Audio Engineering Society

Chicago Section

Meeting Review, April 27, 2004

other meeting reports

4/27/04 Meeting Highlights
by Tom Miller

Mead Killion gave a presentation dubbed ďLost On The Way To The BrainĒ to a joint meeting of the Chicago AES section and the Engineering and Recording Society of Chicago.The presentation was given in the Etymotic training classroom.



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 personís 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.Meadís 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 talkerís mouth rather than at the listenerís 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 listenerís 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.