[Premium-Rx] Headphones

[Barry Hauser]

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




Ken wrote:
> > You may care to take a look at these new head phones just released by
> 'Sennheiser', the model No is, PXC250.
> They are a strange arrangement in so much as the have two miniature
> microphone elements that record the low frequency and thanks to the small
> tube shape unit in which these elements are installed, "remove this low
> frequency noise and then create an, 'anti-sound wave' that cancels out any
> ambient sound " ( Sennheisers words, not mine ).
> They can be viewed in the latest release of the, 'Computer Active'
magazine,
> ( issue No166 ) or you can go direct to the WEB site of  'Sennheiser' at,
> www.sennheiser.co.uk .The price in the UK is £90.00
> They really look the part and from the photo in the magazine seem as
though
> you could wear them all day and suffer no discomfort
> Hope this helps..................cheers all, Ken, G4KIR.