Architectural Acoustics IIArchitectural Acoustics IIIndoor Acoustical PhenomenaIndoor Acoustical Phenomena

Prof S K TangProf S K Tang

PrefacePreface

Concepts on direct and reverberant sound Concepts on direct and reverberant sound fields and reverberation time have been fields and reverberation time have been introduced in past yearsintroduced in past years

Direct sound follows inverse square lawDirect sound follows inverse square law

Reverberant field is uniform (ideal case)Reverberant field is uniform (ideal case)

Enclosure has acoustic responseEnclosure has acoustic response

Human responses to sound is subjective – Human responses to sound is subjective – psychology & physicspsychology & physics

Acoustic Responses of EnclosureAcoustic Responses of Enclosure

Similar to a mechanical & electrical system, an Similar to a mechanical & electrical system, an enclosure responses to acoustic excitationenclosure responses to acoustic excitation

Impulse decay Impulse decay gg((tt) – impulse response function) – impulse response function

gg includes information of direct sound, includes information of direct sound, reflections and the amount of room absorptionreflections and the amount of room absorption

These responses then modify the direct acoustical These responses then modify the direct acoustical signal and signal and contribute directly to human contribute directly to human responsesresponses

Modulated acoustic signal is heard Modulated acoustic signal is heard

Psycho-acoustics ConsiderationPsycho-acoustics Consideration

Reflections cannot be avoidedReflections cannot be avoided

A mixture of direct sound and reflections A mixture of direct sound and reflections reaches human earsreaches human ears

How these reflections are perceived?How these reflections are perceived?

Two questions to deal with :Two questions to deal with :

Psycho-acoustics ConsiderationPsycho-acoustics Consideration

Under what condition is a reflection Under what condition is a reflection perceivable at all, without regard to the perceivable at all, without regard to the way in which its presence is manifested, way in which its presence is manifested, and under what condition is it masked by and under what condition is it masked by the direct sound?the direct sound?

Under what condition does the presence Under what condition does the presence of a reflection rate as a disturbance of of a reflection rate as a disturbance of the listening impression? the listening impression?

Absolute PerceptibilityAbsolute Perceptibility

function of the time delay of the reflectionfunction of the time delay of the reflection

The threshold also depends on type of The threshold also depends on type of soundsound

For a sound of 70dBA and frontal For a sound of 70dBA and frontal incidence :incidence :

86.0 otL

Human is more sensitive to reflections Human is more sensitive to reflections arriving from lateral directions arriving from lateral directions

Threshold of absolute perceptibility for Threshold of absolute perceptibility for speechspeech

Threshold of absolute perceptibility for Threshold of absolute perceptibility for musicmusic

Our hearing is less sensitive to reflection when music is concern

Echoes & ColourationEchoes & Colouration

a reflection may not reach the consciousness a reflection may not reach the consciousness of the listener even it is perceived of the listener even it is perceived At lower levels, it manifests itself by an At lower levels, it manifests itself by an increase of the overall loudness of the total increase of the overall loudness of the total sound sound At higher levels, a reflection can be heard as At higher levels, a reflection can be heard as a separate event – ‘Echo’ a separate event – ‘Echo’ Not welcomed in indoor environment. It may Not welcomed in indoor environment. It may distract listener’s attention, reduce enjoyment distract listener’s attention, reduce enjoyment of music and impair speech intelligibility of music and impair speech intelligibility

Perception of an EchoPerception of an Echo

The superposition of a strong isolated The superposition of a strong isolated reflection or multiple significant regular reflection or multiple significant regular reflections onto the direct sound results in reflections onto the direct sound results in a characteristic change in timbre (spectral a characteristic change in timbre (spectral characteristics) – ‘Colouration’. characteristics) – ‘Colouration’.

This is important when music is concerned This is important when music is concerned

Acoustical ParametersAcoustical Parameters

Echoes are not the only factor affecting Echoes are not the only factor affecting the intelligibility. the intelligibility.

The duration of a sound decay, which is The duration of a sound decay, which is brought about by multiple reflections in the brought about by multiple reflections in the presence of boundary absorption, is of presence of boundary absorption, is of equal importance equal importance

Commonly used parameter : RTCommonly used parameter : RT

Haas EffectHaas Effect

Early reflection, Early reflection, which is the sound which is the sound energy reaches the energy reaches the listener within 50 listener within 50 ms after the arrival ms after the arrival of the direct sound, of the direct sound, enforces enforces intelligibility as it intelligibility as it integrates nicely integrates nicely with the direct with the direct sound sound

Early Decay Time (EDT)Early Decay Time (EDT)

time it takes the sound energy to decay by time it takes the sound energy to decay by 10dB multiplied by 6 = EDT10dB multiplied by 6 = EDT

RTRT equals the time taken for the sound equals the time taken for the sound energy to decay by 60dB energy to decay by 60dB

EDTEDT includes a few isolated early includes a few isolated early reflections reflections

A short A short EDTEDT enhances clarity enhances clarity

Longer Longer RTRT gives the feeling of ‘liveness’ gives the feeling of ‘liveness’

ClarityClarity

Clarity is defined as the difference (in dB) of the Clarity is defined as the difference (in dB) of the sound energy received at a listener in the first sound energy received at a listener in the first 80ms minus the (late) reverberant energy, which 80ms minus the (late) reverberant energy, which is the entire energy arriving at the listener after is the entire energy arriving at the listener after 80ms 80ms

ms80

2

ms80

0

2

1080

)(

)(

log10

dttg

dttg

C

ClarityClarity

characterize the transparency of music in characterize the transparency of music in a concert hall and describe the fullness of a concert hall and describe the fullness of tone tone

A A CC8080 of -3dB is still acceptable. of -3dB is still acceptable.

The typical range of this parameter is from The typical range of this parameter is from -3dB to +5dB -3dB to +5dB

DefinitionDefinition

describes the fullness of tone but it is describes the fullness of tone but it is related to speech intelligibility instead of related to speech intelligibility instead of music. music.

%100

)(

)(

0

2

ms50

0

2

dttg

dttg

D

DefinitionDefinition

Centre TimeCentre Time

0

2

0

2

)(

)(

dttg

tdttg

ts

Gravity / Strength Factor Gravity / Strength Factor

CC8080 and and DD will become meaningless if the will become meaningless if the

sound is weak to be heard at a sound is weak to be heard at a comfortable loudness comfortable loudness

assumption of uniform reverberation field assumption of uniform reverberation field is not valid in halls or long enclosures is not valid in halls or long enclosures

0

2

0

2

10

)(

)(

log10

dttg

tdttg

G

A

45log10 10

Volume

RTG

Initial Time Delay Gap, ITDG Initial Time Delay Gap, ITDG

This parameter This parameter describes intimacy - describes intimacy - subjective impression subjective impression of the size of a hall of the size of a hall

ITDG - interval in ITDG - interval in milliseconds between milliseconds between the arrival of the the arrival of the direct sound and the direct sound and the first reflection at the first reflection at the listener listener

Early Lateral Energy Fraction, LEF Early Lateral Energy Fraction, LEF

correlate with the spaciousness of an correlate with the spaciousness of an enclosure enclosure

independent of other reflections and of the independent of other reflections and of the presence or absence of reverberation presence or absence of reverberation

ms80

0

2

ms80

ms5

2

)(

cos)(

dttg

dttg

LEF

Specular ReflectorSpecular Reflector

Diffuse ReflectionDiffuse Reflection

Convex SurfacesConvex Surfaces

Quadratic Residue DiffuserQuadratic Residue Diffuser

diffusers made from surface modulation diffusers made from surface modulation have two major limitations :have two major limitations :

protrusions and recesses have to be large protrusions and recesses have to be large to provide good diffusion at low to provide good diffusion at low frequencies.frequencies.

no objective method to determine the no objective method to determine the extent of scattering extent of scattering

Consists of an array of linear slits (or Consists of an array of linear slits (or wells) of constant width. wells) of constant width.

Wells are separated by thin rigid walls.Wells are separated by thin rigid walls.

Number of wells inside a period of the Number of wells inside a period of the diffuser is a prime number diffuser is a prime number

Design for SpeechDesign for Speech

Speech should be intelligible without an undue Speech should be intelligible without an undue strain on the listener. strain on the listener. to retain the natural character of the speaker’s to retain the natural character of the speaker’s voice. voice. Providing optimum Providing optimum RTRTEliminating acoustical defects such as echoes and Eliminating acoustical defects such as echoes and flutter echoesflutter echoesMaximizing loudness in the audienceMaximizing loudness in the audienceMinimizing the ambient noise levelMinimizing the ambient noise levelProviding a speech reinforcement system where Providing a speech reinforcement system where neededneeded

Speech Transmission Index STISpeech Transmission Index STI

STISTI is derived from 7 octave band is derived from 7 octave band modulation transfer functions modulation transfer functions mm((FF) )

FF represents a modulation frequency :14 represents a modulation frequency :14 FFs (1/3 octave bands from 0.63Hz to s (1/3 octave bands from 0.63Hz to 12.5Hz). 12.5Hz).

mm((FF) is related to ) is related to RTRT and and S/NS/N : :

110/

5.02

1018.13

21)(

iNSiRTFFm

STISTI

30)15/( appi NSSTI

)(1

)(log10/ 10, Fm

FmNS Fapp

7

1iiiSTIwSTI

STI Quality Speech Intelligibility Loss of Consonants

0 – 0.2 Bad < 50% intelligible > 50%

0.2 – 0.4 Poor 50 – 60% intelligible 40 – 50%

0.4 – 0.6 Fair 60 – 78% intelligible 22 – 40%

0.6 – 0.8 Good 78 – 98% intelligible 2 – 22%

0.8 – 1 Excellent > 98% intelligible < 2%

Articulation Index (AI) Articulation Index (AI)

AIAI is a parameter obtained from the weighted is a parameter obtained from the weighted average of average of S/NS/Ns in the octave bands from 250Hz s in the octave bands from 250Hz to 4kHz to 4kHz

AI < 0.3 0.3 – 0.5 0.5 – 0.7 0.7

Speech intelligibility

Poor Marginal Good Very Good

Percentage Articulation Loss of Percentage Articulation Loss of Consonants (%Consonants (%ALALconscons))

AA = -0.32 log[( = -0.32 log[(LrLr++LnLn)/(10)/(10LdLd++LrLr++LnLn)])]

BB = -0.32 log[ = -0.32 log[LnLn/(10/(10LrLr++LnLn)])]

CC = -0.5 log ( = -0.5 log (RTRT/12)/12)

LdLd = Direct level (energy within 35 ms) = Direct level (energy within 35 ms)

LrLr = Reverberant level = Reverberant level

LnLn = Background noise level = Background noise level

)015.0])[(210(100% ABCBCAALcons

Office DesignOffice Design

For an office, it is the speech privacy which is of the For an office, it is the speech privacy which is of the major concern. major concern. Certainly, the noise level and NC or RC are also in the Certainly, the noise level and NC or RC are also in the design specification.design specification.Speech privacy is basically the opposite of speech Speech privacy is basically the opposite of speech intelligibility.intelligibility.STISTI and and AIAI and even % and even %ALconsALcons can also be used to can also be used to describe it describe it

AI 0 – 0.05 0.05 – 0.15 0.15 – 0.2 > 0.2

Speech privacy Very Good(confidential)

Good(normal)

Marginal(little)

Poor

Privacy between RoomsPrivacy between Rooms

An index called privacy An index called privacy PP is derived from is derived from the isolation property of the partition wall the isolation property of the partition wall ((STCSTC), receiver room characteristics (), receiver room characteristics (RRRR), ), background noise level in receiver room in background noise level in receiver room in dBA (dBA (NN), voice level (), voice level (VLVL) and a source ) and a source room factor (room factor (SRSR) :) :

SRVLNRRSTCP

Privacy in Open-plan OfficePrivacy in Open-plan Office

Acoustic privacy in open-plan offices is achieved Acoustic privacy in open-plan offices is achieved by using absorption partitions (of sufficient by using absorption partitions (of sufficient height), absorptive ceiling/floor and in some height), absorptive ceiling/floor and in some cases by artificial sound masking. cases by artificial sound masking.

Common absorptive ceiling usually has a Common absorptive ceiling usually has a NRCNRC > > 0.75 (0.75 (NRCNRC is the average of the sound is the average of the sound absorption coefficients of the octave bands from absorption coefficients of the octave bands from 250Hz to 2kHz). 250Hz to 2kHz).

HVAC noise provides some masking effectsHVAC noise provides some masking effects

eqSTCRPVLL

MaskingMasking

masking noise level masking noise level cannot be too high cannot be too high and must be and must be acceptable to the acceptable to the occupants.occupants.

In general, a masking In general, a masking noise level of 48dBA noise level of 48dBA is the practical upper is the practical upper limit for open-plan limit for open-plan offices offices

Considerations for MusicConsiderations for Music

The acoustical phenomena related to The acoustical phenomena related to musical attributes may be divided into the musical attributes may be divided into the following two categories :following two categories :

(a) to early sound : reverbance ((a) to early sound : reverbance (EDTEDT), ), Clarity, Intimacy, Spaciousness (laterality)Clarity, Intimacy, Spaciousness (laterality)

(b) to reverberant sound : warmth (b) to reverberant sound : warmth (liveness), brilliance, loudness ((liveness), brilliance, loudness (GG))

Bass Ratio BRBass Ratio BR

Warmth feeling is associated with low Warmth feeling is associated with low frequency reverberation frequency reverberation BRBR is between 1.1 to 1.25 for is between 1.1 to 1.25 for RTRT 2.2s 2.2s Between 1.1 to 1.45 for Between 1.1 to 1.45 for RTRT < 1.8s < 1.8s Larger Larger BRBR than the recommended values than the recommended values should be avoided should be avoided

1000500

250125

RTRT

RTRTBR

BrillianceBrilliance

related to the early decay of high related to the early decay of high frequency sound frequency sound

9.01000500

2000 EDTEDT

EDT

8.01000500

4000 EDTEDT

EDT

Computational ApproachComputational Approach