A SOUND EAR II - Association of British Orchestras

A SOUND EAR II
FREQUENCY SELECTIVITY AND STEREOCILIAR DAMAGE
Threshold of Response
Normal
Frequency
Outer (V-shaped) Stereocilla Damaged
In this state, they do not ‘dance’ effectively and
so there is less amplification of their favourite
frequencies. The result is the red curve, where the
fine tuning response has disappeared.
You would normally have 50% damage to a region of outer stereocilia before seeing any change in threshold shift
(audiogram), and may suffer 75% damage before there is a change in threshold shift. Thus, the first consequence of
noise damage is not deafness but ‘blurred’ hearing. NB: damage to the outer stereocilia can be gauged by otoacoustic
emission testing (OAE) and while OAE results differ markedly from person to person, the change in an individual’s
results over time are useful for monitoring subtle noise damage. (See Health Surveillance).
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Every cell has a write-off point and kills itself when
too damaged. The stereocilia are part of a nerve cell,
and are not replaced after write-off - they are only
filled in by scar tissue.
When an array of stereocilia is damaged, the neighbours will normally also be damaged. A second look at the pink
curve shows that it has shifted away from the original tip. When the brain receives information from damaged
stretches, it can struggle to determine whether it is intensity or frequency that has changed so, for instance, it may
seem that colleagues go sharp during a crescendo.