Re: [LAU] Where do the 60 degrees for stereo come from?

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To: <linux-audio-user@...>
Date: Friday, June 24, 2011 - 1:13 pm

On Fri, Jun 24, 2011 at 01:58:47PM +0200, Philipp Überbacher wrote:

> I found a bit of explanation of wave propagation in one of my books, but

Can't say without seeing your book. P() and V() certainly
are not 'two states', they are two components of a single
state. You can create any combination of P and V at a
given point.

But for a _single source_ they are related, and you
could map them to voltage and current, with the quotient
being the acoustic impedance (as in Ohm's law).

Again, for the P and V fields generated by a single source
at suffient distance, or a plane wave, P and V are in
phase. Their maxima occur at the same points at any time.

It's very common misconception that the energy in a wave
'alternates' between potential energy (at a P maximum)
and kinetic energy (at a V maximum) as it does for e.g.
a pendulum. Even the Wikipedia article on acoustic waves
gets this wrong. In fact the power is proportional to the
product of P and V (as it is to the product of voltage
and current). If the two were 90 degrees out of phase
the average power would be zero.

> I guess this sort of analysis or model is used for more complex systems

Yes. In ambisonics the P/V ratio, divided by its expected value for
a plane wave (i.e. the acoustic impedance), is called 'rV'. A good
decoder is designed to generate rV = 1 for low frequencies. It's
done by adding an antiphase signal in a direction opposite to the
intended source. This increases the vector sum of V, and decreases
the sum of P, so they can be made to match again.

Ciao,

--
FA

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Messages in current thread:
Re: [LAU] Where do the 60 degrees for stereo come from?, Fons Adriaensen, (Thu Jun 16, 8:35 pm)
Re: [LAU] Where do the 60 degrees for stereo come from?, Philipp Überbacher, (Fri Jun 17, 8:58 am)
Re: [LAU] Where do the 60 degrees for stereo come from?, Fons Adriaensen, (Fri Jun 17, 5:48 pm)
Re: [LAU] Where do the 60 degrees for stereo come from?, Philipp Überbacher, (Fri Jun 24, 11:58 am)
Re: [LAU] Where do the 60 degrees for stereo come from?, Fons Adriaensen, (Fri Jun 24, 1:13 pm)
Re: [LAU] Where do the 60 degrees for stereo come from?, Charles Henry, (Mon Jun 20, 9:41 pm)
Re: [LAU] Where do the 60 degrees for stereo come from?, Fons Adriaensen, (Mon Jun 20, 10:33 pm)
Re: [LAU] Where do the 60 degrees for stereo come from?, Philipp Überbacher, (Thu Jun 16, 8:56 pm)
Re: [LAU] Where do the 60 degrees for stereo come from?, Julien Claassen, (Thu Jun 16, 8:21 pm)
Re: [LAU] Where do the 60 degrees for stereo come from?, Philipp Überbacher, (Thu Jun 16, 8:41 pm)
Re: [LAU] Where do the 60 degrees for stereo come from?, Karl Hammar, (Fri Jun 17, 12:45 pm)
Re: [LAU] Where do the 60 degrees for stereo come from?, Monty Montgomery, (Thu Jun 16, 8:43 pm)