Audio Engineering Society
Chicago
Meeting Review,
|
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.
|
