🚀 go-pugleaf

>Demonstration:  Plug one ear with your hand.  Close your eyes.  Click
>the fingernails of your thumb and forefinger on the other hand at 
>various positions around your other ear.  Can you localize the clicks?
>How?  You're only using one ear.
>
>Paul O'Neill                 pvo@oce.orst.edu

Are you satisfied that plugging one ear with your hand is TOTALLY
blocking sound from entering that ear?  I'm not.  Try disconnecting your
auditory nerve :-)

The following is a followup that I composed to the original posting.  I
believe my system had trouble sending it.  If this is a duplicate, please
ignore it (it's very long).  I don't mention many DSP techniques, but
once the concepts are revealed its almost trivial to think of DSP
applications.  I also don't mention the effects of frequency response -
I'll leave that to someone who knows a little more about how sounds are
filtered by the irregular shape of the outer ear.
-------------------------------------------------------------------------

In article <1989Oct31.193130.1685@eddie.mit.edu> rich@eddie.mit.edu (Richard Caloggero) writes:
>     Is anyone out there interested in talking about
>     psycho-acoustics/psycho-acoustic phenomena?
[...]
>                     As it turns out, most of this information is
>spacial in nature.  Psychoacoustics, then, is the study of phenomena
>related to the *realness* of sound.  This *realness* is not only a
>function of frequency response, frequency balance, harmonic
>distortion(s), etc.  It is also a function of things having to do with
>spacial information.  My problem is that I have no name for these
>*spacial things*.  I've heard  them, but I can't talk about my
>experience in concrete terms without the necessary vocabulary.

I think that psycho-acoustics is very relevant to this group because DSP
techniques make it much easier to simulate realistic sound.  It is
currently possible to do a lot of experimenting with sound space using
today's technology, but there is much room for improvement.

>      Can anyone out there shed some light on this muck?  I am
>convinced that it is possible to build a *box* which can take one
>channel and generate a stereo signal which is a representation of the
>original signal plus 3d positional information.  In other words, I can
>generate sound, using just two speakers, which actually comes from
>*behind* the listener even if he/she is facing the speakers.  Does
>anyone agree with this?  Does  anyone know how to build such a thing?

How your brain locates sound sources:

I think that it is very possible with a stationary listener, or a system
which adjusts to the listener's changing position.  (Say, like
headphones?).  In fact, one of my pet peeves is that hi-fi audio salesmen
have been selling *monophonic* subwoofers for years.  They claim that low
frequencies cannot be located by your brain because they are non-
directional.  Actually, your brain uses (at least) two methods of
locating sound sources: delay and amplitude.  Low frequencies are located
by the delay between the arrival of the sound to each ear.  The fact that
low frequencies are less directional helps by presenting approximately
equal amplitudes to each ear, only phase shifted.  Sounds directly ahead
(or behind) arrive simultaneously, while a sound emanating from 90% to
the left or right have the maximum delay, based on the distance between
your ears, the speed of sound, and the period of the frequency.  High
frequencies are more directional (directionality of sound increases with
pitch), and your ear locates higher tones by the difference in amplitude
between each ear.  As with phase shift, amplitude difference is smallest
when straight ahead, and largest to the left or right.  Basically your
head gets in the way of the directional highs, thus lowering the
amplitude as the angle increases.

Why the brain uses two methods:

Both methods of directional sensing are used together because each breaks
down at different frequencies.  Delay, or phase shift, can only be used
for low frequencies, because as you increase pitch the period of the wave
gets smaller and eventually the wavelength is less than the distance
between your ears.  Your brain can determine the delay when comparing two
versions of the same cycle of a wave, but things get muddy and confused
if the sound completes an entire period in one ear before it arrives at
the other.  Amplitude differences start to diminish with lower pitches
because directionality is also decreasing and eventually there is very
little difference in volume.  The moral of the story is that, in truth,
the *mid range* speakers cannot be located by your ears and brain.

Binaural Recording:

Binaural recording was based on recording sound from two microphones
which were the same distance apart as the average persons ears (no jokes
about cranium size, please), thus preserving the natural time delays
between the sound 'images' presented to each ear.  A good analogy is 3D
movies, which record two light images with cameras spaced horizontally
like our eyes.  When the two images are delivered independantly to each
eye (using polarized lenses set at 90 degree angles), the brain
interprets depth that is no longer real.  For some reason binaural
recording is not as popular as it was, even though we probably have the
processing power to synthesize the effect rather than merely recording it
accurately.

Multi-Monophonic Recording:

Standard audio mixers pan sound using volume only.  There was an article
this year in Electronic Musician magazine which referred to this as
'multi-monophonic recording', which is more accurate.  The result is a
distinct lack of 'spacial' clues.  This is probably why mono subwoofers
sell.  The funny thing is that many audiophiles listen to classical music
which doesn't suffer from volume panning since it is recorded via
microphone.  The EM article described how incorrect micing could destroy
phase shift clues (if the mics were spaced too far - often on opposite
sides of the stage) or destroy amplitude cues.  Proper micing results in
good imaging.  Have you ever donned your headphones and panned the sound
all the way to the extreme left or right?  This really bothers me (gives
me a kind of a small headache :-), because my brain is trying to compute
what location this sound could be arriving from such that the other ear
would hear nothing at all.  Just doen't fit into the algorithm.

Drawbacks to 3D audio:

Headphones and loudspeakers are two differect media, yet we pipe the same
source material through them.  Perfect binaural recordings are ruined
when auditioned over free-standing speakers, because each ear hears
*both* channels.  Your brain re-interprets the delays and amplitude
differences and ends up computing WHERE THE SPEAKERS ARE!  It is possible
to make a recording which sounds 3D over loudspeakers, but the effect
would be different through headphones.

How can you experiment with psycho-acoustics today?

As the EM article mentioned, digital synthesizers can precisely repeat
the same sound based on an algorithm.  If a stereo synth (or two mono
synths connected through MIDI) were programmed so that each channel had a
different delay, then you could create 3D soundscapes.  One of the first
things I tried with my EPS (a year before I read the article, BTW) was to
pan two copies of the same digital sample so that one was hard left and
the other was hard right.  This took very little extra memory, since the
data was shared between the two voices.  I set one channel to ignore the
pitch bender so I could change the time delay in real time.  Think of an
analogy to a record player.  If two identical records are playing on
turntables with matched speeds, then the sounds will be in sync.
Grabbing one platter and slowing that channel down for an instant before
letting go would make that record lag with a slight delay after it
returned to normal speed.  The pitch bender on the sampler did the same
so I was able to move a sound around the room (without a volume change)
and leave it there by releasing the pitch bender to standard speed.  If
I needed to make the pitch bender channel advance ahead of the channel
which was ignoring the bender, then moving the pitch up for an instant
and then releasing it would do the trick.  This was pretty cumbersome,
but there is much you can do with this idea.  Many digital delay
processors can listen to MIDI controllers and change their delay in real
time.  If the programmable EPS MIDI controllers were used to affect the
volume of each wave, then one of these delay processors could cause the
phase shift to track along with volume changes.  I sure hope there are
a few synth owners reading comp.dsp

Carver makes a 'Sonic-Hologram' device which tricks your brain into
treating loudspeakers like headphones.  i.e. each ear only hears the
respective channel.  By computing the distance to the optimum listener
position, requiring the speakers to be placed a certain distance apart
and allowing for the speed of sound, Carver delays each channel the
correct amount, inverts the signal's polarity and mixes it with the
opposite channel.  This results in any sound from the *right* speaker
which hits the *left* ear being canceled out by an inverted copy of the
same wave.  The effect is so easy to accomplish that many portables have
a 'stereo wide' switch - although how you could fully appreciate the
effect while both you and the box are moving is beyond me...

Drawback #2:

Any 3D loudspeaker system will be dependant upon the position of the
listener (I think) unless someone designs an adaptive system which
monitors and adjusts to your movements.  Imagine one of those flight
simulator helmets with headphones and a computer which relocates
objects as you turn and walk around (such a thing is in the works,
but it still won't solve the free-standing loudspeaker problem).

Other experimental ideas:

Back when digital delays started to become affordable, I dreamed up a
multi-channel mixer system which had an independant adjustable delay for
each channel which tracked the pan pot.  The delay would be set so that
either channel could be delayed with respect to the other.  Thus, both
amplitude and phase would change is unison - as they do when an object
moves around you.  I thought this would be an expensive device, but I was
thinking of combining an analog mixer and digital delay.  With a totally
digital mixer, it would be interesting to allow very short time delays
in order to synthesize a binaural-style recording.

Other areas:

My experience has only been with time delays and amplitude changes.  I
wonder how much processing your brain does to reverse compute the
changes in the sound as it passes through our irregularly shaped outer
ear.  i.e. is it possible that a sound from behind is distinguished
by the path it takes around your outer ear?  I have read of an individual
who was convinced that he could coax stereo sound out of a monophonic
speaker!  I assume that he thought he knew how the brain interprets
frequency info.  He released a few recordings, perhaps someone else knows
the name of this person?

>     How do you use your ears?  Most people use them for spoken
>communication, and listening to music.  How many out there use them for
>navigation?  Since I am blind, I use audio information to *see* my
>environment.

I have often been walking in the dark down a familiar hallway, where I
was expecting a doorway at some distance, only to stop a few feet short
because I *thought* that I was at the door already.  It is usually around
two feet short of the door, so I figure my unconscious is taking in clues
and warning me to stop at a safe distance.  I've wondered if these
'clues' were sound-based, but sometimes there is faintly detectable
light.  Don't ask me why I occasionally walk around in the dark...

>-- 
>						-- Rich (rich@eddie.mit.edu).

Brian Willoughby
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