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Re: Fw: sursound: speaker phones

At 02:44 PM 10/25/00 -0500, James W. Beauchamp wrote:
While we're on the subject of sound localization, can someone explain
why speaker phones always sound like you're "talking through a tube" to
the person on the other end of the line?
Here is a related problem: [...] When I am
actually there listening, the speech is as clear as a bell; I ignore
all environmental sounds and echoes. [...] Later, when I play it back
through the headphones,
the basic sound is there, but now the echos and environmental sounds swamp
out the speaker, who is rendered barely audible.

As a hearing researcher who has recently (last year) lost all hearing in
one ear, this is a question with which I am very much concerned.  Now that
I no longer have the use of my binaural system to segregate sounds in the
environment, the echoes and noise around me cause great interference with
both detection and identification of sounds.  In a quiet, non-reverberant
environment I don't even notice the loss.  In a crowded room or on the
street however, I can barely have a conversation.

However, it does sound better than a speaker phone.  Why is
this?  Presumably I am missing the binaural cues to segregate the sources
(and the associated echoes?), but I still have the outer ear, head and body
filtering that gives different sources different spectral characteristics
(the so-called head-related transfer function or HRTF.)  The speaker phone
takes away all of these, while the stereo microphone retains the level and
timing differences.

Much work has been done on binaural recordings using artificial heads and
torsos as well as in-ear microphones, but most of it has focused on
veridical perception of location.  Conversely, the work on masking mevel
differences investigated the importance of binaural correlation to
detection but did not include HRTFs and did not focus on speech.  It might
be interesting to combine these and measure speech intelligibility in noisy
environments with and without appropriate HRTFs (which include interaural
differences in time and level.)

Erick Gallun
Graduate Student, Hafter Auditory Perception Lab
UC Berkeley