[Premium-Rx] Headphones

[ken chattenton]

HI Barry, all,
----- Original Message -----
From: "Barry Hauser" <barry at hausernet.com>
To: "ken chattenton" <k.chattenton at btopenworld.com>;
<premium-rx at ml.skirrow.org>; "Blair Batty" <Blair at OntarioRocks.ca>
Sent: Monday, June 28, 2004 11:55 AM
Subject: Re: [Premium-Rx] Headphones


Hi all:

Actually, that situation -- low frequency sound out of phase -- is
time-honored.  It's a primary consideration in speaker design in terms of
what to do with the "back wave" from a woofer.  Basically, you can either
bounce it off the back of the enclosure, hopefully reversing its phase and
send it out a port in the front as with base-reflex designs and their
variants (also "piston" effects), or bottle it up to keep it from escaping,
as with an acoustic suspension design.  It happens all the time by accident
when speaker leads are hooked up "out of phase".  The bass seems to
disappear.  Even with a binaural stereo signal, much of the lows are in
common between the left and right channels, so one woofer will cancel out
much of the sound of the other.

Some "premium" automobiles have noise cancelling incorporated within their
audio sound systems.  Microphones positioned in various places feed a
circuit which reverses the phase of sounds over a certain bandwidth and
blend it into the audio signal to reduce the perception of road noises.  I
believe the process is more readily accomplished with low and lower
mid-frequencies because it's easier to "synch" the reverse phase signal with
longer wavelengths in a semi-controlled, variable environment.  Also, low
frequencies are more omni-directional.  Some of the cancellation effect
occurs directly on actual sound pressure levels, however some of the
"mixing" occurs in the brain, as I recall.  High frequencies are much more
directional.  So, the choice of low frequencies for the "anti-sound wave" is
not so much by choice as it is a limitation imposed by the laws of physics,
methinks.

The most extreme observation occurs when you operate a woofer out of any
enclosure.  As the cone moves forward -- pushing the air at you -- there is
a simultaneous vacuum of nearly equal pressure drawn from around the speaker
basket -- pulling the air back.  That's why they need to be in a box -- or
mounted in a very long, high wall.

Once upon a time, many years ago, I built a pair of humongous speaker
systems of the base reflex kind.  I learned all about this back then, and
resonant frequencies, too.  I will stop here. ;-)

Barry

Thank you for your very interesting reply and explanation'
I did not realise this stemmed from the design of speaker systems.

I think I will give these new 'phones' a try, cheers Barry, all the best to
all on the list, Ken, G4KIR.