Audio Engineering Society
Chicago
Meeting Review,
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Mead began with a description of how ears function,
including the role of the inner and outer hair cells in the cochlea.� He described experiments that isolated the
function of these two groups of cells.�
The outer hair cells amplify weak sounds, while the inner hair cells pass
information to the brain.� If the outer hair cells are damaged, a person�s hearing
can be restored by amplifying sounds, as is done in all hearing aids.� However, if the inner hair cells are
damaged then a person loses discrimination ability.� This loss of discrimination greatly reduces
the ability to understand speech in the presence of noise.� Hearing aids can do little to help.� Inner hair cell damage affects about 20% of
hearing aid wearers. Mead gave attendees a quick SIN test (signal in noise),
where a spoken phrase was partially masked by ever increasing level of
background speech babble.� The examples
were played loud enough that everyone could easily hear the speech.� While most attendees had good scores even
at 0 dB SNR, there were 3 attendees who had great difficulty making out the
words as the background noise was increased. Mead repeated the test, but included a cell phone in the
transmission path.� The phone codec did
very poorly in the presence of background noise, and everybody did poorly on
the SIN test when noise was present.�
Mead�s research showed that 6 dB higher SNR is typically needed with
cell phones to retain intelligibility.�
He claimed that that is why the person sitting next you always speaks so loudly into his or her phone. If the hearing aid microphone is placed at the talker�s
mouth rather than at the listener�s ear, the SNR will improve by up to 35
dB.� This is enough to overcome even a
serious loss of inner hair cells.� Etymotic is developing a system where several microphones
can be linked to a listener�s hearing aid through Bluetooth technology.� The microphones are worn by the talkers,
and the signals are received and mixed by equipment the listener would
wear.�� When asked about interference,
he replied that the Bluetooth system supports 30 transmitters in one room. Mead gave a lively presentation, including many audio demonstrations and video recordings.� After the formal presentation, he answered many questions on the causes of hearing damage.
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