Re: 2 source interference

["Richard W. Tarara" <rtarara@SAINTMARYS.EDU>]

An additional comment about stereo imaging.  Before computers became the
primary drain on _my_ wallet, the passion was stereo equipment.  Even in
graduate school I had the first Bose 901 speakers, a Crown 300 Watt
amplifier, and a Revox tape player.  (Other nuts will understand)  Later I
went on to Dahlquist speakers and other goodies (like a 'click & pop'
remover).  However, the most dramatic piece of equipment I ever added was
Bob Carver's 'Sonic Holography' circuit.  This $300 box did wondrous things
to the sound (although not always realistic things).  The basis of it's
operation is some real, and accessible physics.

In two speaker stereo reproduction, sound sources can be spread across the
'sound stage' by varying the loudness of that source as delivered to each
speaker.  But consider a source that should be centered.  The obvious choice
is to send the same volume (and phase) to each speaker.  It the sound source
was really at center stage (and you are sitting in the middle of the
auditorium, club, etc.) then the sound from that source reaches each ear
simultaneously and with the same intensity.  You easily locate the sound to
be directly in front of you.  Now when the stereo speaker try to reproduce
this, they also deliver the sound simultaneously to each ear and at the same
loudness and phase.  HOWEVER, the left speaker also delivers a signal to the
right ear that arrives a few microseconds after that from the right speaker.
The same thing happens for the left ear from the right speaker.  These
'crossover' signals that occur from the speakers are not present in the live
sound.  The ear/brain is sophisticated enough to know the difference.  The
generalized result is that sound reproduction from two speakers tends to be
very 2-dimensional.  You do hear sound sources spread between the speakers,
but they seem to come from a plane formed by those speakers.

Now for the holography circuit.  This box takes some of the left channel
signal, conditions it spectrally due to the damping effects of the head, del
ays it a few microsecond, reverses the phase and sends it to the right
speaker.  Likewise for the other channel.  For a listener positioned
centrally between the speakers, the effect is to cancel the unnatural
crossover signals.  The aural effect is a more 3-dimensional sound.
Instruments are not just positioned right/left but also forward and back.
Some sound sources seem to be located to the left of the left speaker or to
the right of the right speaker.  The stability of the images is quite good.

The drawback of the system is that it does require a central seating
position and is highly dependent on the nature of the recording.  I tended
to find that with smaller groups the sound stage was often reversed--with
the vocalist seemingly to the back of the stage.  However on some recordings
the effects were spectacular.  The original recording of Jesus Christ
Superstar, despite being a studio album, really presented a staged aural
experience.  The different singers were spaced across the stage and when
appropriate would move during a number.  I too spent a lot of time moving
speakers around and even covered the rear wall of my listening room with
anechoic material (since reflected sound could interfere with the holography
effects).  Anyway, short of true binaural recordings, this technique--later
coupled with some rear channel time delay/reverberation effects to simulate
larger listening spaces, provided the most realistic sound reproduction that
I've experienced.  {Still have all the equipment, but no longer have the
freedom to experiment with the 'room acoustics'.}

Rick

*********************************************
Richard W. Tarara
Associate Professor of Physics
Department of Chemistry & Physics
Saint Mary's College
Notre Dame, IN 46556
219-284-4664
rtarara@saintmarys.edu

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*************************************************
----- Original Message -----
From: "Michael Edmiston" <edmiston@BLUFFTON.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Tuesday, April 11, 2000 9:15 AM
Subject: Re: 2 source interference


> (1)  On one hand I agree with Leigh that worrying too much about speaker
> phasing is "a bit of a joke."  On the other hand, playing around with
> phasing is one of several audio variables that can make a very significant
> audible difference.  The difference is stereo imaging.  When I am
listening
> to the "hi-fi," can I close my eyes and pretend I am at the live concert?
> Is the violin sound coming from the correct position?  The trumpets?  The
> percussion?
>
> I have some demonstration tapes and records in which the engineers have
> deliberately played games with the phasing.  Aside from switching the
> location from which you think the sound is coming, they can also make it
so
> you can't you can't place the sound.  They can also make it sound like the
> source of the sound wanders around as the instrument plays different
notes,
> even though they aren't changing anything after the initial set-up.
>
> I have also played around with speaker placement in my living room
including
> combinations of direct and reflected sound, center speakers, rear
speakers,
> sub woofers, simulated surround sound using the Dynaco wiring scheme with
6
> db blended stereo, etc.