Re: [ARSCLIST] Absolute Polarity

[Michael Shoshani <mshoshani@xxxxxxxxxxxxx>]

On Mon, 2008-10-20 at 12:29 -0700, Lou Judson wrote:

> 
> Think of the sond emanating from the bottom of a piano, and how it  
> affects the listener in the same room (and what if it were recorded  
> with mics underneath - then which polarity should it have "on tape?"  
> Or snare, tympani, literally every percussion instrument? - how much  
> sound comes from the bottom?

Wouldn't matter so much; sound from the bottom either gets absorbed or
diffused. An audience listening to a piano is for the most part
listening to a waveform that has been reflected twice: initially off the
soundboard below the strings, then that reflection is reflected off the
upraised lid. The soundboard has holes to provide resonance from below,
but as a general rule any sound that one would hear from the bottom is
dull and unfocused. Pianos are designed to project up and out...well,
grand pianos anyway. 


> Just thoughts on the topic. Makes me smile and think of the French  
> horn. Would its attack be positive or negative, since it faces away  
> from the audience?

I am neither a scientist nor an acoustician, but I would guess positive
since the waveform is really projected upward rather than away from the
audience. 


> I am one of those persons whose eyes tear up when presented with out  
> of phase stereo speakers. 

It should be immediately apparent to anyone; the sound "in the
middle" (ie, that common to both channels) partially cancels itself out
as the sound reaches your ears. That's how those "stereo wide" settings
work on consumer televisions and portable boxes, by reversing the
polarity of one channel.

There were at least two talking machines marketed in the late 19th/early
20th century that claimed that every other talking machine only gave the
listener half the sound, but theirs (the Conn Double-Bell Wonder and the
Duplex) gave the listener both sides of the sound wave thanks to having
two independent diaphragms actuated by a common stylus, and feeding
their sound to two horns.  I should actually think that the volume would
be diminished somewhat, because these were playing a mono signal
together with its out of phase self.  I suspect that imperfections in
the playback process (horn resonances, tracking errors) actually worked
to the advantage of these machines.

Michael Shoshani
Chicago