Hearing Aids and Psychoacoustics

Ozarks Technical Community CollegeHIS 110

Some hearing aid basics…

•All hearing aids have a microphone(s) to pick up sounds, an amplifier to make sounds louder, and a speaker to put the amplified sound into the ear canal

•All hearing aids are powered by a battery

•Note: hearing aids are much more complicated than this, but you will learn more about that in future courses

Some hearing aid basics…

•Hearing aids come in a variety of styles and sizes:▫In-the-ear hearing aids▫Behind-the-ear hearing aids

Hearing Aids

• In-the-ear (ITE) hearing aids

Hearing Aids

• Behind-the-ear (BTE) hearing aids

The Goal of Hearing Aids

•To amplify the sounds in the hearing aid user’s environment so those sounds are audible and comfortable

•All of the amplified sounds from the hearing aid must fit within the patient’s dynamic range (DR, the dB range from threshold of hearing to loudness discomfort level)

SNHL and Hearing Aids

•Even when hearing aids amplify sounds so that they are audible to the hearing-impaired listener, the perception of those sounds is usually abnormal due to distortion in the cochlea•Patients will report:▫That the sounds are unclear and distorted▫That it is hard to hear comfortably over a

wide range of sound levels▫Difficulty in understanding speech in noisy

situations or reverberant rooms

SNHL and Hearing Aids

•Much of these complaints are due to the more linear response of the cochlea/basilar membrane due to outer hair cell loss

•So how do hearing aids overcome the loss of the non-linearity of the cochlea?▫By using non-linear sound processing!!!▫Modern hearing aids attempt to mimic the

non-linearity of a healthy cochlea

Non-Linear Sound Processing in Hearing Aids• Modern hearing aids employ a combination of

compression, expansion, and linear processing to fit sounds within the patient’s DR

• Expansion is a reduction of very soft/low intensity sounds▫Purpose is to reduce circuit/microphone noise

generated by the hearing aids operation • Compression can be thought of as an “automatic

volume control”. It makes soft sounds louder (turns them up) and loud sounds softer (turns them down) without the patient ever having to touch the volume control. Compression maintains audibility and comfort.

Overcoming Reduced Frequency Selectivity

• Many patients have the greatest degree of hearing loss in the high frequencies, since damage due to aging and noise exposure affects the high frequency hearing first.

• In patients with this sloping, high-frequency hearing loss, the amount of gain or amplification in hearing aids is greater at high frequencies than at low frequencies.

• This rising frequency-gain characteristic will help to alleviate the effects of the upward spread of masking.

Cochlear Dead Regions• Cochlear damage may result in complete loss of

function of inner hair cells over a certain area of the cochlea, called a dead region.▫The nerve cells that are “dead” cannot transmit

any sounds of the specific frequencies that those nerve cells are responsible for up to the brain

▫People with extensive dead regions often do not benefit much from a hearing aid May benefit from amplifying frequencies up to an

octave above the edge frequency of a high-frequency dead region

May also benefit from frequency transposition

Frequency Transposition in HAs

•For patient’s with very severe-to-profound hearing loss in the high frequencies OR for patient’s with cochlear dead regions, amplification at those frequency regions will often be ineffective at making those sounds audible to the patient.

•Frequency transposition or frequency shifting literally moves the frequencies that are inaudible down to a lower octave, so they can be perceived by the patient.

Loss of the Pinna Effect/Ear Canal Resonance with Hearing Aid Use•Recall that the

resonance of our ear canal and the pinna effect result in our most sensitive hearing from 2-5 kHz▫Our ears have a

12–15 dB advantage near 3000 Hz

Image from: http://hyperphysics.phy-astr.gsu.edu/hbase/sound/

Loss of the Pinna Effect/Ear Canal Resonance with Hearing Aid Use

When a hearing aid is blocking the ear canal and/or the microphone placement of the hearing aid is outside of the ear canal, it affects the natural resonance of our earo This natural resonance is lost for a behind-the-ear aid

with a closed mold,oReduced for an in-the-ear aid,o Shifted to higher frequencies for a completely-in-the-

canal aid. This should be considered when fitting hearing aids,

though many hearing aid manufacturers have “corrections” for this change in the software.

Hearing Aids and Conductive Hearing Loss•Because the cochlea is healthy and there is no

distortion in the processing of sound in the cochlea, CHLs are very easy to fit with hearing aids

•These patient’s just need an increase in volume at the affected frequencies

•Of course, hearing aids should only be considered AFTER medical options for treatment have been exhausted and the patient is medically cleared for the fitting of hearing aids