Greetings again to all,
My second post (“Great responses to my post--thanks!”) elicited some noteworthy
responses, particularly regarding my comment that most binaural recordings that
I’ve listened to don’t give a sense of “open space.” Naturally, we all have a
unique HRTF, and recordings or IRs made with an acoustical test fixture (e.g.
KEMAR) probably won’t match our own HRTF.
Recordings made with KEMAR (Knowles Electronic Manikin for Acoustic Research)
have the microphones deeply seated in this fixture. Such recordings will have a
“naturally occurring” resonant peak around 3 kHz because of the KEMAR’s pseudo
ear canal (which, for KEMAR, is just a straight tube, with or without Zwislocki
couplers). A naturally occurring resonant peak exists in open-ear listening
situations, and this adds to the sense of openness. The style of headphones we
use may destroy the ear canal’s natural resonant peak, particularly if the
headphones are of the insert type. If the recording includes a peak, then
insert phones may not be a problem. Otherwise, we may have to use a peaking
filter to re-create an open-ear type of response. Of course, not all headphones
seal off the canal. So how do these headphones affect listening? My
off-the-cuff answer follows:
I’d estimate that the earcup volume of circumaural headphones is around 6 cm2.
But because headphones include active drivers, computing the combined resonance
of the ear canal with the earcup’s volume may not be so simple: There’s an
issue of “equivalent volume” when dealing with active elements (for example,
consider the equivalent volume of a B&K acoustic calibrator). The point to all
of this is that HRTF, pinna transfer functions, open-ear frequency response,
etc. are dependent not only on the individual, but on the headphones used for
playback.
I made one recording using in-the-ear microphones and it was eerily realistic
in one way: I was slowly moving on a squeaky floor while making the recording,
and when I played the recording I found myself looking at my feet because it
made me feel as though something was moving at my feet. This is the result of a
full-body transfer function, and was the most out-of-the-head sensation I've
experienced with headphones. The rest of the recording wasn’t this impressive.
I have listened to Hector’s recording using AKG K240 studio phones (semi-open).
(Thanks to Hector for making his recording available.) The sounds and child
coming from the extreme left gave the sense of a distant source--this is good.
But I believe I experience what others discovered: None of the sounds appeared
to come from behind or in front of me; it was though the child was running
through my head. This may not be the case with all headphones. I have a pair of
ER-3A insert phones that will probably yield a different effect. I’m currently
using my ER-3A’s for an otoacoustic emission (OAE) study, but will report back
once I have a chance to listen to the recording via insert-type phones and my
Sennheiser HDA-200 headphones.
Again, many thanks to all for sharing thoughts, recordings, references, and
wisdom.
Sincerely,
Eric
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