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 University Visiting 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

30% 15% 4% 1% 35% Relative importance

Speech spectrum at 1 foot 35% 30% 15% 4% 1% 65% 95%

Speech spectrum at 1 foot Distance 1 feet Average level = 72 dBSPL Distance 1 feet Average level = 72 dBSPL 30 dB Range of most useful information = 30 dB (from about 40 to about 70 dBHL)

2. Background Noise

Speech Speech Audibility and the signal-to-noise ratio Speech level increasing Speech Noise 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 Direct speech at 4 ft = 60 dBSPL Direct speech at 1 ft = 72 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 field Enclosed space The reverberation effect Direct sound Reverberation

Direct sound The shower 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 (RT 60 ) is the time taken for the sound level to drop by 60 dB after the source is turned off

Time in milliseconds Amplitude in dBSPL Amplitude in dBSPL Time in milliseconds 60 dB 500 msec 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 (RT 60 ) 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)

Reverberation is not all bad - - early reflections can increase loudness without reducing clarity

Early reverberation Reverberation is the persistence of sound in an enclosed space because of multiple, repeated reflections from the boundaries Reverberation time (RT 60 ) 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

Direct speech signal Early reverberation Combined signal The early components of reverberation Increase the level of speech at a distance Critical distance Direct speech negligible

Early reverberation Reverberation is the persistence of sound in an enclosed space because of multiple, repeated reflections from the boundaries Reverberation time (RT 60 ) 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

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 analogy 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 Early

Severe reverberation Severe reverberation Time in seconds Frequency in kHz Output from talker Input to 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 Speech signal 30 dB Self 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 Effective signal-to-noise ratio in dB Percent Audibility Phoneme recognition in CVC words in % 30dB

Part B The 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) Effective signal = ( direct speech & early reverberation} Effective noise = (actual noise & late reverberation) Effective signal-to-noise ratio = (effective signal – effective noise) Speech Audibility Index (%) = (effective signal-to-noise ratio+15)/30*100 (Max = 100; Min = 0)

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 C Effective 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 C 1 V C 2 Probability of recognizing a phoneme = p Probability of recognizing whole word = w w = p j j = 3 for unfamiliar words j = 2 for familiar words

CVC words in isolation Familiar Unfamiliar 13 %points 2 dB

Words in sentences Lamb Would you like Lamb for dinner Probability of recognizing a word in isolation = w Probability of recognizing a word in a sentence = s s = 1 – (1-w) k k < 2 for complex sentences k > 5 for simple sentences

Words in sentences Normal Hearing 95% Simple sentences Familiar words Complex sentences Unfamiliar words 38% 11.5dB

Words in sentences Aided 50 dB Sensorineural Loss 95% 36% Simple sentences Familiar words Complex sentences Unfamiliar words 6.5 dB 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)

The Sound Field solution

Sound-field amplification FM link Amplifier Loudspeakers

Increasing volume DOES increase signal-to-noise ratio Increasing volume DOES increase signal-to-noise ratio

Amplifier Distance from loudspeaker in feet dBSPL NOISE Increasing volume DOES increase signal-to-noise ratio Increasing volume DOES increase signal-to-noise ratio Direct sound

Amplifier Distance from loudspeaker in feet dBSPL NOISE Increasing volume DOES increase signal-to-noise ratio Increasing volume DOES increase signal-to-noise ratio Direct sound

Amplifier Distance from loudspeaker in feet dBSPL NOISE Increasing volume DOES increase signal-to-noise ratio Increasing volume DOES increase signal-to-noise ratio Direct sound

Amplifier Distance from loudspeaker in feet dBSPL NOISE Increasing volume DOES increase signal-to-noise ratio Increasing volume DOES increase signal-to-noise ratio Direct sound

Increasing volume does NOT increase signal-to-reverberation ratio Increasing volume does NOT increase signal-to-reverberation ratio

Amplifier Distance from loudspeaker in feet dBSPL Early reflections Direct sound Late reflections Increasing volume does NOT increase signal-to-reverberation ratio Increasing volume does NOT increase signal-to-reverberation ratio

dBSPL Early reflections Direct sound Late reflections Amplifier Distance from loudspeaker in feet Increasing volume does NOT increase signal-to-reverberation ratio Increasing volume does NOT increase signal-to-reverberation ratio

dBSPL Early reflections Direct sound Late reflections Amplifier Distance from loudspeaker in feet Increasing volume does NOT increase signal-to-reverberation ratio Increasing volume does NOT increase signal-to-reverberation ratio

dBSPL Early reflections Direct sound Late reflections Amplifier Distance from loudspeaker in feet Increasing volume does NOT increase signal-to-reverberation ratio Increasing volume does NOT increase signal-to-reverberation ratio

Improving proximity DOES increase signal-to-reverberation ratio Improving proximity DOES increase signal-to-reverberation ratio

Amplifier Distance from loudspeaker in feet dBSPL Early reflections Improving proximity DOES increase signal-to-reverberation ratio Improving proximity DOES increase signal-to-reverberation ratio Direct sound Late 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 with Standard error)

Improving directionality (Q) DOES increase Signal to reverberation ratio Improving directionality (Q) DOES increase Signal to reverberation ratio

Amplifier Distance from loudspeaker in feet dBSPL Early reflections Low Directionality (Q) Direct sound Late reflections

dBSPL Amplifier Distance from loudspeaker in feet Early reflections Direct sound Late reflections High directionality (Q)

Summary re Sound-Field 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. 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.

Measuring Reverberation Times

Estimating Reverberation Times

Simulating Sound-Field Installation

Demonstrations Measuring reverberation time Simulating the effects of room acoustics and Sound-Field amplification (SFWIZ – downloadable free of charge from either or Additional reading Boothroyd A (2003). Room Acoustics and Speech Perception. Seminars in Hearing (in press). Draft available on