PSYCHOACOUSTICS OF THE PATHOLOGIC EAROzarks Technical Community College

HIS 110

Psychoacoustics and HL

Recall that acoustics are physical properties of a sound that are measureable (intensity, frequency, wavelength)

Having a hearing loss does not change the acoustics of sound or the soundwave itself

Hearing loss changes our psychoacoustic perceptions of sound

Important Terms to Understand

Dynamic Range (DR)=the range of intensities from the softest sounds we can hear to the loudest sounds we can hear

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• In normal-hearing individuals, the DR of our ears is 140 dB SPL (from 0-140 dB)

• When we are referring to hearing loss and the fitting of HAs, the dynamic range refers to the range of intensities from the threshold of hearing (red circles) to the loudness discomfort level (“L”=loudest tolerable sound intensity)• On the audiogram at

right, the DR at 500 Hz is 80 dB HL and at 4000 Hz is 55 dB HL

Important Terms to Understand Linear vs. Non-linear

Linear refers to an equal input-output system. For example, in a linear hearing aid, for every additional 10 dB that comes into the microphone, the amount of gain or volume coming out of the speaker is increased by 10 dB.

Non-linear refers to a system in which the output of a system in not equal to the amount of input to the system. For example, in a non-linear hearing aid, for every additional 10 dB into the microphone, only an extra 5 dB of gain or volume may be put out by speaker.

Non-Linearity of the Basilar Membrane

In NORMAL hearing individuals, the basilar membrane is NON-LINEAR in the way that it responds to different sound intensities and frequencies In other words, what comes in is not what comes

out If you double the input to the basilar membrane, the

output less than doubles If you add a second tone at a different frequency, the

response to the first tone decreases (Two-tone suppression)

If you play two tones (say 1000 & 1200 Hz) a third tone can appear (at 800 Hz) (Cubic Difference Tone)

Non-Linearity to Sound Intensities

In the cochlea, specifically, on the basilar membrane: when sound intensity

(dB) is increased, the magnitude of the movement of the basilar membrane does not grow directly in proportion to that sound intensity

If it had a linear response, the plot at right would result in a perfectly-straight, diagonal line

Image from: sciencedirect.com

The Purpose of the Cochlea’s Non-Linearity

….to fit the huge range of sound pressures that our ears are capable of detecting (dynamic range) into the auditory system

Sensorineural Hearing Loss

Most common type of hearing loss Diagnosed by elevated

air- and bone-conduction thresholds (worse than 20 dBHL) on the audiogram

SNHL is usually associated with damage to the outer hair cells of the cochlea

What happens when there is OHC loss in the cochlea?

The response of the basilar membrane becomes more linear Loud sounds are not compressed as they

once were As a result, loudness recruitment occurs

Loudness Recruitment

Recruitment is an abnormal loudness perception in individuals with hearing loss Oftentimes, patient’s with hearing loss report

that sounds that were once a comfortable volume are now uncomfortably loud

Patient’s with SNHL have an elevated threshold (sound has to be louder for them to hear it); however, the loudness discomfort level does not change (it is the same as it was when they had normal hearing) As a result, the rate of loudness growth to their

ears is much more rapid This results in loudness recruitment

What else happens when there is OHC loss in the cochlea?

In a healthy cochlea, the basilar membrane is very sharply tuned to specific frequencies (i.e. a 2000 Hz tone results in activation of a very narrow, specific area on the basilar membrane)

In cochlear hearing loss, the loss of outer hair cells results in a broadening of frequency tuning on the basilar membrane (i.e. a 2000 Hz tone activates a broader area on the basilar membrane) This results in reduced frequency selectivity (usually

occurring in the high frequencies most significantly), which results in difficulty understanding speech, especially in noise.

SNHL and Timbre

Timbre is composed of the whole spectrum of sound—not just the pitch Timbre is what allows us to distinguish two

musical instruments, even when they are playing the same note of identical frequency

Due to the reduction in frequency selectivity in SNHL, the ability to hear changes in timbre is impaired. it will be more difficult for the to tell the

difference between different vowel sounds or to distinguish musical instruments

Sound Localization in HL

Sound localization abilities are reduced with hearing loss Most patients show a reduced ability to use

interaural time and intensity differences This is especially true in individuals with

asymmetrical hearing loss In addition, people with high-frequency

hearing losses are usually not able to make use of the directional information provided by the pinna

Conductive Hearing Loss

•CHL is due to a problem with transmission/conduction of sound from the outer ear to the inner ear

•Common causes: wax, fluid, otosclerosis

•Remember CHL can often be treated medically or surgically

Psychoacoustics and CHL

In cases of CHL, the cochlea is healthy As a result, patient’s with conductive hearing loss do not

experience distortion of sounds because they still have normal frequency tuning/selectivity on the basilar membrane

As long as sounds are loud enough, they hear clearly Patients will report:

Sounds are softer than normal Different tonal quality from normal, depending on the

frequencies affected If low frequency CHL, patient’s might say sound is tinny or

Mickey Mouse-like due to mainly hearing high frequency input If high frequency CHL, they might say that sounds are muffled,

mumbly, or have too much bass due to loss of consonant information