Lindborg RoomAcou Summary

Barron, Michael (2010). Auditorium Acoustics and Architectural Design. 2nd edition. Spon Press.

Summary (& some extensions)by PerMagnus Lindborg (2011).

(1) Introduction

Barron: Auditorium Acoustics and Arcitectural Design. (Lindborg)

(not part of course requirements)

(2) Sound and roomsWaves


Direct sound

Reverberation time

Reflection

Absorption

Scattering

Background noise

Sound level distribution

Seat-dip effect

Diffraction

Waves


The propagation of sound energy in an elastic medium, such as air, is mainly in the form of longitudinal waves…

…rather than as transverse waves, which is more typical of e.g. string vibration.

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

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

More: http://www.physicsclassroom.com/mmedia/waves/lw.cfmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/modepropagation.htm





http://www.physicsclassroom.com/mmedia/waves/lw.cfm

http://www.physicsclassroom.com/mmedia/waves/lw.cfm

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/modepropagation.htm




Waves


Illustrations on this page: Rumsey, F & McCormick, T. (2006). What is sound?. Chapter 1 (pp. 1-24) in Sound and Recording. Focal Press 2006.

One way to understand what is happening physically is to visualise a ball or a ballon that is expand, contracting, expanding, contracting…

The wall of the ballon pushes the surrounding molecules in the air away.

In some volume, molecules find themselves packed together:

<<-- Let’s look at a snapshot of a portion of space.

this is a compression, with a higher than usual pressure.

At another point, there are fewer molecules:

this is a rarefaction, with a lower than usual pressure.

The energy is propagated in all directions of space (3D).

These molecules travel a distance, before bumping into other moleculelike colliding balls on a pool table, or Newton’s cradle.

http://en.wikipedia.org/wiki/Newton's_cradle

http://en.wikipedia.org/wiki/Newton's_cradle

Waves


decibel is a ratio, indicating the relationship between two quantities.

0.000 020 N/m2 200 N/m2

Our hearing apparatus can handle sound intensity in a wide range, approximately

The hearing range in terms of intensity is fourteen powers of ten = 14 bel = 140 decibel.

and the lower limit is by convention the reference level in acoustics (ANSI-1969/ISO-1963).

0.000 000 000 001 W 100 W(1 picoW)

(20 µPa)

In terms of pressure, the hearing range expressed above corresponds rather well to

A sound level is indicated i.e. 60 dB re 20 µPa, or more succinctly, 60 dBSPL.

SPL = “sound pressure level”.

Acoustic pressure is proportional to the square root of the intensity: “real mean square”, RMS.

More: http://en.flossmanuals.net/csound/ch008_c-intensities/

http://en.flossmanuals.net/csound/ch008_c-intensities/

http://en.flossmanuals.net/csound/ch008_c-intensities/

Direct sound


Direct sound is the sound that travels from a source to a listener in a straight path (without reflections). It is the part of sound that arrives first.

Direct sound in an enclosed space (e.g. a concert hall) behaves much the same way as in a free field (e.g. outdoors). For every doubling in distance, the sound level decreases by 6 decibel. This follows from the inverse square law of propagation.

In a concert hall, a listener will generally receive good direct sound if she has an unobstructed sightline.

Borb (2008). http://en.wikipedia.org/wiki/File:Inverse_square_law.svg

More: http://www.acoustics.salford.ac.uk/feschools/waves/propagation.htm

With increasing distance from the stage, the floor needs to be (increasingly) raked.

I = P/ 4πr2

Intensity= Pressure/Area

http://en.wikipedia.org/wiki/File:Inverse_square_law.svg

http://en.wikipedia.org/wiki/File:Inverse_square_law.svg

http://www.acoustics.salford.ac.uk/feschools/waves/propagation.htm

http://www.acoustics.salford.ac.uk/feschools/waves/propagation.htm

Seat-dip effect


It was discovered in 1962 that audience members receive a harmonically distorted direct sound, i.e. the propagation does not have a flat frequency response. Direct sound travelling at grazing incidence will be subject to absorption over and above the inverse square law, but the effect is different at low and mid frequencies.

The strong attenuation around 125 Hz is assumed to be due to resonance effects happening between rows of seats. It is sometimes referred to as a seat-dip effect, and would be related to Helmholtz resonators. Remarkably, The attenuation does not depend on distance but is uniformly distributed.

More: http://www.rexresearch.com/helmholtz/helmholtzresonators.htmhttp://physics.kenyon.edu/EarlyApparatus/Rudolf_Koenig_Apparatus/Helmholtz_Resonator/Helmholtz_Resonator.html

At low frequencies, our hearing is less sensitive to inter-aural time differences. Deficiency in direct sound can thus be compensated by late sound, e.g. a long enough reverberation time at low frequencies.

The broad-band attenuation around 800 Hz would be due to absorbent seating materials.

The diagram also indicates that lateral reflections are unaffected by the broad-band seating material absorption, but the seat-dip effect is equally pronounced as for direct sound.

http://physics.kenyon.edu/EarlyApparatus/Rudolf_Koenig_Apparatus/Helmholtz_Resonator/Helmholtz_Resonator.html




Reflection


Geometries for reflection for light and sound are identical.

Pure reflection (specular) of a sound wave on a finite-size reflector happens for frequencies that are higher (that is, with shorter wavelength) than a certain number. This cutoff frequency depends on the apparent size of the reflector from the vantage points of both the source and the listener. (See Barron p. 457 for details).

Reflection between a source and a listener can happen when a reflector is placed as a tangential to an ellips with Fsource and Flistener as foci.

Good concert hall design maximises the density of the subjectively important early reflections. One way of achieving this is to have shape and dimensions similar to an ellipse.

(Note: 50 ms is has been chosen based on psychoacoustic thresholds. Walls should generally should not be curved, to avoid slap-back echoes at points near the ellipse’s two axes.)

Diffraction


Diffraction of sound waves is normally a low-frequency phenomenon.

At low frequencies, sound waves recombine behind a small obstacle, while higher frequencies are reflected away (see page on reflection).

Diffraction occurs whenever a propagating sound wave encounters an obstacle. The bending or blocking effects are most pronounced for waves where the wavelength is roughly similar to the dimensions of the diffracting object.

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

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

Scattering


Scattering takes place when a reflecting surface is irregular.

A Quadratic Residue Diffuser can be constructed to have specific scattering properties. The amount of scattering and frequency range are a function of the depth and shape of the profile. Both plane (1-dimensional) and 2-dimensional QRDs exist.

More: http://www.subwoofer-builder.com/qrdude.htmhttp://www.digitalaudiorock.com/cgi-bin/qrd.cgihttp://www.pmerecords.com/Diffusor.cfm

http://www.subwoofer-builder.com/qrdude.htm




http://www.digitalaudiorock.com/cgi-bin/qrd.cgi








Absorption


Absorption is the energy loss from the sound wave travelling in the propagation medium (e.g. the air) into the reflecting medium (e.g. a wall).

In practice, all building materials are scattering and absorptive to some degree.

More: http://www.sae.edu/reference_material/pages/Coefficient%20Chart.htm

The absorptivity of a material for construction or interior design is normally measured in a laboratory. Standard tables are published and much in used in prediction of reverberation time.

Get acquainted with different materials with the AbsorptivityViewer! (Lindborg 2011)

http://www.sae.edu/reference_material/pages/Coefficient%20Chart.htm

http://www.sae.edu/reference_material/pages/Coefficient%20Chart.htm

http://www.permagnus.net


Background noise


Concert halls are often constructed on urban sites, where traffic and other noise sources “necessitate substantial expenditure to achieve inaudibility inside the auditorium”. (Barron p. 27)

The main contribution to background noise - in concert halls, laboratories and recording studios alike - comes from ventilation systems. This is particularly the case in places where air conditioning is used a lot (such as in Singapore).

As a measure of background noise, the NR (Noise Rating) criterion is most common in Europe. (Other measures include NC, PNC and RC).

http://www.engineeringtoolbox.com/nr-noise-rating-d_60.html

NR is estimated by measuring SPL of ambient noise at 9 octave-band filters. The resulting curve is compared with the template (see diagram) and moved ‘upwards’ or ‘downwards’ so that no measured band exceeds the template’s at a corresponding frequency. The rating is determined by the minimal distance between measured curve and a NR template curve.

Large concert hallOpera houseSmall auditorium

NR15NR20NR25

Criteria are strictest for large concert halls:



Reverberation time


The most important acoustic measure for a room is its response. The sound that can be heard when a loud source is turned off is call terminal reverberation. Subjectively, in situations of speech or music, the running reverberation, i.e. the early part of the reverberation, has been shown to be more salient.


The RT60 measure is the time in seconds it takes for a terminal reverberation to recede to a millionth of its energy, i.e. by 60 decibel. In practice, it is calculated from the slope between -5 and -35 dB below steady-state maximum. This assumes a linear decay, which is not always the case. The presence of coupled acoustic spaces with different reverberation characteristics can give rise to non-linear decay. In auditorium acoustics, such coupled spaces can be found between the main hall and a reverberant flytower, or an orchestra pit, or under large balconies.

Drama theatreChamber music and operaClassicist musicRomantic musicOrgan

0.85 +- 0.151.55 +- 0.21.7 +- 0.12.0 +- 0.2>2.5

Recommended RT60 values (in seconds) for auditoria depend on programme:

Early decay time approaches RT60 for rooms that are highly diffuse.

Reverberation time


Wallace Clement Sabine is the founder of modern acoustics. He discovered around 1900 that a room’s reverberation time depends on two things only:- room volume (V)- total absorption (A)


The total absorption is the sum over all elements of the product of each element’s size and absorptivity:

A = ∑ S*aThe equation for reverberation time (in metric units):

0.161*V A

T =

In auditoria, the major absorbing surface is people. Beranekt (1969) proposed a model where reverberation time is estimated from volume and ST, the total floor area of audience, orchestra, and chorus (Kinsler & Frey p. 345):

1/T = 0.1 + 5.4*ST/VThe discussion of how to find optimal measurement method and expression for a prediction is on-going. The economic gains with a more exact prediction are obvious. For example, prediction methods currently in use depend on precise values for the absorptivity of seating upholstery.

At higher frequencies, reverberation time also depends on air absorption, which in its turn depends on temperature and humidity.

Sabine’s formula for predicting total absorbtion was not correct, mainly because he was calculating seat absorption on a per-seat basis rather than on per-square-meter basis. This caused him to grossly overestimate the reverberation time in Boston Symphony Hall (Barron p. 89).

Sound level distribution


The subjective sense of loudness is determined by the objective sound level.


The theoretical behaviour for total sound level takes in the distance between source and listener, but not the room characteristics apart from reverberation time. Theory assumes uniformity of propagation paths, which is rarely the case in actual auditoria. For example, sound pressure decreases substantially below balcony overhangs.

We have seen before that the total sound level a listener experiences can be split in two parts: direct sound and reflected sound. The distance at which these components are of equal intensity is called the reverberation radius.

Based on measurements in a number of 15 British halls, Barron proposed an expression for reflected sound level as a function of source power (SWL, in decibels), A, T and r (source-listener distance):

reflected level = SWL + 10*log 4A

0.174*r T

-

The formula leads to a revised theory of reflected sound.

Reverberation time


Check predicted RT60, frequencies for standing waves, Schroeder frequency etc with the RoomAcousticsCalculator! (Lindborg 2011)


http://www.permagnus.net/





Reverberation time


Calculate reverberation time from a soundfile using the RT60 estimator! (Lindborg 2011)




(3) Acoustics for the symphony concert hall

Subjective dimensions


Initial Time Delay Gap

Three Questionnaires for subjective ratings

Objective descriptors

ISO 3382

Design recommendations

Design for performers

Subjective dimensions


“Good acoustic design for good speech is easy… speech must be intelligible!”

With music, no direct assessment exists.

The appreciation of music acoustics is multi-dimensional.

clarity

reverberant response

impression of space

intimacy

loudness

These five dimensions have been derived from questionnaires (we assume, principal component analysis from multiple descriptors).

(quantitative assessment with e.g. BKB-SIN, HINT, QuickSIN, or WIN)

5D Cube

A starting point is to consider the following dimensions:

http://www.youtube.com/watch?v=-yHIAw5izqU

http://www.youtube.com/watch?v=-yHIAw5izqU



Tone colouration is strong (according to Barron, in particular for strings) with a reflection delay of around 20 ms.

Marshall (1967) proposed that reflections from the sides are preferred to those from above.

Lateral reflections gives the listener a sensation of the source broadening, of being involved in a three-dimensional experience.

Morimoto and Maekawa (1989) provided empirical evidence about the temporal aspect of laterality:

Beranek (1962) initiated a survey of 54 international concert halls (conductors, performers, music critics) to estimate a subjective rating of each hall. The highest explanatory power (for preference) was found for a component associated with initial time delay gap, that is, the delay of the first reflection.

Early lateral reflections are associated with a sense of source broadening;

Late lateral reverberation is associated with a sense of listener envelopment.

For the best-liked halls (incl. classical rectangular): gap ≤ 21 ms.

apparent source width (ASW)

listener envelopment (LE)Hearing is believing: try the “TimeDelayGapTester” (Lindborg 2011)

source broadening also depends on loudness





Hearing is believing: try the “TimeDelayGapTester”



Barron’s questionnaire for subjective rating of concert hall acoustics


= ability to hear musical detail

= degree of perceived reverberation in a temporal sense

= degree of perceived reverberation in a spatial sense

= degree of identification with the performance (feeling acoustically involved or not)

~ assessed relative to orchestral forces involved

apparent source width

listener envelopment

Questionnaire for concert hall rating (paper based)


1. DESIGNATION of this room _____________________

2. For each dimension, place a mark to indicate YOUR SUBJECTIVE IMPRESSION of the acoustics.

CLARITY Muddy |---------|---------|---------|---------|---------|---------|---------|---------|---------|---------| Clear(how well can you hear details of the source sound?)

REVERBERANCE Dead |---------|---------|---------|---------|---------|---------|---------|---------|---------|---------| Live(how would you describe the reverberation in a temporal sense?)

SOURCE WIDTH Large |---------|---------|---------|---------|---------|---------|---------|---------|---------|---------| Small(how would you describe the source broadening?)

ENVELOPMENT Expansive |---------|---------|---------|---------|---------|---------|---------|---------|---------|---------| Constricted(to what degree do you feel surrounded by sound?)

INTIMACY Remote |---------|---------|---------|---------|---------|---------|---------|---------|---------|---------| Intimate(do you feel acoustically involved and able to identify with the source?)

LOUDNESS Loud |---------|---------|---------|---------|---------|---------|---------|---------|---------|---------| Quiet(relatively speaking, how loud does the room seem to be?)

3. Indicate how you perceive the BALANCE between MID-range, and BASS and HIGH ranges:

TREBLE to MID Weak -------------|-----------------|------------------|------------------|----------------|------------Treble louder

BASS to MID Weak -------------|-----------------|------------------|------------------|----------------|------------ Bass louder (balanced)

4. BACKGROUND |---------|---------|---------|---------|---------|---------|---------|---------|---------|---------|NOISE (inaudible) (Acceptable) (Tolerable) (Intolerable)

5. OVERALL IMPRESSION: indicate by circling one of the words:

5. Add a SHORT COMMENT about this room:

A questionnaire developed for a pilot study (N=20) of subjective and objective measurement of 3 rooms (Lindborg 2011) with Barron’s template as a starting point.

Questionnaire for soundscape rating (software based)


An interface for subjective rating of soundscapes (Lindborg 2011) using the Swedish Soundscape Quality Questionnaire (Axelsson, Nilsson et al 2010).

… in parenthesis, questionnaire for a study in a related topic (not concert hall acoustics!)

3.3 Objective descriptors


Reverberation (RT or T) and early decay time (EDT)

Objective clarity (early-to-late index C80)

T is RT60 is calculated from the reverberation tail from -5 dB to -35 dB

EDT is RT60 is calculated from the reverberation tail from 0 dB to -10 dB

Time (s)0.806991 1.57667

24.12

54.34

Inte

nsity (

dB

)

Time (s)0.12237 0.755128

24.92

54.57

Inte

nsity (

dB

)

Time (s)0.134193 0.763125

24.01

54.58

Inte

nsity (

dB

)

0.2 0.3 0.4 0.5 0.6 0.7

30

36

42

48

54

Auditorium, 3 reverberation tails (white noise)

http://pcfarina.eng.unipr.it/Aurora/SAW/RoomSim.html

C80 = energy 0…0.08 senergy 0.08…+∞ s

For music, the breakpoint 80 milliseconds is standard. For speech, 50 ms is used, which corresponds to normal speech rates (measured by e.g. r- and n-PVI). Average inter-vocalic durations for several languages are in the range 40--75 ms.

http://wwwu.uni-klu.ac.at/gfenk/posterLondon2.pdf

http://students.uta.edu/MS/msb3676/THE%20CONTINUUM%20OF%20SPEECH%20RHYTHM%20posterFinal.pdf

http://wwwhomes.uni-bielefeld.de/~gibbon/AK-Phon/Rhythmus/Grabe/Grabe_Low-reformatted.pdf

An alternative measure is centre time, i.e. the energy ‘equilibrium point’ in time (∫0∞)

∫ t p2(t) ∫ p2(t)

centre time =

EDT is calculated from the reverse-time integrated impulse response. It correlates better with the subjective judgement of reverberance than T.

Barron suggests four objective descriptors. All can be calculated from impulse-response recordings.

Using impulse response has advantages to steady-state noise, in that random error at turn-off is eliminated. The time domain IR can be converted into a frequency response curve with a Fourier transform.

http://www.acoustics.salford.ac.uk/acoustics_info/concert_hall_acoustics/?content=testing










3.3 Objective descriptors


Objective source broadening (early lateral energy fraction, LF)

Reflectors and objective support

LF80 = lateral energy 0…0.08 stotal energy 0…0.08 s

∫0.0050.08 p2(t) cos2 theta ∫00.08 p2(t)

= [theta ≈ axis of ears of a listener. The measurement can be made with a Blumlein + omni configuration]

source broadening = LF80 + early level 60

The perceived degree of source broadening also depends on direct sound level, and Barron writes:

L - L0 = 10*log(d+er+l)

d = 100/r2

er = (31200 / V) e-0.04r/T * (1 - e-1.11/T)

l = (31200 T / V) e-0.04r/T * e-1.11/T)

L - L0 = 10*log (100/r2 + 31200 T / V)

Using Sabine’s equation for the relationship between total absorption and reverberation time, the dB relationship between the total sound level, L, and the direct level at 10 meters, is:

where r is the distance between source and receiver.

Barron’s research showed that total sound level cannot be considered as uniform in an enclosed space, but is reduces with increased distance. as revised theory gives for the revised total sound level:

[with 10 m as standard, L - L0

is in a range around 0 dB]

exponential function of distance (r)

Design recommendations


Barron discusses recommended values for the objective characteristics that acoustic designers should aim for.

Early Decay Time (EDT) 1.8…2.2 secondsObjective Clarity -2…2 dBObjective Source Broadening 0.1…0.35Total sound level(symphonic orchestra) ≥ 0 dB

(varies with position in hall) EDT / RT mean = 0.96, range = 0.79…1.06.

(early-to-late energy ratio)

(lateral-to-omni energy ratio) mean = 0.18, range = 0.05…0.50

In Barron’s study of British halls:

(re direct sound @ 10 m)

Example of graphic presentation of objective measurements (Royal Concert Hall, Nottingham, UK)

Reverberation time

ISO 3382-1:2009


http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=40979



3.8 Design for performers


- early reflections

An ‘orchestral shell’ is often advantageous. EDT onstage should be ≥ 70% of RT in the main hall.

- floor materials

- hearing damage is unfortunately common amongst orchestra musicians (around 25%, as quoted in the PDF above)

The acoustic needs of musicians on stage have only been systematically investigated since the mid-1970s.

Main concerns for performing instrumentalists: - ease of hearing each other (chamber music!) - acoustic support for own instrument - avoiding tone colouration (often introduced with overhead reflectors)

Designers will want to carefully consider:- size and shape of the stage

With 8 meter distance, the delay (around 20 ms) causes problem for synchronisation (again, see the TimeDelayGap tester)

- high SPL close to e.g. timpani and brasses

Use of plexi-glas screens is common, especially in opera pits. Wall absorption reduces early reflection, important for support!

- adequate floor space

Chorist ≈ 0.5 m2, small instrument ≈ 1.25 m2, larger instrument 1.5 m2, percussion up to 20 m2, 100-piece orchestra ≈ 150 m2.

A thin wooden floor with air-filled cavity can act 1) as a resonator (Askenfelt 1986) for instruments in direct contact

http://www.speech.kth.se/music/publications/kma/papers/kma52-ocr.pdf

, but also 2) as an (Helmholtz) absorber for low-frequency sounds reaching it through the air.





(4) The development of the concert hall


Study of a success

Study of a failure

A lateral directed reflection sequence hall

Design factors

Historic overview

Baroque theatre

Historic overview

Fan shape


Classical rectangular plan

Arena

“The width of a hall might be related to the maximum acceptable width for a stage, the length to visual distance, and the height to reverberation time considerations…” (p. 75). While it seems that the overall dimensions of an auditorium are set, (luckily enough for eager designers) the shape isn’t.

Until around 1900, auditorium design was based on (serendipitous) precedence. With Sabine, enter science.

The four ‘traditional forms’, in order of historical appearance:

Concertgebow (Amsterdam)

Historic overview

Grosser Musikvereinssall (Vienna)


Symphony Hall (Boston)

Neues Gewandhaus (Leipzig)

In small halls, the room form matters relatively little for the acoustics.

The ‘four most renowned classical concert halls’ (Beranek 1962), are

Construction around 1900

Volume 17000 m3

Seats 2000 pax

Length 48 m

Width 22 m

Height 17 m

T 1.85 s

H : W : L 1 : 1.22 : 2.70

The dimensions of these 4 halls are similar (see p. 80 for details). Averages are:

Design factors


The subjective acoustic experience is not due to any single factor. Risking absurd reductionism, which are the design factors to keep in mind for good concert hall acoustics?

“It is much easier to determine the reason for failure than for success” (Barron p. 90)

• all seats are close to a reflecting surface

• parallel side walls produce high density of (early) reflection

higher lateral energy fraction

higher early-to-late index

more envelopment

higher clarity

• height under ceiling sufficient reverberation time more warmth, envelopment

• choice of material sufficient low-frequency response more warmth

more late lateral reverberation more envelopment• reflecting surfaces close to seats, choice of materials

Which are the design factors that may cause bad concert hall acoustics?

• too much reliance on overhead reflectors

colouration of higher frequencies

instruments (e.g. strings) sound sharp

• bad sightlines low direct sound level lack of intimacy

• further away seats not raked

lacking direct sound level at low frequencies

lack of power

Study of a failure


New York Philharmonic (1959) - “the most publicized acoustic disaster of the twentieth century”.

main floor rake is remarkably shallow (bad sightlines + obstructed direct sound)

rear wall is concave (slap echoes)

many seats have receive no lateral reflection

stage too large = orchestra too widely spaced(lack of intimacy and self-sound support for musicians)

double layer of small and regular suspended reflectors (“some 30,000 small dowels”, http://en.wikipedia.org/wiki/Avery_Fisher_Hall)

(did not reflect low frequencies)

good thing: discovery of seat-dip effect on low frequencies (direct sound passing over seat rows at grazing incidence)

level of bass to mid-frequency sound approaching -20 dB (criterion is -2 dB)

“a poor frequency response affecting audibility of cellos and double basses, a lack of subjectively felt reverberation, echoes from the rear, inadequate sound diffusion and poor hearing conditions for musicians on stage” (Schroeder 1966)

critics described sound as “clear, a little dry, with not much reverberation and a decided lack of bass”… “steely hardness, the fiddles sounded harsh, and the orchestra’s sections failed to blend, as if invisible walls stood between strings, woodwind and brasses”

Study of a success


Berlin Philharmonie (1963) - “people always gather around in circles when listening to music informally”.

“space acts as a positive force upon the life it contains”

“Music as the focal point… no segregation between ‘producers’ and ‘consumers’, but rather a community of listeners… the construction follows the pattern of a landscape. with the auditorium as a valley… the ceiling, resembling a tent, encounters this ‘landscape’ like a ‘skyscape’” (Scharoun ca. 1963)

with the arena form there are two major acoustic concerns:- the voice, and many instruments, are directional- surfaces are needed to provide early lateral reflections

"…product of the Expressionist movement and of organic architecture, this concert hall in which the audience is seated around the orchestra was worked out in accordance with the laws of acoustics." (Stierlin http://www.GreatBuildings.com/buildings/Berlin_Philharmonic_Hall.html)

solutions:- not fully central stage, directionally biased- ‘vineyard’ terraced concert hall

critic to balance: acoustic uniformity somewhat uneven

tent-like profile of convex surfaces assist diffusion

pyramidal diffusers with slits acting as Helmholtz resonators (limiting low-frequency reverberation: 1.9 s at mid, 2.1 s at low)


In the 1970s, Harold Marshall developed a theory that “lateral reflections were the most important single component of the early reflection series”

http://www.vbase.co.nz/town_hall_for_performing_arts/facilities

A lateral directed reflection sequence hall

Marshall Day Acoustics have recently completed a major work for Guangzhou opera House (architect: Zaha Hadid)…

http://marshallday.com/project/christchurch-town-hall-christchurch-new-zealand

http://marshallday.com/project/guangzhou-opera-house-china

http://www.guardian.co.uk/artanddesign/2011/feb/28/guangzhou-opera-house-zaha-hadid

…receiving raving reviews from a.o. The Guardian.

The designers have accorded primordial attention to early lateral reflections (click image for PDF design article)

Christchurch Town Hall (New Zealand)

inclined balcony soffitslarge suspended reflectors

Both scale (using ultrasound) and computer models were employed to optimise the designEarly decay time is only 82% of RT60 reverberation, explaining the subjective impression of rich reverberation but not excessive, despite T=2.3 s when occupied!

absorbent

All seats receive a reflection within 20 ms.









http://marshallday.com/system/files/The%2520Room%2520Acoustic%2520Design%2520of%2520the%2520Guangzhou%2520Opera%2520House%255B2%255D.pdf

http://marshallday.com/system/files/The%2520Room%2520Acoustic%2520Design%2520of%2520the%2520Guangzhou%2520Opera%2520House%255B2%255D.pdf

(5) British concert halls and conclusions for concert hall acoustics


Royal Festival Hall

Acoustical design of concert halls in subjective and objective terms

Royal Albert Hall

Royal Albert Hall


The infamous ‘slap-back’ of the Royal Albert Hall was noticed already at the opening ceremony in 1871, an “echo which seemed to be suddenly awoke from the organ or picture gallery”.

It was found that the oval shape of the plan and concave ceiling (the largest unsupported span roof at the time) in iron and glass, and generally hard wall surfaces, created perfect conditions for a focussed echo.

In 1941, an enormous ‘velarium’ was suspended 10m below the ceiling, which almost cured the echo. However, the solution was deemed too risky.

In 1968-70, a set of 134 diffusers - known as the ‘mushrooms’ or ‘flying saucers’ were suspended.

The distance from stage to back wall being almost 60 meters, a person on stage or in first row would have received a 300 ms delayed echo!

Subjective ratings of the hall are generally poor:- quiet sound (due to added absorbent material)- lack of clarity (least intimate of all Hall’s in Barron’s study)- low sense of envelopment (most early reflections from above)

At RT60 = 2.4 s, despite the efforts for acoustic retrofitting, the reverberation time is the largest of all Halls in Barron’s study.

More: http://peutz.fr/lacoustique/articles/salles/PaperIOA02.pdf

http://peutz.fr/lacoustique/articles/salles/PaperIOA02.pdf


Royal Festival Hall


The Royal Festival Hall (London) was constructed in only 2.5 years and completed 1951. Acoustic consultant was Hope Bagenal. Inspired by the rise of cinema, a fan-shaped layout was chosen.

Haunted by the echo of Royal Albert Hall, the designers had chosen highly absorbent materials at the back wall. Even more important (2/3 of the prediction error), they had seriously underestimated audience absorption. (see earlier slides).

More: http://www.hughpearman.com/2007/08.html

The design aim was RT60 = 2.2 s…

In 1965, the world’s first electronic ‘assisted resonance’ system was invented by Parkin for the Hall, improving subjective

However, a solution without electronics was sought. 2005-7, Pearman et al. modified the layout, removed absorbing surfaces, replaced the suspended ceiling with a thicker one, and replaced the overstage reflector with one having special reflective, adjustable material.

A cavity wall was built to insulate against street noise.

There is a single, very deep balcony.

To achieve the goal of 2900 seats, and with no seat further away than 40 meters, the hall had to be 32 m wide, and splayed from the stage outwards.

What had happened?

Resulting in an occupied mid frequency RT = 1.65 s, and a longed-for significant bass rise.

Despite the shortcomings, the Hall is very well liked.

…but turned out to be 1.45 s.

http://www.hughpearman.com/2007/08.html

http://www.hughpearman.com/2007/08.html

Acoustical design in subjective and objective terms


Listeners divide in three main groups following their preference for either clarity, reverberance or intimacy.

Barron (1988) found that listeners subdivided into two groups: those that preferred intimacy and those that preferred reverberation. He refers to Cremer and Müller (1981) who reported a subdivision between clarity and loudness.

To some extent, clarity and reverberation are inversely related.

High clarity seemed to be preferred by those with a musical background.

The strongest correlations between subjective quality and objective measure:

clarity

reverberance

source broadening

intimacy

loudness

warmth

mid-frequency early-to-late index

early decay time

early lateral energy fraction

total sound level

mean frequency total sound level

bass level balance

+ mean frequency total sound level

+ source-receiver distance

However, when it comes to concert halls, they are independent measures.

We have earlier seen the five dimensions for perception of concert hall acoustic quality. Warmth is added to this list.

Acoustical design in subjective and objective terms


provide sufficient early reflection

Aiming for good clarity, reverberance, envelopment and intimacy, our design needs to:

have adequate early decay time occupied EDT ≥ 1.8 s

ensure significant early lateral reflections

have adequate sound levels level ≥ 0 dB

early lateral energy + sound level + reverberation time --> envelopment

Problematic spots include:- lacking sound level at seats under balconies- lacking direct sound at side seats with awkward sight lines

Barron identifies the ‘large concert hall problem’: the difficulty of achieving good acoustics in a concert hall rises sharply when seating numbers > 1500. Very good halls with > 2000 seats are rare.

directed reflection approach = modulating reflecting surfaces, avoiding large unbroken plane surfaces

vineyard terrace design = acoustically reflective bounding walls and balcony fronts

parallel-sided plan with side walls not too far apart, has less audience capacity

Note: orchestra musicians will be helped by an on-stage surrounding shell.

… and proposes three ‘solutions’:

(6) Chamber music and recital halls

Differences between symphony halls and chamber music halls



(7) Acoustics for speech


Speech in open spaces and rooms

Early reflection ratio


Theatre acoustics

(8) Theatre acoustics

Classical amphitheatre



Elisabethan theatre

Proscenium

Thrust-stage

Open stage

In the round

(9) Acoustics for opera

The orchestra pit


Architectural design

Objective assessment of opera houses

Opera

The singer’s voice

Acoustic criteria

Breaking the horseshoe

http://oxidonegro.tumblr.com/post/6791492770/betonbabe-j-rn-utzon-sydney-opera-house

http://oxidonegro.tumblr.com/post/6791492770/betonbabe-j-rn-utzon-sydney-opera-house

Opera


Opera is “an extravagant art form” (Barron p. 333) and was at first limited to wealthy courts. When brought to public consumption the demands resulted in a “theatre-building boom”. In the 20th century, opera (as well as its various spin-offs: variété, cabaret, musical, rock show etc) came to fully exploit the stage, auditorium, electronics, screens, and all the other media to extend its palette of expression.

“Of all auditorium types, the architectural form of opera houses has changed the least during its existence as a public space for entertainment.” (Barron p. 380)

Today, opera is a curious mixture of conservatism and renewal. “Of the 1000+ composers programmed over the last five seasons, over 500 are alive — not bad for an art-form that is sometimes thoughtlessly described as ready for the museum.”

http://operabase.com/top.cgi?lang=en&

#opera performances per million citizens.

#opera productions globally, last 5 seasons

Opera house is highly constrained in terms of design.- sightlines between singers and conductor, & musicians and conductor- sightlines from audience positions to (proscenium) stage- maximum distance ≤ 30 m- …

The baroque theatre (“horseshoe”) layout has historically been dominant.

SS Giovanni e Paolo Glyndebourne

1638 1993

http://operabase.com/top.cgi?lang=en&show=alive&no=0&break=25

http://operabase.com/top.cgi?lang=en&show=alive&no=0&break=25



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

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

http://www.hopkins.co.uk/projects/_1,14/

http://www.hopkins.co.uk/projects/_1,14/

The singer’s voice


http://rs2007.limsi.fr/index.php/Measurement_of_3D_Phoneme-Specific_Radiation_Patterns_in_Speech_and_Singing

Noticeable differences in timbre occurs when a singer turns more than 40˚, and becomes detrimental beyond 80˚.

The human voice is highly directional, radiating forwards (≈180˚) and slightly downwards (≈20˚).

Formants - more correctly referred to as vocal tract resonant peaks - are broad, band-pass-like, spectral regions that can be controlled by the muscles of the throat and mouth cavities; this is how the human voice produces different vowels.

Sundberg (1977, 1995) has shown that the “singers’ formant” refers to a mode of voice production where singers train to ‘tune’ their 3rd and 4th formants to coincide, and create a region of strong resonance in a range where the orchestra is relatively weak. This allows the singer to be heard on top of an orchestral forte.

In normal speech, the 1st and 2nd formants determine the vowel, and higher formants indicate (in mostly subconscious ways) more subtle aspects such as mood, and personality.

The acoustic behaviour of the human voice is very rich. The sound output depends on subglottal pressure (produced by muscles around the lungs), detailed characteristics of the vocal folds, the size and shape of the laryngial tract, the nasal cavity, the position and shape of the tongue, and finally that of the lips.

On stage, the voice projection is further enhanced by early reflections from the stage, and in the opera auditorium.





The orchestra pit


In comparison to symhonic orchestras, opera orchestras show a characteristic lack of brilliance. This is obvious during overtures, but when accompanying singers presents an advantage.

Note the pit design at Wagner’s Beyreuth, using a “hood” making the orchestra invisible to the audience.

The lack is due to the diffraction (“bending”) of sound that has to escape the orchestra pit and travel into the stalls. The attenuation is frequency dependent; at 125 Hz it is -6 dB, at 4 kHz it is near -20 dB.

High-frequency reflectors can be placed directly above the pit…

… but for early reflections to arrive ≤ 35 ms (see the TimeDelayGapTester!) reflector height ≤ 6 m.

A large survey (Mackenzie 1985) showed that opera-goers invariably consider the orchestra too loud.Balance can be improved by partly covering the pit, but this becomes expensive as it eats into seating area.With a covered pits there are serious ergonomic problems which is why they are disliked by musicians:- musical communication with singers become difficult- often lack of space- SPL rises to dangerous levelsRecommendations: - degree of overhang ≤ 40% of total pit depth- height is ≥ 2.5 m

Overhead early reflections tend to distort the sound more than lateral reflections.



Breaking the horseshoe


http://unesdoc.unesco.org/images/0006/000645/064581eo.pdf

Richard Wagner was deeply involved in the design of the Festspeilhaus Bayreuth (architect was Otto Brückwald) and opened in 1876

“For Wagner, no distraction should interrupt the view of the stage” (Barron p. 352)

“Der Ring des Nibelungen… was conceived in terms of the theatre which was built to house it. Not by chance is the orchestra in Bayreuth covered and the distance from stage to auditorium - over what Wagner called the ‘mystischer Abgrund’ (‘mystical depths’) - greater than in the normal opera house.” (Bornoff 1968)

Reverberation time = 1.55 s (longer than most pre-1900 opera houses), with a bass rise to 1.75 s.

The ‘Wagner pit’ is controversial, and not used anywhere else.

Contrary to most preceding and contemporary opera layouts, it has an amphitheatrical setting.

See the video guides at http://www.bayreuther-festspiele.de/english/english_156.html, in particular for the orchestra pit.



http://www.bayreuther-festspiele.de/english/english_156.html




Acoustic criteria


Opera house design has to resolve with many contending requirements, including:

Primary aim in opera house design is to enhance the singers’ sound relative to the orchestra, without distorting the latter.

Barron argues that the performer-audience relationship is different in symphonic music and opera:

- reverberation time (optimal speech intelligibility ≈ 1.0 s, optimal music ≈ 2.0 s)

- loudness balance between singers and orchestra

- source broadening (more important for orchestra than singers)

auditorium stage

concert hall

stage

opera house

auditorium

Length of opera houses ≤ 30 m implies larger & deeper balconies.

Traditional use of horseshoe layout is not good for lateral reflections.

Architectural design


Plan design in an opera has to originate from the proscenium opening.

More: http://peutz.fr/lacoustique/articles/salles/PaperIOA02.pdf

To create early lateral reflections (for both singers and orchestra), proscenium splays are helpful.

By contrast, an overhead ceiling splay does not add reflections where they are needed!

For visual reasons, the proscenium is typically more narrow than the width of the hall, leading to flat angle of the side splays. This produces few early reflections.

A solution is to place a retreated splay with a more parallel angle behind a visually opaque but acoustically transparent curtain. This will produce a more dense set of early reflections

Curved surfaces, in particular at the back of the hall, bring a risk for echo focusing. Balcony boxes are there for social reasons, not acoustical!

La Scala (Milano) was renovated 2001-4. See: http://www.gingerfoot.de/La_Scala_en.htm



http://www.gingerfoot.de/La_Scala_en.htm




Objective assessment of opera houses


A key difference between opera and orchestral music (as well as to other forms) is that two different sound sources have to be considered: - singers onstage- orchestra in a pit

Measures:- reverberation time (RT)- early decay time (EDT)

Recommendations:RT = 1.3…1.8 seconds (+ slight bass rise; shorter for Mozart, longer for Verdi)EDT ≈ RT (watch out for seats under balconies, with low EDT/RT ratio)

- singer / orchestra balance

- objective voice clarity, intelligibility- voice total sound level

D50 > 0.5 (early to late energy, time cutoff = 50 ms)totalvoice ≥ 0 dB (re directvoice @ 10 m)

- objective orchestral clarity- orchestral source broadening (ASW, LE)- orchestra total sound level

C80 = -1…3 dB (slightly higher than for orchestral halls)as for orchestral halls (though expected to be generally higher)L - L0 ≥ -2 dB (less stringent than for orchestral halls)

Lsinger - Lorchestra = 1…4 dB (no agreed-upon psychoacoustic measure exists)

Voice and orchestra have different acoustic power. Spectral differences make up for some of the difference. More complex is the influence of multimodal (i.e. visual) sensation, expectations etc affect the audience experience.

(10) Acoustics for multi-purpose use

Variable acoustic elements


Electronics

Derngate Centre

Espace de projection, IRCAM


(11) Multi-purpose halls in Britain



(12) The art and science of acoustics

Speech


Concert

Opera


Multi-purpose halls

Documents

Lindborg RoomAcou Summary