If all you mean by "speed of sound" is "how fast this particular
disturbance travelled from A to B", then you're right, but that isn't
the usual (or useful) definition, because it only applies to the one
case. The usual definition of a shock wave is one where the entropy
density is significantly increased by its passage.
I was merely complaining that Wikipedia presents the small-signal
approximation of the speed of sound (and the rest of the small signal
sound properties) without properly explaining what the approximation
is and what its limits of validity are. Not to mention a complete
lack of explanation as to why the speed of sound is what it is.
A drawback of the standard convention of referring to the small-signal
speed of sound as the speed of sound is that there is a tendency to
apply it (and the other small-signal sound properties) to all sound
with the possible exception of shock waves or sound loud enough to
clip at zero absolute pressure. This is like assuming that the small
signal response of an amplifier applies right up to clipping at the
power supply rails - it just isn't so.
Real finite amplitude sound has losses. The primary losses, or
"increase in entropy density" if you prefer, are due to the conduction
of heat from the higher pressure, hotter part of the wave (peaks) to
the lower pressure, cooler part of the wave (troughs), converting
mechanical energy into heat. These losses are insignificant over
short distances within the frequencies of human hearing and at
comfortable listening levels, but they become significant at high
frequencies because the peaks and troughs get close, putting an upper
limit on the frequency of ultrasound which will propogate a
significant distance in air. At high enough frequencies a transducer
will simply heat the air in front of it, increasing entropy density.
Over long distances this effect selectively attenuates higher
frequencies even within the range of hearing, again increasing entropy
density. Likewise where the sound pressure approaches atmospheric
pressure the temperature differences between peaks and troughs
increase, losses increase significantly - and the speed of sound
becomes significantly non-constant.
So your "usual" definition of a shock wave applies to non-shock finite
amplitude sound also, and is probably why no source I consider to be
authoritative on the subject uses it (e.g. Shapiro - The Dynamics and
Thermodynamics of Compressible Fluid flow, Thompson - Compressible
Fluid Dynamics, Blackstock - Fundamentals of Physical Acoustics). All
of these sources use what I regard as the usual definition,
essentially a "large" change in state variables (pressure,
temperature, density ..) in a very "thin" layer (or "short" time
depending on frame of reference). Entropy density also increases, not
because that is a defining characteristic of shock waves but for the
exact same reasons other finite amplitude waves increase entropy
density - primarily due to the conduction of heat.
The thickness of the shock layer is not independent of the magnitude
of pressure change across it; as a shock wave weakens with propogation
(and usually expansion) its thickness increases until it is no longer
a shock wave but rather an oridinary finite amplitude sound wave, with
no clear dividing line between the two, and no sudden change in the
significance of the entropy density increase.
Yes, all of which are very very constant on the time scales of sound
waves (see subject line).
But not constant between NYC and Denver (see original question and my
response re: air impedance, the significant change with altitude.)
On a slightly longer time scale, the atmospheric CO2 increase has
decreased the small-signal speed of sound at STP from 1126.91 Feet/Sec
in 1976 to 1126.89 Feet/Sec in 2003. Retune those instruments
.