diameter of microphones used for earlier designs. Designs of the 1980s often used microphones with diameters of approximately 13 mm instead of the 25-mm diameters of older designs. Advancements in technology permitted the smaller microphones to have the same or similar sensitivity as was achieved by the larger microphones.

Some instruments adopted a digital display of the sound level (for example, the Cesva SC-10) while others retained the old (but less expensive at that time) d ial-and-pointer analog display (for example, the Cirrus 2.22). What do we mean by time-av eraged sound lev el?

In the late 1970s and early 1980s, digital technology became readily available and manufacturers of sound level meters developed instruments that were capable of forming the time average of a fluctuating sound-pressure signal from a microphone and averaged over a user-selected averaging time.

Digital technology provided the capability to approximate the time average of a squared frequency-weighted sound-pressure signal. The fo llowing equations illustrate the process that is the basis for operation of a practical sound level meter.

Time-averaged frequency-weighted sound pressure is determined according to

( ) ( )2 2A A1/ d

t

t Tp T p ξ ξ

−= ∫ (3)

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The time integral of the square of the instantaneous A-weighted sound-pressure signal runs from the time at t-T to the time of the observation at time t where T is a preset averaging time interval. Symbol ξ is a dummy variable of time integration as in Equation (1).

Time-averaged (or time-mean-square) A-frequency-weighted sound pressure 2Ap is obtained

by dividing the result of the time integration by the averaging time T. The unit of 2Ap is the

pascal squared if sound pressure is in pascals. In principle, t ime-averaged A-frequency-weighted sound pressure level, LA,T or LAeq , T, is

determined, n decibels, according to

2A

A, 20

10 lgTpLp

=

(4)

In Equation (4), the numerator of the ratio of which the logarithm is taken is from Equation (3). The denominator is the square of the reference value of 20 micropascals (20 μPa). When reporting the result of a measurement, it is always necessary to identify the averaging t ime as well as the frequency weighting.

A sound level meter that displays time-averaged frequency-weighted sound levels is called an integrating-averaging sound level meter in consideration of fundamental operation as described by Equation (3). The first IEC standard for such an instrument was issued in 1985 as IEC 804:1985 [9]. See the cover page in Figure 17. An integrating-av eraging and integrating sound lev el meter of the 1980s

In the mid 1980s, B&K introduced the model 2231 as an integrating-averaging sound level meter that also provided the capability to function as an integrating sound level meter to measure sound exposure levels. Figure 18 is a photograph of the B&K 2231. Push-buttons were used to select the quantities to be measured as well as the various control settings.

A key feature of the 2231 was the length of the cylindrical element that contained the microphone preamplifier. The length of the cylinder was chosen to further reduce the influence of sounds reflected from the instrument case to the microphone, in addition to the use of a tapered case above the digital display device. What do we mean by sound exposure lev el?

Time-averaged sound level is useful for characterizing the level of sound at a location over a specified averaging time that could be as long as an hour or even 24 hours. For such measurements, all sounds occurring at the location of the microphone are included in the t ime integral.

However, if it is desired to characterize a sound that has a short duration, then it is often useful to consider measurements of frequency-weighted sound exposure level which accounts for the amplitude and duration of short-duration sounds. Such sounds include the discharge of a weapon, the drive-by of a motorcycle, or the over-flight of an aircraft. Instruments that measure sound exposure levels are known as integrating sound level meters.

Sound exposure for sound from any short-duration source is like t ime-averaged sound pressure, but without the time averaging as shown in Equation (5) where symbol EA,T represents the A-weighted exposure occurring in integration time T starting at time t1 and ending at time t2.

( )2

1

2A, A d

t

T tE p t t= ∫ (5)

With sound pressure in pascals and time in seconds, the unit of frequency-weighted sound exposure is pascal-squared seconds. The duration of the integration time is chosen as

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appropriate for the short-duration sound source so as to ensure that all significant contributions are included in the time integral. The duration of the integration time is inherently included in the integral and need not be reported explicitly.

A-frequency-weighted sound exposure level, LAE,T, is determined according to the following.

A ,A ,

0

10 lg E TE T

EL

E

=

(6)

In Equation (6), the reference value for sound exposure level, E0, is defined as the product of the square of the reference value for sound level, 2

0p , and a reference value of T0 = 1 s for sound exposure level. Thus, the reference value is 2 12 2

0 0 0 400 x 10 Pa sE p T −= = .

A multi-func tion sound lev el meter of the 1990s

Figure 19 shows the B&K model 2250 mult i-function sound level meter introduced in the 1990s. The long tapered case helped reduce the influence of reflected sound at the microphone, but was hard to hold steady in a hand. A key feature of the model 2250 was the capability to display the one-third-octave-band sound pressure level spectrum of the sound, with or without frequency weighting.

The various buttons are marked with symbols, not words or letters, to permit the manufacture of one control panel for marketing in any country. An online Instruction Manual provided instruction and information in various languages.

2000s to present

IEC 61672, International Standard for sound lev el meters

In 1990, work was init iated to develop a consolidated revision of IEC 651:1979 for time-weighting sound level meters and IEC 804:1985 for integrating-averaging and integrating sound level meters.

The result of a decade of development yielded a 3-part International Standard for all types of sound level meters in IEC 61672 [10].

Part 1 of the series was issued in 2002 as IEC 61672-1:2002; it provided specifications for performance design goals. Acceptable allowances for deviations from the design goals were provided for class 1 and class 2 performance. Parts 2 and 3 deal with test procedures and reporting.

Figure 20 shows the cover page for IEC 61672-1:2002. In 2012, work was underway to update all three parts of the IEC 61672 series.

Current v ersions of multi-function sound lev el meters conforming to IEC 61672-1

Figures 21 through 24 show photographs of hand-held sound level meters from eight manufacturers of class 1 mult i-function instruments. These instruments represent designs that were on the market in 2012. Information about the capabilities of the instruments is availab le from the manufacturer’s Internet website.

All designs use a tapered instrument case with a relatively long cylindrical device to mount the capacitor microphone and preamplifier away from the body of the instrument case. This design feature helps the instrument conform to the requirements within allowed deviations from the design goal of an omni-directional response.

Digital technology permits the option to measure a large number of acoustical quantities in addition to sound levels for characterizing the sound of a source or event.

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Technology even permits the incorporation of designs for displays that can be read outdoors as well as indoors without the power required by a background light for the display. What are key electroacoustica l performance requirements in IEC 61672-1?

Design goals and allowed deviations from the design goals are provided in IEC 61672-1 for several characteristics important for sound level meters intended for use in a wide variety of applications. Design goals include the following:

• An omni-directional response at any frequency in the range of interest for sounds incident on the microphone from any sound-incidence angle

• Response for frequency weightings A, C, and Z • Linearity of sound levels indicated in response to electrical input signals

• Time weightings F and S • Response to bursts of 4-kHz tones Figure 25 shows the design-goal response for frequency weightings A, C, and the new Z for

zero weighting at any frequency in the range from 10 Hz to 20 kHz. Many capacitor microphones that are used on even class 1 sound level meters do not have

useful output at frequencies less than about 16 Hz or greater than about 16 kHz. The allowances for deviations from design goals recognize these limitations.

Figure 26 shows the linearity of the response of a hypothetical sound level meter. There is a central region where changes in the level of steady electrical signals applied to the input of the microphone preamplifier result in identical changes in the indicated sound levels.

Figure 26 also indicates the regions where the response deviates from the intended linear response. In region 2, the self-generated background electrical noise provides a floor to the lower extent of the linear reg ion. In region 3, various sources within the instrument cause overload conditions and serve to limit the upper extent of the linear region.

A key goal of a modern design is to provide linear operating ranges with the largest practical extent at any frequency in the range of the instrument as applicable to the performance class for a sound level meter.

Figure 27 shows the design-goal response to 4-kHz tonebursts of durations from 1000 ms to 0.25 ms for a single cycle from the init ial to the final zero crossing. Toneburst response is specified as the difference between the sound exposure level of a burst and the time-averaged sound level of the corresponding steady sinusoidal signal from which a burst is extracted.

Examination of Equations (6) and (5) for sound exposure level and Equations (4) and (3) for the corresponding time-averaged sound level shows that the difference is given by

A A,

0

10 lgE TTL LT

− =

(7)

For example, a single cycle of a 4-kHz toneburst has a total duration of 0.25 ms. For the reference value of T0 = 1000 ms for time-averaged sound level, the difference is 10 lg(0.25/1000) or -36.0 dB as shown in Figure 27.

Concluding remarks

The capabilit ies of sound level meters have evolved significantly since the need for such instruments was first identified in about 1930. The weight of a sound level meter is now of the

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order of 0.5 kg to 1 kg instead of as much as 3 kg. The size of a hand-held sound level meter has been reduced significantly while increasing the functionality of the instrument.

The design of capacitor microphones has not changed greatly since capacitor microphones replaced microphones that used crystals for the sensing element.

The extent of the range over which the response of a sound level meter is a linear function of the input signal is much greater than it was even for instruments introduced in the 1980s. Similarly, the response to short-duration signals has also greatly improved.

International Standards for the electroacoustical performance of sound level meters have evolved, and continue to evolve, for all types of, and implementations for, sound level meters.

Acknowledgments

Digital files for many of the photographs shown in this article were kindly provided by representatives of manufacturers of sound level meters. Receipt of those files is acknowledged with thanks.

References

[1[ Wallace Waterfall, “A loudness scale; How the audiometer scale can be applied to problems of practical acoustics in buildings,” Engineering News Record, 1929 January 10. [2] Z24.3-1936, “American Tentative Standards for Sound Level Meters For Measurement of Noise and Other Sounds,” J. Acoust. Soc. Am. 8, 147-152, October 1936. [3] Z24.2-1936, “American Tentative Standards for Noise Measurement,” J. Acoust. Soc. Am. 8, 143-146, October 1936. [4] Z24.3-1944, “American Standard Sound Level Meters for Measurement of Noise and Other Sounds, American Standards Association, Inc., New York, N.Y. (1944 July 28). [5] IEC 123:1961, “Recommendations for sound level meters” [Intended for general purpose (survey) instruments; frequency weightings were revised from those given in Z24.3-1944.] [6] IEC 179:1965, “Precision sound level meters” [tighter tolerances than for survey meters] [7] IEC 179:1973 [Revision of 1965 issue with tighter tolerance limits around design goals for frequency weightings in the range from 10 Hz to 80 Hz.] [8] IEC 651:1979, “Sound level meters” [Consolidated revision of IEC 123 and IEC 179 for sound level meters measuring frequency-weighted and time-weighted sound levels.] [9] IEC 804:1985, “Integrating-averaging sound level meters” [For sound level meters designed to measure time-averaged sound levels and sound exposure levels.] [10] IEC 61672-1:2002, “Electroacoustics — Sound level meters — Part 1: Specifications”

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Figure 1 — Loudness “thermometer” scale with rankings for common sound sources. Scale, in

“sensation units”, runs from the threshold of audibil ity at 0 units at the bottom of the scale to 108 units at the threshold of feeling. The rustle of leaves in a gentle breeze is shown at 10 units. Noise in an airplane is at about 96 units. Il lustration is from an article by Wallace Waterfall in the “Engineering News Record” of 1929 January 10.

Figure 2 — Truck and equipment used for survey of “noise” in New York City in 1929. Sign on side of

truck reads: “Official Measuring Apparatus; Noise Abatement Commission.” Weight of the “noise meter” was in excess of 25 kg, including the batteries.

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Figure 3 — Cover page of first Standard with design goals and tolerances for the electroacoustical performance of sound level meters. The Standard was sponsored for the American Standards Association by the Acoustical Society of America (founded in 1929). The organizing meeting occurred in 1932 May for the committee that was established to draft a Standard. A draft Standard was discussed in 1934 May, revised, and approved by the American Standards Association in 1936 February 17. The Z24.3-1936 Standard was published in the Journal of the Acoustical Society of America in 1936.

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Figure 4 — Sound level meter, octave-band fi lter set, and magnetic recorder as used for 1947 noise survey of Chicago; instruments were mounted in the back of a station wagon.

Figure 5 — Portable hand-held sound level meter introduced by the General Radio Company (USA) as

model 1551A in the early 1950s. Scale intervals are in decibel steps.

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Figure 6 — Illustration of recommended use of GR 1551A sound level meter. Instrument was supported

by a strap held by the right hand with controls operated by the left hand.

Figure 7 — GR model 1555A sound survey meter introduced in the mid 1950s. Logarithmically spaced

markings on indicator dial for display of frequency-weighted sound level in decibels, but with variable resolution.

Figure 8 — Rion N1101 sound meter introduced in the 1950s. Display indicates frequency-weighted

sound level in phons because of requirements as given in a German National Standard of 1942.

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Figure 9 — Cover page for first international document issued in 1961 with recommendations for design goals and tolerances for the electroacoustical performance of general purpose sound level meters measuring frequency-weighted and time-weighted sound levels, International Recommendation IEC 123:1961.

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Figure 10 — Model S.S.T.1 sound level meter from LEA in France introduced in the 1960s. Instrument

was held by the handle and pointed at the source of sound with the display toward the observer; microphone was in the case opposite the display of sound levels in decibels. Logarithmically spaced markings on display.

Figure 11 — Design was by Peekel of Holland, but packaged and sold in the 1960s by Advance in

England as model SPM1. No frequency weightings provided; instrument only measured octave-band sound pressure levels. Microphone was mounted on a short extension rod and was flush with the case when not in use. Pushing the button above the word “microphone” near the top left of the case released a spring that extended the microphone. Logarithmically spaced markings shown on the display.

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Figure 12 — Rion NA-07 sound level meter (Japan) from the 1960s for measuring frequency-weighted

sound level in phons; later modified to indicate sound levels in decibels. Toggle switch above display device to select F or S time weighting.

Figure 13 — B&K model 2203 sound level meter (Denmark); weight about 3 kg (1960s). 25-mm-diameter capacitor microphone. Logarithmically spaced markings shown on the display. Time weightings F and S were provided.

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Figure 14 — Cover page for International Standard issued in 1979 with design goals and tolerances for the electroacoustical performance of three classes of sound level meters for measurement of f requen cy-weighted a nd time -weighted sound le vel s, Inte rnational Sta nda rd IEC 651:1979 .

Page 26 of 34


Figure 15 — Class 2 sound level meters from the 1970s: Cirrus 2.21 (England) and Rion NA-09 (Japan).

Both indicated frequency-weighted sound level in decibels on a scale with logarithmically spaced markings. F or S time weightings were provided. The Cirrus 2.21 had a cavity on the left side of the upper case where the microphone on the gooseneck could be inserted. The cavity had a loudspeaker to produce a sound-pressure signal that could be used to check acoustical sensitivity because many customers refused to purchase a sound calibrator.

Figure 16 — Integrating-averaging sound level meters from late 1980s. Left: Cesva SC-10 (Spain); right:

Cirrus 2.22 (England). SC-10 only measured A-weighted sound levels. 2.22 measured A-weighted, C-weighted, or flat-weighted sound levels as time-averaged sound levels or as sound exposure levels. SC-10 has digital indicator scale; display for 2.22 has equally-spaced markings in decibels. Thickness of instrument set by diameter of AA batteries. Microphone and preamplifier relatively close to tapered case of instrument.

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Figure 17 — Cover page for International Standard issued in 1985 with design goals and tolerances for the electroacoustical performance of three classes of sound level meters for measurement of frequency-weighted and time-averaged sound levels or frequency-weighted sound exposure levels, International Standard IEC 804:1985.

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Figure 18 — Multi-function B&K model 2231 class 1 sound level meter of the 1980s. 13-mm-diameter

microphone and preamplifier mounted away from tapered case of instrument. Digital display for level of selected quantity. Instrument measures time-averaged sound levels; F or S time-weighted sound levels, maximum time-weighted sound level in a time period, and sound exposure levels, all with choice of frequency weightings A, C, or flat.

Figure 19 — Multi-function B&K model 2260 class 1 sound level meter introduced in the 1990s. Long

tapered case minimizes effect of sound reflected from case to microphone and helps to conform to directional response requirements of international performance standards. 13-mm-diameter capacitor microphone and preamplifier. Screen can display a spectrum of one-third-octave-band sound pressure levels with or without frequency weighting.

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Figure 20 — Cover page for International Standard IEC 61672-1 issued by the International

Electrotechnical Commission in 2002 May with electroacoustical performance specifications for class 1 and class 2 sound level meters measuring frequency-weighted and time-weighted sound levels, frequency-weighted and time-averaged sound levels, and frequency-weighted sound exposure levels.

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Figure 21 — Modern designs for multi-function class 1 sound level meters–Larson Davis model 831

(Provo, Utah, USA on left); Quest Technologies/3M model SoundPro (Oconomowoc, Wisconsin, USA on right).

Figure 22 — Modern designs for multi-function class 1 sound level meters: Cirrus Research model 171B Optimus (North Yorkshire, England on left); Svantek model Svan 979 (Warsa w, Poland on right).

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Figure 23 — Modern designs for multi-function class 1 sound level meters: Cesva model SC 310

(Barcelona, Spain on left); Rion model NL-62 (Tokyo, Japan on right).

Figure 24 — Modern-designs for multi-function class 1 sound level meters: Norsonic model Nor 140

(Oslo, Norway on left); B&K model 2270 (Naerum, Denmark on right).

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Figure 25 — Design-goal response for frequency weightings A, C, and Z as specified in

IEC 61672-1 for sound level meters.

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Figure 26 — Example of l inearity of indicated level of output signal levels in response to a range of input

electrical signal levels. In region 1, the design goal for the output signal level is a l inear response to changes in input signal levels; the extent of the linear response is the linear operating range. In region 2, the self-generated noise within a sound level meter l imits the level of the indicated output signal. In region 3, elements within a sound level meter (including those that cause overload conditions) l imit the indicated level of the output signal. According to IEC 61672-1, the extent of the linear operating range at 1 kHz is required to be at least 60 dB.

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Figure 27 — Design-goal response of a sound level meter to bursts of 4-kHz sinusoidal electrical input

signals of various burst durations. A burst with a duration of 0.25 ms contains one complete cycle of a 4-kHz signal from the initial zero crossing to the final zero crossing. Toneburst response is defined as the difference between an indicated frequency-weighted sound exposure level of a toneburst and the time-averaged frequency-weighted sound level of the corresponding steady signal from which the toneburst is extracted.

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Sound level meters: 1928 to 2012 level meters 1928 to 2012.pdf · Sound level meters: 1928 to 2012 . This article reviews the development of sound level meters, their principal