Hearing Aids and Hearing Impairments

Meena Ramani

02/21/05

Dramatic Decrease In Audibility & Intelligibility

Original Speech

40dB conductive loss

P. Duchnowski and P. M. Zurek, “Villchur revisited: Another look at automatic gain control simulation of recruiting hearing loss,” J. Acoust. Soc. Am., vol. 98, no. 6, pp. 3170-3181, Dec. 1995

Outline Facts on Hearing Loss Hearing Aids Cochlea-IHC and OHC Presbycusis

Decreased Audibility Decreased Frequency Resolution Decreased Temporal resolution Decreased Dynamic Range

Amplification Techniques Linear Compressive-Single/MultiBand

Facts on Hearing Loss in Adults One in every ten (28 million) Americans has hearing loss and the

prevalence of hearing loss increases with age. While hearing aids can help about 95% (26 million) of them, only 6

million use hearing aids.

WHY? Stigma associated with wearing a Hearing Aid (HA) Denial about one’s Hearing Loss (HL) Exorbitant cost (eg. A pair of Widex Senso Diva BTEs cost around $11,000) Current HAs do not meet user expectations

Hearing Aids- An Engineering perspective

Area where vast improvement are possible 28 million Hearing Impaired people Huge Market($$$$) Circuit design and Signal processing personnel

Circuit design: Low power: 1.3V Fast acting (delay < 10ms) Small size Lower cost

Signal Processing: Biologically inspired/smarter algorithms Restore all effects of hearing impairment.

Anatomy of a Hearing Aid

Microphone Tone hook Volume control On/off switch Battery compartment

Types of Hearing aids

Behind The ear

BTEIn the Ear

ITE

In the Canal

ITC

Completely in the canal

CIC

Cochlea-IHC and OHC

Organ of corti: IHC/OHC 3 times more OHC

Inner Hair Cells (IHC) Afferent <to brain>

Outer Hair Cells (OHC) Efferent <from brain> Sharpen the traveling wave Provide an amplification for

soft sounds(40-50 dB SPL)

Damage in OHC/IHC Sensorineural Hearing Loss (SNHL)

Presbycusis Type of Sensorineural Hearing Loss HL in aging ears; occurs due to damage in OHCs

Mild 25-39 dBHL Moderate   40-68 dBHL Severe 70-94 dBHL

Problems faced by people with presbycusis: Decreased Audibility Decreased Frequency Resolution Decreased Temporal resolution Decreased Dynamic Range

Decreased Audibility

90% of HI adults loose frequencies between 500Hz-4KHz

HF components of speech (consonants) are weaker than the LFs.

Loudness dominated by the LFs “Speech is loud enough but not clear enough!”

To overcome this: HA has to provide more gain at HFs.

Decreased Frequency Resolution Asymmetry of traveling wave Eg. Reverse Audiogram OHCs do not sharpen the traveling wave. Decreases the ability to distinguish close

frequencies Upward spread of masking low frequencies mask

more than high frequencies Normals and HI: Poor resolution at high intensities To overcome this:

HAs less gain at LFs Try to remove noise before entering HA. Beamforming

Decreased Temporal Resolution Intense sounds mask weaker sounds that

immediately follow them. To overcome this:

Fast acting compression <Problem: Changes the speech cues; decreases

intelligibility though it increases audibility!>

Dynamic Range of Hearing The practical dynamic range could be said to be from the threshold of hearing to the threshold of pain

Sound level measurements in decibels are generally referenced to a standard threshold of hearing at 1000 Hz for the human ear which can be stated in terms of sound intensity:

Equal Loudness Contours

Decreased Dynamic Range/Recruitment

SNHL increases threshold of hearing much more than the threshold of pain; thus decreases the Dynamic Range of the ear.

To overcome this:HA has to provide Compression; cut down amplification as sound gets louder.

Decreased Dynamic Range/Recruitment

Figure 7.1. Typical loudness growth functions for a normal-hearing person (solid line) and a hearing-impaired person (dashed line). The abscissa is the sound pressure level of a narrowband sound and the ordinate is the loudness category applied to the signal. VS, very soft; S, soft; C, comfortable; L, loud; VL, very loud; TL, too loud.

Figure 7.2. The response of a healthy basilar membrane (solid line) and one with deadened outer hair cells (dashed line) to best-frequency tone at different sound pressure levels (replotted from Ruggero and Rich 1991).The slope reduction in the mid-level region of the solid line indicates compression; this compression is lost in the response of the damaged cochlea.

Linear Amplification

Figure 7.3. Loudness growth functions for a normal-hearing listener (solid line), a hearing-impaired listener wearing a linear hearing aid (short dashed line), and a hearing-impaired listener wearing a compression hearing aid (long dashed line with symbol).

HA wearer adjusts gain, using volume control, as the level of environment changes.

Compressive Amplification

Slope=1/Compression Ratio Imitates compression carried

out by OHCs Fast Acting/Syllabic

Compression: Attack time~5ms Release time~60ms

Choose release time To avoid distortion To normalize loudness from

phoneme to adjacent phoneme for syllabic compression

Figure 7.4. Typical input-output function of a compression hearing aid measured with a pure tone stimulus at multiple levels. The function depicted shows linear operation at low and high input levels, and 3 : 1 compression at mid-levels. Different compression hearing aids have different compression ratios and different levels over which compression occurs.

Time Constants: Overshoot and Undershoot

Figure 7.5. A demonstration of the dynamic behavior of a compressor. Top: Level of the input signal Middle: Gain that will be applied to the input signal for 3 : 1 compression, incorporating the dynamics of the attack and release time constants. Bottom: The level of the output signal, demonstrating overshoot (at 0.05 second) and undershoot (at 0.15 second).

Overshoot:

•Affects Intelligibility

•Makes consonants be identified as plosives

Reduce effects:•Clipping overshoot•Delaying gain

Undershoot:

When release time isn't that large, then forward masking lowers the affect of undershoot

Singleband/Wideband

Adjust gain across all frequencies equally

Preserves spectral shape over short time scales; speech cues

Choose gain based on highest level; Spectral peak

Speech has multiple spectral peaks. Inadequate selection of gains.

Figure 7.7. Amount of compression applied to music by a wideband compressor (squares) and a multiband compressor (circles).The compression was measured by comparing the peak/root mean square (rms) ratio of the music into and out of the compressor over different frequency regions. The open symbols on the left show the compression ratio calculated from the change to the broadband peak/rms ratio. The filled symbols show the change to the peak/rms ratio in localized frequencyregions.

Multiband Compressor Normally upto 20 bands are used with varying compression ratio per

band. Adjust gain/compression in each band independent from other Change in spectral contrast across bands may cause perceptual

consequences though it restores normal loudness. STI<Speech Intelligibility Measure> of compressed speech does not

correlate to Listening tests. With more experience people who use multiband HAs get adjusted

to the change in spectral shape/cues. Typically vowel perception is not affected as much as consonant

perception Overamplification occurs at crossover between bands<To avoid this

increase overlap between bands so that gain at a frequency is controlled by more than 2 bands>