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Stupid Audio Question

A

Al

Jan 1, 1970
0
Has anyone gone to the same concert at both say, a mile high venue, like
in Denver, and then in NYC? If so, does the music sound the same, or
does the altitude make a difference? I suppose it wouldn't for
electronic instruments, but how about acoustic instruments? Is the
density of the air a factor?

Al
 
E

Eeyore

Jan 1, 1970
0
Al said:
Has anyone gone to the same concert at both say, a mile high venue, like
in Denver, and then in NYC? If so, does the music sound the same, or
does the altitude make a difference? I suppose it wouldn't for
electronic instruments, but how about acoustic instruments? Is the
density of the air a factor?

I think it affects the speed of sound. Not sure what difference that would make
to listen to.

Graham
 
J

John Larkin

Jan 1, 1970
0
Has anyone gone to the same concert at both say, a mile high venue, like
in Denver, and then in NYC? If so, does the music sound the same, or
does the altitude make a difference? I suppose it wouldn't for
electronic instruments, but how about acoustic instruments? Is the
density of the air a factor?

Al


Air pressure doesn't affect sound velocity, but temperature does. I'd
imagine that a change of temperature would detune wind instruments to
some extent, certainly with more effect than, say, using Monster
cables or tube DACs or picosecond jitter reducers.

Music sellers should be required to note ambient recording temperature
for every track. That would give the audiophools something new to
argue over.

John
 
J

Jim Thompson

Jan 1, 1970
0
Air pressure doesn't affect sound velocity,

Eh, John? Are you sure about that? Maybe, as atmospherics are
concerned, but how about 200PSI versus 20PSI ??
but temperature does. I'd
imagine that a change of temperature would detune wind instruments to
some extent, certainly with more effect than, say, using Monster
cables or tube DACs or picosecond jitter reducers.

Music sellers should be required to note ambient recording temperature
for every track. That would give the audiophools something new to
argue over.

John


...Jim Thompson
 
J

John Larkin

Jan 1, 1970
0
Eh, John? Are you sure about that? Maybe, as atmospherics are
concerned, but how about 200PSI versus 20PSI ??

Wikipedia never lies!

John
 
P

Phil Hobbs

Jan 1, 1970
0
John said:
Wikipedia never lies!

John

It got it right this time, anyway. Look up some more controversial
topics, and you'll get a very different picture of its accuracy.

Within the range where ideal gas behaviour applies, i.e. mean free path
between molecule-molecule collisions is much greater than the molecular
diameter but much smaller than a breadbox, the speed of sound depends
only on temperature.

This is because sound is transmitted by those collisions, and it can't
go faster than the mean velocity of the molecules (there's a factor of,
iirc, 1/sqrt(3) because the collisions randomize the particle directions).

Shock waves are what you get when the sound is strong enough to
significantly change the mean molecular velocity, and they can go much
faster than the speed of sound.

Cheers,

Phil Hobbs
 
G

Glen Walpert

Jan 1, 1970
0
It got it right this time, anyway. Look up some more controversial
topics, and you'll get a very different picture of its accuracy.

Within the range where ideal gas behaviour applies, i.e. mean free path
between molecule-molecule collisions is much greater than the molecular
diameter but much smaller than a breadbox, the speed of sound depends
only on temperature.

This is because sound is transmitted by those collisions, and it can't
go faster than the mean velocity of the molecules (there's a factor of,
iirc, 1/sqrt(3) because the collisions randomize the particle directions).

Shock waves are what you get when the sound is strong enough to
significantly change the mean molecular velocity, and they can go much
faster than the speed of sound.

Wikipedia has cleaned up its speed of sound info since the last time I
looked, but it is still only a presentation of the infinitesimal
("small signal") equations (provided with the vague caveat "This
equation applies only when the sound wave is a small perturbation on
the ambient condition") without any derivations. For derivations or
the finite amplitude sound equations you still need a book. IMO the
best is Blackstock, Fundamentals of Physical Acoustics (presumes
comprehension of partial differential equations). The introduction
chapter defines wave propogation, presents some simple examples
(electrical transmission line and plucked string) and derives the
lossless one dimensional wave equations for sound in ideal gasses from
conservation of mass and momentum on an infintessimal control volume.
When the pesky nonlinear terms are dropped (valid only for
infintessimal amplitude sound) you get the Wikipedia version of sound.
When they are not dropped you have the finite amplitude sound
equations, in which the speed of sound is not a constant - the higher
pressure parts of the wave travel faster because they are hotter
(adiabatic compression required by the lossless assumption). Shock
waves do not really go faster than the speed of sound (how can sound
go faster than sound?), they only go faster than the infintessimal
amplitude speed of sound.

BTW, the speed of infinitesimal amplitude sound varies with the mean
molecular weight of the air, which changes with humidity (~.4%
increase dry to wet at STP). And the characteristic impedance of air
varies directly as infinitesimal amplitude speed * density, a function
of pressure, temperature and humidity.

And don't forget the effect of the increasing concentration of CO2 in
the atmosphere :).
 
P

Phil Hobbs

Jan 1, 1970
0
Glen said:
Shock
waves do not really go faster than the speed of sound (how can sound
go faster than sound?), they only go faster than the infintessimal
amplitude speed of sound.

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.
BTW, the speed of infinitesimal amplitude sound varies with the mean
molecular weight of the air, which changes with humidity (~.4%
increase dry to wet at STP). And the characteristic impedance of air
varies directly as infinitesimal amplitude speed * density, a function
of pressure, temperature and humidity.

Yes, all of which are very very constant on the time scales of sound
waves (see subject line).

Back on your heads.

Cheers,

Phil Hobbs
 
J

John

Jan 1, 1970
0
Al said:
Has anyone gone to the same concert at both say, a mile high venue, like
in Denver, and then in NYC? If so, does the music sound the same, or
does the altitude make a difference? I suppose it wouldn't for
electronic instruments, but how about acoustic instruments? Is the
density of the air a factor?

Al



If you ever heard the effect of helium on changing the voice by
breathing a little of it due to the decrease in air density, the same
thing would happen to all the wind instruments but to a lesser extent.
The string instruments would not change due to the difference in air
density. How much the freq. would change is up to you to find out.

John
 
A

Al

Jan 1, 1970
0
John said:
If you ever heard the effect of helium on changing the voice by
breathing a little of it due to the decrease in air density, the same
thing would happen to all the wind instruments but to a lesser extent.
The string instruments would not change due to the difference in air
density. How much the freq. would change is up to you to find out.

John

Ah, the best answer yet. Air density is the key.

Al
 
J

John Larkin

Jan 1, 1970
0
Ah, the best answer yet. Air density is the key.

Al

If the composition doesn't change, density does not change sound
velocity. Temperature does.

John
 
G

Glen Walpert

Jan 1, 1970
0
If the composition doesn't change, density does not change sound
velocity. Temperature does.

John

Right, a change in density due to a change in pressure with constant
temperature and composition has absolutely no effect whatsoever on the
(small-signal) speed of sound in air. A change in temperature or to a
lesser extent humidity (which changes the composition of the air) will
require retuning of instruments, a change in pressure will not.

What does change with pressure is the impedance of air, which is
directly proportional to air density. Impedance can be thought of as
the ratio of "push" to "flow". In a DC electrical circuit the
impedance (resistance) is push (volts) / flow (amps); R = E/I. In an
acoustic "circuit", impedance is push (sound pressure) / flow
(particle displacement). So as air pressure decreases from NYC (14.7
PSIA) to Denver (12.1 PSIA), the sound pressure produced by the same
instrument surface vibration amplitude will be reduced by a factor of
12.1/14.7, or about 82% of the sound pressure level produced by the
same amplitude surface vibration at sea level.

Glen
 
R

Rich Grise

Jan 1, 1970
0
Air pressure doesn't affect sound velocity, but temperature does. I'd
imagine that a change of temperature would detune wind instruments to
some extent, certainly with more effect than, say, using Monster
cables or tube DACs or picosecond jitter reducers.

I'd have guessed that lower pressure air, which is lower density, would
raise the resonant freq. of an instrument, kinda like helium makes your
sinuses resonate higher.

Or am I just blowing smoke up my own headbone?

Thanks,
Rich
 
G

Glen Walpert

Jan 1, 1970
0
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 :).
 
J

John Larkin

Jan 1, 1970
0
Right, a change in density due to a change in pressure with constant
temperature and composition has absolutely no effect whatsoever on the
(small-signal) speed of sound in air. A change in temperature or to a
lesser extent humidity (which changes the composition of the air) will
require retuning of instruments, a change in pressure will not.

What does change with pressure is the impedance of air, which is
directly proportional to air density. Impedance can be thought of as
the ratio of "push" to "flow". In a DC electrical circuit the
impedance (resistance) is push (volts) / flow (amps); R = E/I. In an
acoustic "circuit", impedance is push (sound pressure) / flow
(particle displacement). So as air pressure decreases from NYC (14.7
PSIA) to Denver (12.1 PSIA), the sound pressure produced by the same
instrument surface vibration amplitude will be reduced by a factor of
12.1/14.7, or about 82% of the sound pressure level produced by the
same amplitude surface vibration at sea level.

Glen

With, maybe, a corresponding increase in Q.

John
 
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