Room Acoustics: implications for speech reception and perception by hearing aid and cochlear implant...

Room Acoustics: implications for speech reception and perception

by hearing aid and cochlear implant users

2003

Arthur Boothroyd, Ph.D.Distinguished Professor Emeritus, City University of New York

Scholar in Residence, San Diego State UniversityVisiting Scientist, House Ear Institute

Part A

Room acoustics and speech

audibility

Critical Factors

1.Speech spectrum

2.Noise

3.Distance

4.Early reverberation(early reflections)

5.Late reverberation

1.The spectral

distribution of useful information in the

original speech signal

1 foot

70 dBSPL

Speech spectrum at 1 foot

Speech spectrum at 1 footSpeech spectrum at 1 foot

30% 15%15%4% 1%35%

Relativeimportance


35%30% 15%15%4% 1%

65%65%95%95%


Distance 1 feetAverage level = 72 dBSPL

Distance 1 feetAverage level = 72 dBSPL

30 dB

Range of most useful information = 30 dB

(from about 40 to about 70 dBHL)

2.Background Noise

Signal-to-noise ratio in dB

-20 -15 -10 -5 0 5 10 15 20

Pe

rce

nt

Au

dib

ility

0

20

40

60

80

100

Signal-to-noise ratio in dB

-20 -15 -10 -5 0 5 10 15 20

Pe

rce

nt

Au

dib

ility

0

20

40

60

80

100

Speech

Speech

Speech Audibility and the signal-to-noise ratio

Speech Audibility and the signal-to-noise ratio

Speech level increasing

SpeechSpeech

Speech

Noise

30dB

30dB

The spectral distribution of useful information in the original speech signal

• Most important range is from around 700 to around 3000 Hz - contains 65% of the useful information

• Extending down to 350 Hz and up to 6000 Hz adds another 30% (total = 95%).

• Useful information covers a range of 30 dB• Signal-to- Speech Phoneme

noise ratio Audibility Recognition -15 dB 0% 0% 0 dB 50% 85%+15 dB 100% 98%

3.The distance between

listener and talker

The distance between listener and talker

• The level of the direct speech signal falls by 6 dB for every doubling of distance

16 ft

48 dBSPL

32 ft

42 dBSPL

4 ft

60dBSPL

8 ft

54 dBSPL

Direct speech at 32 ft= 42 dBSPL



4.The early components of

reverberation (early reflections)

Early reverberation

• Reverberation is the persistence of sound in an enclosed space because of multiple, repeated reflections from the boundaries

Free fieldEnclosed space

The reverberation effectThe reverberation effect

Direct sound

Reverberation

Direct sound

Theshower analogy

The reverberation effect

Reverberation

Early reverberation

• - the persistence of sound in an enclosed space because of multiple, repeated reflections from the boundaries

• Reverberation time (RT60) is the time taken for the sound level to drop by 60 dB after the source is turned off

Time in milliseconds

80

60

40

20

0

Amplitudein

dBSPL

0 100 200 300 400 500 80

60

40

20

0

Amplitudein

dBSPL

0 100 200 300 400 500 Time in milliseconds

60 dB

500 msec

Reverberation time (RT60)Reverberation time (RT60)

Early reverberation



• The early components of reverberation are those reflections that arrive soon enough to be integrated with the direct sound, and with each other, so as to enhance perception (less than 1/20 of a second)

Reverberation is not all bad -

- early reflections can increaseloudness without reducing clarity

Early reverberation



• The early components of reverberation are those reflections that arrive soon enough to be integrated with the direct sound, and with each other, so as to enhance perception

• The early components of reverberation increase the level of speech at a distance

Distance from talker in feet

0 5 10 15 20 25 30 35

Spe

ech

leve

l in

dBS

PL

20

30

40

50

60

70

80

Distance from talker in feet0 5 10 15 20 25 30 35

Spe

ech

leve

l in

dBS

PL

20

30

40

50

60

70

80

Direct speech signal Early reverberation Combined signal

The early components of reverberationIncrease the level of speech at a distance

Criticaldistance

Direct speechnegligible

Early reverberation



• The early components of reverberation are those reflections that arrive soon enough to be integrated with the direct sound, and with each other, so as to enhance perception

• The early components of reverberation increase the level of speech at a distance

5.The late components of

reverberation

The late components of reverberation

• They arrive too late to be integrated with the direct signal or the early components (more than 1/10 of a second)

• If their level is still high enough, they interfere with the current sound by both physical and perceptual masking

Text analogyThe following is a list of Farmer’s markets to be held in the surrounding areas

The following is a list of Farmer’s markets to be held in the surrounding areas

Late



Early

Severereverberation

Severereverberation

Time in seconds0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

8

6

4

2

0

6

4

2

0

8

Fre

quen

cy in

kH

zOutputfromtalker

Inputto

listener

Where can I get my suit cleaned?




• Effectively, the reverberant speech signal generates its own masking


Speech signal

30 dB Selfmasking




• Effectively, the reverberant speech signal generates its own masking

• The effective signal-to-noise ratio depends on the reverberation time


Reverberation time in seconds 0.25 0.5 1.0 2.0 4.0 8.0

12 7 2 -3 -8 -13 Effective signal-to-noise ratio in dB 90 73 57 40 23 7 Percent Audibility

98 95 91 81 62 24 Phoneme recognition in CVC words in %

30dB

Part BThe

Speech Audibility Index(SAI)

Speech Audibility Index (SAI)

Combines effects of:

•Direct Speech Signal

•Early Reverberation

•Noise

•Late Reverberation

Effective signal&

Effective noise&

-

Effective s/n ratio

Speech Audibility Index (SAI)

• Speech Audibility Index is the proportion of the combined direct speech signal and early reverberation that exceeds the combined noise and late reverberation.

• It rises from 0 (no useful speech audible) to 100% (all useful speech audible) as the effective signal to noise ratio rises from -15 dB to +15 dB.

Part CEffective signal-to-

noise ratio, Speech Audibility Index and Speech

Perception

Speech Perception

1.Phonemes (vowels and consonants)

2.Words in isolation

3.Words in sentences

Phoneme recognition in CVC words

CVC words in isolation

C1 V C2

Probability of recognizinga phoneme = p

Probability of recognizingwhole word = w

w = pj

j = 3 for unfamiliarwords

j = 2 for familiarwords

CVC words in isolation

FamiliarUnfamiliar

13 %points

2 dB

Words in sentences

Lamb

Would you like Lamb for dinner

Probability of recognizing aword in isolation = w

Probability of recognizing aword in a sentence = s

s = 1 – (1-w)k

k < 2 for complexsentences

k > 5 for simplesentences

Words in sentences

Normal hearing

Signal-to-noise ratio in dB-20 -10 0 10 20

Wor

d re

cogn

ition

prob

abili

ty in

%

020406080

100

Speech Audibility Index in %0 20 40 60 80 100

Word recognition

probability

0.00.20.40.60.81.0

11.5 dB

38 %

Simple sentences,familiar wordsComplex sentences,unfamiliar words

Normal Hearing

95%Simple sentencesFamiliar words

Complex sentencesUnfamiliar words38%

11.5dB

50 dB unaidedhearing loss

(plus amplification)

Signal-to-noise ratio in dB-20 -10 0 10 20

Wor

d re

cogn

ition

prob

abili

ty in

%

020406080

100

Speech Audibility Index in %0 20 40 60 80 100

Word recognition

probability

0.00.20.40.60.81.0

6.5 dB

36 %

Words in sentencesAided 50 dB Sensorineural Loss

95%

36%

Simple sentencesFamiliar words

Complex sentencesUnfamiliar words

6.5dB

Normal

Speech Perception• Speech Audibility Index phoneme

recognition isolated word recognition sentence perception

• Relationship between SAI and sentence perception depends on word familiarity, sentence complexity, cochlear pathology, listener knowledge, listener skills

• But optimization of Speech Audibility Index is a crucial first step

Optimizing SAI for hearing aid and cochlear implant users

• Reduce Background noise• Reduce reverberation time (- but)• Enhance early reflections (- but)• Use directional microphones (- but)• Use remote (wireless) microphones (- but)• Use beam-forming (super-directional)

microphones (- but)• Use Sound-Field amplification (- but)

TheSound Field

solution

Sound-field amplification

FM linkAmplifier Loudspeakers

Increasing volumeDOES increasesignal-to-noise

ratio

Increasing volumeDOES increasesignal-to-noise

ratio

Amplifier

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40

Distance from loudspeaker in feet

dBSPL

90

80

70

60

50

40

30

20

10

NOISENOISE

Increasing volume DOES increasesignal-to-noise ratio

Increasing volume DOES increasesignal-to-noise ratio

Direct soundDirect sound

Increasing volumedoes NOT increase

signal-to-reverberationratio

Increasing volumedoes NOT increase


Amplifier

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40


dBSPL

90

80

70

60

50

40

30

20

10

Early reflectionsEarly reflections

Direct soundDirect soundLate reflectionsLate reflections

Increasing volume does NOT increasesignal-to-reverberation ratio


dBSPL

90

80

70

60

50

40

30

20

10



Amplifier

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40




Improving proximityDOES increase


Improving proximityDOES increase


Amplifier

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40


dBSPL

90

80

70

60

50

40

30

20

10


Improving proximity DOES increasesignal-to-reverberation ratio

Improving proximity DOES increasesignal-to-reverberation ratio


Phoneme recognition in CVC words by 14 child implant users.(data courtesy of Frank Iglehart of the Clarke School for the Deaf)Phoneme recognition in CVC words by 14 child implant users.(data courtesy of Frank Iglehart of the Clarke School for the Deaf)

(Bars show mean withStandard error)

Improving directionality(Q) DOES increase

Signal to reverberationratio

Improving directionality(Q) DOES increase

Signal to reverberationratio

Amplifier

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40


dBSPL

90

80

70

60

50

40

30

20

10


Low Directionality (Q)Low Directionality (Q)


dBSPL

90

80

70

60

50

40

30

20

10

Amplifier

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40




High directionality (Q)High directionality (Q)

Summary re Sound-FieldSummary re Sound-Field15 to 20 dB increase of s/noise and s/reverb at mic.Redistributed with possible increase of overall level.Benefit to listener reduced by:

Distance to nearest loudspeaker (6 dB rule).Room reverberation (increases with additional sources).Room noise.

Increasing system gain increases signal/noise,BUT NOT signal/reverberation.

Improving proximity increases signal/noise,AND signal/reverberation.

Increasing loudspeaker directionality increases s/reverb.If primary problem is noise, place speakers high for coverage.If primary problem is reverb., place speakers low for proximity.

(for individuals, desk-top system may be the best)Priority should always be to minimize reverberation.Sound-field system can then address distance and noise.Directional speakers are to be preferred.

15 to 20 dB increase of s/noise and s/reverb at mic.Redistributed with possible increase of overall level.Benefit to listener reduced by:

Distance to nearest loudspeaker (6 dB rule).Room reverberation (increases with additional sources).Room noise.

Increasing system gain increases signal/noise,BUT NOT signal/reverberation.

Improving proximity increases signal/noise,AND signal/reverberation.

Increasing loudspeaker directionality increases s/reverb.If primary problem is noise, place speakers high for coverage.If primary problem is reverb., place speakers low for proximity.

(for individuals, desk-top system may be the best)Priority should always be to minimize reverberation.Sound-field system can then address distance and noise.Directional speakers are to be preferred.

MeasuringReverberation

Times

EstimatingReverberation

Times

SimulatingSound-FieldInstallation

Demonstrations

• Measuring reverberation time

• Simulating the effects of room acoustics and Sound-Field amplification (SFWIZ – downloadable free of charge from either www.arthurboothroyd.com or www.phonicear.com

Additional readingBoothroyd A (2003). Room Acoustics and Speech Perception. Seminars in Hearing (in press). Draft available on www.arthurboothroyd.com

http://www.arthurboothroyd.com/

http://www.phonicear.com/

Room Acoustics: implications for speech reception and perception by hearing aid and cochlear implant...

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