Improving audibility as
a foundation for better
speech understanding
Pamela Souza, PhD
Northwestern University
Evanston, IL
Outline
O The talker: producing clear speech
O The listener: effects of hearing loss
O Speech audibility
O Distance, noise and echoes
O 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
Audibility is determined by the level of the
speech, the level of any noise, and the
listener’s audiogram
Sensitive
hearing
Mild
Moderate
Severe
Understanding the audiogram
Amount of hearing loss
Mild
Rehabilitation choices
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
Louder
Pink line shows upside-down audiogram
(high-frequency loss)
Soft speech, no
hearing aid, 29%
audible
Louder
Audibility is higher when
the talker’s voice is
louder (or closer)
Loud speech, no
hearing aid, 57%
audible
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 directly
O 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 signal
O Reducing distance
O Eliminating noise and reverberation
O Hearing aids and cochlear implants
O Assistive listening devices with remote
microphones
What does the hearing aid do to
improve audibility?
O Customized frequency-gain response
O Multichannel compression
O Frequency lowering
O Noise reduction
O 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 audiogram
O Different mathematical formulas or
“prescriptions” (such as DSL and NAL) are
used to determine desired response
Improving audibility: frequency-gain
response Audiogram
shows highfrequency
hearing loss
Hearing aid gain should be
greater in high frequencies
Speech audibility is improved
in high frequencies
Without hearing aid
With linear hearing aid
Louder
Soft speech
29% audible
Loud speech
57% audible
Linear aids improve
audibility, but may cause
loudness discomfort
Soft speech
51% audible
Too loud
Loud speech
79% audible
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
Without hearing aid With compression hearing aid
Louder
Soft speech
29% audible
Loud speech
57% audible
Compression hearing aids
improve audibility and
loudness comfort (without
volume adjustments)
Soft speech
59% audible
Loud speech
79% audible
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?
with sensitive
O Adults
When background
hearing
are similar to each
noise limits
other
– ifspeech,
speech is at least
audible
adultaudible,
listenersit is usually
60%
with sensitive
understood
hearing need
about 50%
audibility to
understand 80%
of sentences
Results from Souza, Boike, Witherell, Tremblay, 2007
How much audibility is enough?
O When
background
With
hearing
loss, good
noise limits
audibility
does not always
audible
mean
goodspeech,
speech
adult listeners with
understanding
hearing loss need
about 80%
audibility to
understand 80%
of sentences
Results from Souza, Boike, Witherell, Tremblay, 2007
Better understanding
Children need greater audibility than
adults
When children and adultsAdults
are
getting the same amountAge
of 9-12 years
audible information, children
Age 5-8 years
have more difficulty
understanding speech than
adults do
Better audibility
McCreery & Stelmachowicz, 2011
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
Summary
O Talkers should be close to the listener,
O
O
O
O
visible, and producing clear speech
Reduce background noise!
Hearing aids (with appropriate settings and
features)
Assistive listening devices for more difficult
(noisy or distant) situations
Conversation is a two-part experience
Thank you
Email contact: p-souza@northwestern.edu
Research web site: halab.northwestern.edu