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Stupid Audio Question

Discussion in 'Electronic Design' started by Al, Mar 9, 2007.

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  1. Al

    Al Guest

    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
     
  2. Eeyore

    Eeyore Guest

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

    Graham
     
  3. John  Larkin

    John Larkin Guest


    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
     
  4. Jim Thompson

    Jim Thompson Guest

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

    ...Jim Thompson
     
  5. John Larkin

    John Larkin Guest

    Wikipedia never lies!

    John
     
  6. Phil Hobbs

    Phil Hobbs Guest

    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
     
  7. Glen Walpert

    Glen Walpert Guest

    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 :).
     
  8. Phil Hobbs

    Phil Hobbs Guest

    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.
    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
     
  9. John

    John Guest



    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
     
  10. Al

    Al Guest

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

    Al
     
  11. John  Larkin

    John Larkin Guest

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

    John
     
  12. Glen Walpert

    Glen Walpert Guest

    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
     
  13. Rich Grise

    Rich Grise Guest

    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
     
  14. Glen Walpert

    Glen Walpert Guest

    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.
    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 :).
     
  15. John  Larkin

    John Larkin Guest

    With, maybe, a corresponding increase in Q.

    John
     
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