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
Critical Factors
1.Speech spectrum
2.Noise
3.Distance
4.Early reverberation(early reflections)
5.Late reverberation
Speech spectrum at 1 footSpeech spectrum at 1 foot
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)
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%
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
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
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
• Reverberation is 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
• 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)
Early reverberation
• Reverberation is 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
• 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
• Reverberation is 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
• 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
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
The 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
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?
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
• Effectively, the reverberant speech signal generates its own masking
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
• Effectively, the reverberant speech signal generates its own masking
• The effective signal-to-noise ratio depends on the reverberation time
The late components of reverberation
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
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.
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
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)
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
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
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
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
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
Distance from loudspeaker in feet
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
Increasing volume does NOT increasesignal-to-reverberation ratio
dBSPL
90
80
70
60
50
40
30
20
10
Early reflectionsEarly reflections
Direct soundDirect soundLate reflectionsLate reflections
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
Increasing volume does NOT increasesignal-to-reverberation ratio
Increasing volume does NOT increasesignal-to-reverberation ratio
dBSPL
90
80
70
60
50
40
30
20
10
Early reflectionsEarly reflections
Direct soundDirect soundLate reflectionsLate reflections
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
Increasing volume does NOT increasesignal-to-reverberation ratio
Increasing volume does NOT increasesignal-to-reverberation ratio
dBSPL
90
80
70
60
50
40
30
20
10
Early reflectionsEarly reflections
Direct soundDirect soundLate reflectionsLate reflections
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
Increasing volume does NOT increasesignal-to-reverberation ratio
Increasing volume does NOT increasesignal-to-reverberation ratio
Improving proximityDOES increase
signal-to-reverberationratio
Improving proximityDOES 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
Distance from loudspeaker in feet
dBSPL
90
80
70
60
50
40
30
20
10
Early reflectionsEarly reflections
Improving proximity DOES increasesignal-to-reverberation ratio
Improving proximity DOES increasesignal-to-reverberation ratio
Direct soundDirect soundLate reflectionsLate reflections
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
Distance from loudspeaker in feet
dBSPL
90
80
70
60
50
40
30
20
10
Early reflectionsEarly reflections
Low Directionality (Q)Low Directionality (Q)
Direct soundDirect soundLate reflectionsLate reflections
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
Distance from loudspeaker in feet
Early reflectionsEarly reflections
Direct soundDirect soundLate reflectionsLate reflections
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.
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