Acoustical Measurements

1 © 2006 Nokia Acoustical measurements.ppt / 2006-04-19 / IJ

Acoustical measurementsIiro JantunenNokia Research Center19.4.2006

S-108.4010 Licentiate course in measurement science and technology


Contents• Principles of acoustics• Acoustics measurements • Microphone• Sound pressure level measurements

• Sound intensity measurements

• Calibration• SoundField


Principles of acoustics• Sound waves in gas or liquid

• No shear forces → no transverse waves → purely longitudinal waves

• Audible sound range 20 Hz – 20 kHz• Fully described by 3 variables

• Pressure• Particle velocity• Density


Wave equations of sound• Euler’s equation

• Newton’s 2nd law (F=ma) applied to fluid

• Continuity equation• Bringing extra air to a

volume increases density

• State equation• Relates pressure changes

to density


Wave equation of sound• Previous wave equations used pressure, density and particle velocity

• Eliminating density and particle velocity the wave equation of sound is obtained

• Two basic solutions:• Plane wave

• Spherical wave


Free field acoustics• Sound propagates to all directions without diffraction, reflection or absorption

• Spherical waves• In principle, infinite, empty space without reflections

• In practice, anechoic chamber, with near 100% absorptive walls


Free field microphone• Intended to measure the

sound pressure as it existed before the microphone was introduced

• Microphone pointed to source• Microphone tip causes an

increase in sound pressure• Taken care of by internal

acoustical damping to achieve flat frequency response


Diffuse field – random incidence microphone• Sound reflects from many

directions → sound comes to microphone from every direction

• In practice achieved in a reverberation room with 100% reflective and unparallel walls

• Microphone diffracts the sound waves from different directions in different ways

• Combined influence depends on directional distribution of sound waves

• Standard distribution based on statistical considerations used for random incidence microphone


Closed coupler• Chamber with small dimensions compared to sound wavelength

• Special case: standing wave tube

• Diameter smaller than sound wavelength

• Source at the end• Possible to calculate the

sound field• Used in calibration

• Used in microphone calibration


Pressure microphone

• Measuring the actual pressure on a wall

• Typically used in closed coupler for calibration


Microphone directionality

• Directionality indicates the sensitiveness of a microphone to sound coming from different directions

• No microphone is perfectly omnidirectional• Cardioid or hypercardioid commonly used to record vocals

• Most ribbon microphones are bi-directional• Shotgun directionality used outdoors for TV/film production and wildlife recordings


Parabolic microphone• Parabolic reflector used to collect sound waves to microphone

• Very directional• For eavesdropping in e.g. spying


Microphone transducers• Condenser microphones• Electret capacitor microphones

• Dynamic microphones• Ribbon microphones• Carbon microphones• Piezoelectric microphones• Laser microphones


Condenser microphone• Diaphragm and backplate form a plate capacitor

• Charge kept constant → voltage varies as pressure actuates the diaphragm

• External voltage supply or pre-charged diaphragm

• Acoustical performance determined by physical dimensions


Condenser microphone – cont• The larger the diaphragm, the

more sensitive the microphone

• Upper limit is defined by diaphragm touching the backplate

• The smaller the microphone, the greater the frequency range

• Increasing tension extends range but decreases sensitivity

• Optimum size of a measurement microphone is (up to 20 kHz) is about 12.6 mm (1/2’’)

• Damping effect of air reduced by drilling holes in the backplate


Electret microphone• Invented at Bell Labs in 1962

by Gerhard Sessler and Jim West

• Diaphragm permanently polarized the same way as permanent magnets magnetized (electrostatic magnet)

• Once considered low price and low quality

• Now most common microphone type


Dynamic microphone• A movable coil is attached to

the diaphragm• An unmovable magnet

produces a magnetic field• Moving diaphragm moves the

coil in the magnetic field, inducing a measurable current

• Exactly same principle as in loudspeakers, only reversed

• Poor low-frequency response → reduces handling noise

• Robust, relatively inexpensive and resistant to moisture→ widely used on-stage


Ribbon microphones• Revolutionized recording and

broadcast industry in the 30’s• Special type of dynamic

microphones• Thin metal ribbon between

poles of magnet• Voltage output typically low

compared to normal dynamic microphones

• Bidirectional• Very sensitive and accurate• Generally delicate and

expensive


Carbon microphones• Invented by David Hughes in

1878• Very important in the history

of telephone• Sound pressure (AP) presses

the diaphragm (2) to a bed of carbon granules (1). Contact resistance depends on the pressure → resitance R changes

• Also an amplifier• Extremely low-quality sound

reproduction• Very limited frequency range• Very robust


Piezo microphones• Piezoelectric material• Diaphragm moves the

armature to bend piezoelectric crystal over a fulcrum

• Small size, cheap, low quality• Have replaced carbon

microphones• Often used as

• contact microphones to sound instruments

• underwater or other unusual environments


Laser microphones• Window of a room acting as

diaphragm• Reading with laser beam

reflected from the window• Two laser beams for common

mode rejection of large window movements and path disturbances

• For eavesdropping• Works best with one-glass

windows


Sound level measurements• Measurement of sound pressure filtered by

• frequency (A-weighting)• time-domain (RMS)

• Mimics response of human ear to noise


Human hearing frequency response

A-weighting curveFor subjective responses in special cases there are B-, C- and D-weighting curves•very high or low level•special noise, e.g., of aircraft


Sound level measurements• IEC International Standard 651 ”Sound Level Meters”

• Tolerances per frequency band defined for 4 classes of accuracy

• Type 0: precision laboratory use• Type 1: general purpose• Type 2: low price• Type 3: not used in practice (too wide tolerances)


Sound intensity measurements

no. x/r y/r z/r

1 -0.99 0 0.15

2 0.5 -0.86 0.15

3 0.5 0.86 0.15

4 -0.45 0.77 0.45

5 -0.45 -0.77 0.45

6 0.89 0 0.45

7 0.33 0.57 0.75

8 -0.66 0 0.75

9 0.33 -0.57 0.75

10 0 0 1.00

ISO Standard 3745 “Acoustics — Determination of sound power levels of noise sources — Precision method for anechoic and semi-anechoic rooms”


Two-microphone probe• Measures the sound intensity

in two directions• Pressure is mean of the two

measured pressures• Air particle velocity calculated

from the two pressures

• All intensity is in radial direction, no intensity in perpendicular

• Powerful tool to locate noise sources


Calibration techniques• Reciprocity calibration method• Comparison or substitution methods• Pistonphone (closed coupler)• Sound pressure calibrator• Electrostatic actuation


Reciprocity calibration method• Microphone can be used as a loudspeaker

• Three test microphones measured against each other alternating the function

• As a result a set of 3 equations with microphone sensitivities as unknowns

• Very accurate• Rather tedious• Requires well-controlled environment

• Seldom used in practical situations


Comparison/substitution methods

• Microphone measured related to a reference microphone

• Comparison method: microphone and reference at the same time

• Substitution method: microphone put in the lace of the reference

• Sound source stability


Pistonphone• Closed coupler• Well-defined sound pressure level

• Relatively simple mechanically, very stable

• Used often as the sound source in comparison/subsitution calibration

• Accuracy around 0.1 dB

• Depends on• Volume of the coupler• Volume displacement• Barometric pressure• Humidity• Heat dissipation


Sound pressure calibrator• Small, self-contained• Comparison calibrator• Closed coupler• Small loudspeaker produces single-frequency signal

• Reference microphone gives feedback signal

• Well-defined, provided that reference microphone and feedback gain are stable

• For field-calibration of microphones

• Normally not for laboratory calibrations


Electrostatic calibration• Direct use of electrostatic actuator to drive the diaphragm

• 800 V DC• 50-150 V AC signal

• Generally used to measure frequency response of microphones

• Widely used as a convenient and accurate test method

• For production and final calibration of measurement microphones


SoundField microphone• 3D view of the sound with a

single device• 4-channel measurement of

sound: B-format• The spatial pattern can be

decided later• Mono, stereo, 5.1, …

• Fairly expensive, but replaces effectively a system of many microphones

• http://www.soundfield.com

http://www.soundfield.com/

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Acoustical Measurements