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Re: 2 source interference



After reading John Denker's and Leigh Palmer's responses, I'll back off
partly from what I said about crossovers. I'll back off from my statement
that the phasing through the crossover is important for this experiment.
Yes, if each speaker of a 2-way systems is getting a sine wave, then the
linear combination of the two is also a sine wave, no matter what the
phasing.

However, I still believe getting the phasing correct is very important for
stereo imaging. I have done experiments in which the phasing is
intentionally altered. This can be done within each speaker box (for
two-way and three-way or ported designs) and can also be done between the
two stereo speakers. When the phasing is not correct, the imaging
(perceived location of the sound source) wanders around as the music is
played. An important word in the last sentence is "music." When playing
music (as opposed to pure and steady-frequency sine waves) the wavelength is
always changing. Speaker phasing that is not reproducing the phase
relationships picked up by the microphones (in a true stereo recording)
makes the interference maxima/minima change locations in the room in a
different way than they should, and that ruins the stereo image.

I was erroneously extending this idea to this experiment. This experiment
is different than music because pure sine waves are used.

However, there seems to be some consensus that it would be good to have a
"single source" for each of the two speakers (not the two speakers in a
two-way system, but the two speakers between which we want to hear
interference). In this sense the crossover is still the villain because the
crossover is not perfect. It does not send all frequencies below the
crossover frequency to one speaker and all those above to the other speaker.
If it did, then for a particular frequency only one speaker of the two-way
system would be playing. Since the crossover allows both speakers to
produce sound (for frequencies not too far from the crossover frequency) the
two-way system involves two spatially-separated sources of sound. As others
have point out, that coupled with ground reflections might make the
interference pattern more complicated than expected.

A tuned port also serves as a second (spatially separated) source of sound.
I had already recommended, in my first posting, avoiding this kind of
speaker system. I said to use an "air suspension" speaker system. John
Denker said to use an "infinite baffle" speaker system. For small portable
speakers, these are the same thing. That is, there is no port in the
speaker case; the speaker case is sealed "airtight." This means it is an
infinite baffle. Unless the box is very large, the air in the case becomes
too stiff for a typical speaker, so the speaker has a very compliant
surround and makes use of the air stiffness to help dampen it. Hence the
"air suspension" designation. Therefore the infinite baffle system must be
an air suspension system if the box is small. Only if the sealed box is
very large can we use a non-air-suspension speaker design in the sealed box,
because the large air volume doesn't appreciably dampen the speaker.


Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817