improving audibility as a foundation for better speech understanding

Improving audibility as a foundation for better speech understanding

Pamela Souza, PhDNorthwestern University

Evanston, IL

OutlineO The talker: producing clear speechO The listener: effects of hearing lossO Speech audibilityO Distance, noise and echoesO Improving audibility: hearing aids

and cochlear implants

Communication: “send and receive”

Speech varies rapidly in time, frequency (pitch), and intensity

(loudness)

Speech is redundant – we don’t need to hear every sound. But the more

information is received, the less effort is needed to listen

High redundancy: quiet, visual cues, high context

Low redundancy: noisy, no visual cues, low context

Severe

Audibility is determined by the level of the speech, the level of any noise,

and the listener’s audiogram

Sensitive hearing

Mild

Moderate

Understanding the audiogramAmount of hearing loss

Rehabilitation choices

Mild May use hearing aid if communication is affected

Moderate Partial audibility of conversational speech; hearing aid recommended

Moderately severe Poor audibility of conversational speech; hearing aid recommended

Severe Inaudibility of conversational speech; hearing aid, cochlear implant, or alternative communication mode

Profound

Soft speech, no hearing aid, 29% audible

Loud speech, no hearing aid, 57% audible

Pink line shows upside-down audiogram (high-frequency loss)

Loud

erLo

uder

Audibility is higher when the talker’s voice is louder (or closer)

Effects of distance and position

The further the talker is from the listener, the lower audibility will be

Effects of background noise

Effects of background noise

O Energetic masking: reduces audibility when noise overlaps in pitch and timing with the speech, blocking the speech from being heard

O Informational masking: when noise does not overlap with speech energy, but causes confusion or draws attention away from the talker

O More effort is required to listen in noise

Effects of reverberation (echoes)

Effects of reverberation

O Sound reaches the listener directlyO A delayed version of the same sound

reaches the listener after reflecting from a hard surface

O Overlap masking: the delayed sound is still present and energetically masks sounds that follow

O More effort is required to listen in reverberation

Improving audibility

O Improving signalO Reducing distanceO Eliminating noise and reverberationO Hearing aids and cochlear implantsO Assistive listening devices with

remote microphones

What does the hearing aid do to improve audibility?

O Customized frequency-gain responseO Multichannel compressionO Frequency loweringO Noise reductionO Suppressing reverberation (echoes)

Frequency-gain response

O Provides more amplification (“gain”) at frequencies where there is more hearing loss

O Like a mirror of the audiogramO Different mathematical formulas or

“prescriptions” (such as DSL and NAL) are used to determine desired response

Improving audibility: frequency-gain responseAudiogram

shows high-frequency hearing loss

Hearing aid gain should be greater in high frequencies

Speech audibility is improved in high frequencies

Soft speech29% audible

Loud speech57% audible

Without hearing aid

With linear hearing aid

Soft speech51% audible

Loud speech79% audible

Loud

er

Too loud

Linear aids improve audibility, but may cause loudness discomfort

Multichannel compression

O Within each frequency band, soft sounds are amplified more than loud sounds

O Reduces the dynamic range from soft to loud

O Soft sounds should be made more audible without making loud sounds too loud

O But –more extreme compression parameters may distort speech

Soft speech29% audible

Loud speech57% audible

Without hearing aid

With compression hearing aid

Loud

er

Soft speech59% audible

Loud speech79% audible

Compression hearing aids improve audibility and loudness comfort (without volume adjustments)

Audible bandwidth and frequency lowering amplification

O Listeners with sensitive hearing receive sounds to 8000 Hz or higher; most listeners wearing hearing aids receive sounds up to 3000-5000 Hz

O A wider bandwidth may improve speech understanding

O A wider bandwidth may make it easier for children to learn new information

O A wider bandwidth supports speech production

Gustafson & Pittman, 2011; Pittman, 2008; Stelmachowicz et al., 2004;

Frequency lowering hearing aids

O Used to improve high-frequency sound audibility

O High-frequency sounds are shifted to a lower frequency range

O May be more beneficial for children than adults (due to adults’ greater linguistic experience)

O Should be used selectively (more studies are in progress)

Souza et al. 2013; McCreery et al., 2014; Bentler et al., 2014

Hearing aid noise reduction: directional microphones

O Apply less gain to noise that is from a different location than the talker

O Improve the relative levels of the talker (“signal”) and the background (“noise”)

O Can improve speech audibility (and understanding) if signal and noise are spatially separated

Hearing aid noise reduction: digital noise reduction

O Attempts to determine what is “noise” and what is “speech” based on their sound patterns

O Mathematically removes the pattern of the noise

O May not improve speech understanding, but can reduce listening effort and improve listening comfort

Assistive listening devices

O Overcome distance by placing the microphone close to the talker’s lips and transmitting that signal to the listener’s hearing aid

O Suppress noise and reverberation by transmitting the “clean” signal direct from the talker

O Can be used anywhere distance or noise is an issue: classroom, automobile, restaurant

How much audibility is enough?

O When background noise limits audible speech, adult listeners with sensitive hearing need about 50% audibility to understand 80% of sentences

Results from Souza, Boike, Witherell, Tremblay, 2007

Adults with sensitive hearing are similar to each other – if speech is at least 60% audible, it is usually understood

How much audibility is enough?

O When background noise limits audible speech, adult listeners with hearing loss need about 80% audibility to understand 80% of sentencesResults from Souza, Boike, Witherell,

Tremblay, 2007

With hearing loss, good audibility does not always mean good speech understanding

Children need greater audibility than adults

McCreery & Stelmachowicz, 2011

Bette

r un

ders

tand

ing

Better audibility

Adults

Age 5-8 years

Age 9-12 years

When children and adults are getting the same amount of audible information, children have more difficulty understanding speech than adults do

Audibility for children

O Child-centric hearing aid fitting procedures emphasize audibility

O Child audibility needs are assumed to be different from adults

O Audibility may be addressed differently by pediatric audiologists than by adult audiologists

Using cognitive ability to “fill in” inaudible information

O We unconsciously use memory and knowledge to extract meaningful information from a partially audible signal

O Adults are good at using context; children have more difficulty (due to limited linguistic experience)

O This process uses cognitive resources

SummaryO Talkers should be close to the

listener, visible, and producing clear speech

O Reduce background noise!O Hearing aids (with appropriate

settings and features)O Assistive listening devices for more

difficult (noisy or distant) situationsO Conversation is a two-part

experience

Thank you

Email contact: p-souza@northwestern.eduResearch web site: halab.northwestern.edu