>> To get
 >>the payload 20dB above the noise, and the crypto-stream 20dB
 >>above the payload, we need an audio carrier with a signal-to-
 >>noise of 40dB, which is 10,000-to-1 in sound power.
 >
 > IIRC, when I went to school, audio volume was measured in dB - spl
 > (sound pressure level, which I noted in the pervious post) which is
 > 20 x log10 and not 10 x log10 which is a measure for comparing power

What did you think "10,000-to-1 in sound power" meant?

A bel is, by definition, a factor-of-10 in power.  Sound
pressure increases as the square root of power, so a bel implies
a factor of the square root of 10 in pressure. They are not two
different measures.

'SPL' is an absolute scale, fixed by specifying dB values from a
0dB reference approximately at the edge of human hearing. Saying

     +20db(spl) above the ambient noise

doesn't make sense. It's 20dB above the ambient noise, which is
a factor of 10 in pressure and a factor of 100 in power. If we
want to say how loud the ambient noise is in an absolute sense,
there we'd use SPL.

 > - - you did not do your basic research.  Try -
 >
 > http://en.wikipedia.org/wiki/Sound_unit
 >
 > Forty dB is a factor of 100.

Forty dB is a factor of 100 in sound pressure and a factor of
10,000-to-1 in sound power. Click the "Decibel" link on the page
you cited and read the paragraph under "Definition".

The way the notation works, one does not specify decibels of
pressure or decibels of power. A decibel figure implies both the
pressure multiple and the power multiple. When one goes to say
what the multiple is, then one must say power or pressure,
because the power multiple is the square of the pressure
multiple.

[...]
 > The OP implied the transmition of the sound data should occur in real
 > time when he said:

Which step do you think a cheap PC cannot do in real time?

 > As another poster pointed out a speaker used for acoustic transmition
 > has all sorts of phase shifts

Not all sorts, mostly just one sort: speakers send different
frequencies to different physical sound-generators.

 > which will be unpredictable and
 > unrepeatable unless you want to specify exactly which speaker (not
 > which brand - which one), sound card, microphone and which
 > temperature, pressure and humidity conditions apply.

Why should those have phase-shift problems?

 > You could only
 > use the most crude forms of phase shift modulation, if any.  You'll
 > be *lucky* if you get an unreliable 20k bits per second.

A made-up figure, right?

--
--Bryan