martin@cod.NOSC.MIL (Douglas W. Martin) (11/16/89)
In the past few weeks, there have been several articles
about human perception of sound, and how such
psychoacoustic cues are interpreted spatially.
The two areas I wish to address are those of binaural recording,
and the obstacle detection sense used by many blind people
for navigation. I myself am totally blind,
use this obstacle sense extensively, and have a MS
in acoustics from Penn State.
The ability to detect obstacles, find doorways, estimate the size of
rooms, etc., was first discussed in the literature by
D. Diderot in 1749. He thought that a blind person could
judge the proximity of bodies by "the action of air on his face."
The sensation of approaching an obstacle is somewhat like light
pressure on the face. Thus, this sense has been misnamed
"facial vision" in much of the early literature.
The obstacle sense is more accurately referred to as
echolocation; it is an auditory perception. Confirmation that this is an
auditory sense and not some kind of "facial vision" was first
obtained by researchers at Cornell University in the late
1940's. Obstacles can be detected either
passively (using reflections of ambient sound in the room) or
actively (using a self-generated noise such as a click or
whistle). Learning to use this echolocation is
sudden and insightful rather than gradual; a person merely needs to learn
what to listen for. Of course, the use of this perception
is not limited to blind people. Anyone can easily demonstrate
this perception. Simply close your eyes, and walk with hard shoes
on a hard floor toward a wall. You should sense the presence of the
wall before actually contacting it. However, if walking barefoot across a
carpeted floor, will usually result in impact with the wall, because
there is much less reflected sound to work with.
Many of the parameters of this echo detection capability were
quantified by Charles Rice and his colleagues at Stanford in
the mid and late 1960's. The ability to detect an object depends on its
size, distance, and reflectivity. Rice found that blind people could
detect obstacles spanning an angle of about four degrees.
Area ratios between disks as small as 1.06 to one, could be discriminated.
Some subjects could also reliably discriminate circles, squares, and
triangles using their echolocation. Large obstacles can be detected at
distances exceeding ten to fifteen metres. Distance cues appear to be related to
both pitch and loudness, and directional cues result from the
same auditory localization phenomena described by
earlier articles in this group, mainly interaural
time and amplitude differences.
It was mentioned in an earlier article that binaural
recordings can be made by separating two microphones
by a distance equal to the diameter of the head. Actually, this
is not sufficient to make a binaural recording; it will
only make a stereo recording. In order to obtain the
binaural effect of localization, an obstacle (like a head) must be
present between the microphones. This is necessary to create the auditory
shadow which is critical for high-frequency localization.
When listening to a stereo recording through headphones, the sound
image is "lateralized" as opposed to the image
being "localized" using headphones with a true binaural recording.
In lateralization, with stereo headphones, the sound image appears
to be coming from somewhere inside the head, often closer to
either left or right, but still within the head.
When listening with headphones to a true binaural
recording, the sound image is "out there in space" with a
perceived distance and direction. Again, to make a binaural
recording, it is necessary to have a head-sized obstacle
between the microphones. The actual shape of the obstacle, the presence
of hair or facial features, and other similar factors are not
very critical. However, if sounds are to be localized
in elevation as well as in azimuth, there must be a reflecting
surface below the head, e.g. a torso.
It has been mentioned that the ear has a different frequency
response for sounds arriving from different angles. In fact, the
structure of the pinna (outer ear) is such that an impinging sound wave
undergoes multiple reflections in the pinna before reaching the eardrum.
The amplitudes and relative time delays associated with these multiple
reflections are, of course, angle dependent.
An excellent paper on this topic was published by Wayne
Batteau, 1965, in Proceedings of the Royal Society,
London.
I have hundreds of references in all these areas: blind
echo location, binaural hearing and recording with dummy heads,
and sound transformations in the outer ear.
If there is interest, I will compile a bibliography
as I have time, and will send it to anyone who wants it.
Doug Martin martin@nosc.mil
Naval Ocean Systems Center,
San Diego, ca 92152.
phone: (619) 553-3659.