90--110 DB (very OT)

[Nathan Bower]

Hello Sarah,

I have heard that organs can create beat frequencies below the normal
percieved cutoff of the human ear (5-10hz). Does this fit in with what you
are saying? Do we just 'imagine' the very low frequency, or does it really
exist?

I watched a program on tomorrows world where a man had designed a speaker
that was highly directional. He used frequencies in the range of 100s of
kilohertz that either interfered or created beat frequencies that could be
heard by someone at a distance, but not the people around him (sound becomes
more directional as the frequency rises). The technology was going to be
used in PA speaker systems in order that you could have loud music where you
wanted it, and still have the other parts of the room quiet enough to talk
in (the speakers were ceiling mounted firing downwards).

I thought I understood beat frequencies :/

N


----- Original Message -----
From: "Sarah Fox" <sarah@gendernet.org>
To: "Pianotech" <pianotech@ptg.org>
Sent: Sunday, December 01, 2002 8:23 PM
Subject: Re: 90--110 DB (very OT)


> Hi Richard,
>
> > Was this a sound system for an large venue?
>
> No, actually for a very small venue -- a 4' x 4' "anechoic" chamber used
for
> research purposes.  The subjects?  Frogs. <shrug>
>
> >  I have always
> > wondered if audible resultants can be gotten from supersonic
> > frequencies.  While standby once I was talking to a sound
> > technician about it.  Just as he said "come on lets find out",
> > they said the piano was ready.  So I have been "standing by" ever
> > since for another opportunity to find out.
> > For example, would the frequencies of 20,440 and 20,000 give a
> > resultant of 440 ?
>
> When two tones of slightly differing frequency are summated, there is a
> *perceived* beat frequency equal to the difference of the two frequencies.
> Each beat represents a complete "drift cycle" in which the two tones cycle
> from adding constructively to adding destructively to adding
constructively
> again.  However there really isn't any acoustic energy at the difference
> frequency.  That is, 20,440 plus 20,000 is nothing more than 20,440 plus
> 20,000.  Since both frequencies are beyond our sensory limits (for all
> practical purposes), I don't think we would hear the beat frequency.  Of
> course there could be some weird nonlinearities at play, in which case all
> bets would be off.
>
> Beat frequency detection is really a perceptual phenomenon that lets us
> determine the periodicity (i.e. the fundamental) of a sound without energy
> being present at the fundamental.  If we are presented with 400+500+600,
> provided the relative phasing of these components doesn't drift, we hear
> 100.  This is called the "missing fundamental" effect.  Even with digital
> filtration -- even with computational filtration -- the effect remains.
> However, in order to experience this perceptual phenomenon, we must first
be
> able to hear 400-600 Hz.
>
> Hope that helps!
>
> Peace,
> Sarah
>
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