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How is sound transmitted through air so clearly

  1. Oct 24, 2011 #1
    How is sound able to be transmitted through the air with such extreme clarity. What I find puzzling is that air has no stiffness to it at all. There are just countless numbers of molecules travelling at the speed of sound in random directions.

    How can a single speaker cone set up pressure waves that can simultaneously play bass instruments, voice or a bell through a medium that is so easily brushed aside by my hand such that I cannot even tell it is there.

    When the aether was invented as a medium for the propagation of light waves, it necessarily had to have a property of almost infinite stiffness. However unlike light waves, sound waves are transmitted through a medium that waves.

    I can understand how such clarity could be transmitted through something solid, less so for liquid, but not at all through something as insubstantial as air.
  2. jcsd
  3. Oct 24, 2011 #2
    The "extreme clarity" is quite meaningless. Compared to what? How do you you know that is extremely "clear"? Maybe is very distorted but we've got used to it as all our hearing happens usually through air.

    Leaving this aside, the air has stiffness or rather elasticity, as any gas. Ever used a bicycle pump? Or tried to push the piston of syringe while closing the tip? When suddenly compressed (for example by the membrane of a speaker) the air near the membrane exerts an extra pressure on the air a little farther away, which is compressed a little too, and so on. This wave of compressions (and expansions) propagate with the speed of sound.

    The random (thermal) motion of the air molecules is not done at the speed of sound. The molecules have actually various speeds, in a quite wide range. The average speed for air at room temperature is higher than the speed of sound. But this is independent of the existence of a sound wave. The disturbance produced by the sound wave is an effect on top of the background of this thermal motion.
  4. Oct 24, 2011 #3


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    Well, rationalizing it as "getting used to it" doesn't explain how it can faithfully transmit detailed information such as a voice, music or data.

    Fairly certain that I could put a modem on a quality loudspeaker and a receiver some arbitrary distance away - and the intervening air gap would not be a significant source of noise in the signal. It would pass checksum with 100%. That's pretty good clarity.
  5. Oct 24, 2011 #4


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    I think this is the crux of the question. To wit: how can a medium - that is permeated with perturbations on the order of the speed of sound - faithfully transmit a signal at the speed of sound - without distortion.

    I know a spring can transmit waves along its length without them interfering with each other. I'm not so sure it would transmit them faithfully if I were grabbing and yanking the spring all over the place.
  6. Oct 25, 2011 #5
    I was half teasing and half trying to have a better formulated problem. Now you use "faithfully" which is again kind of fuzzy. The sound transmission through any medium induces some distortion. For one, the attenuation depends on frequency so the spectral composition of the received signal is different than that of the original.

    I see "clear" more like a physiological sensation which has some amount of subjectivity and I suppose may have some evolutionary component. I think we consider that we hear someone "clearly" on the phone (and even recognize the voice after some practice) even though there is some distortion, the spectral composition is altered.
    If the air would introduce a similar (or even worse) distortion, which was there forever, I guess we will still call what we hear as "clear".

    I think you are mixing the transmission of the information with transmission of the signal quality. Don't we use digital signals so the information is recovered completely even from a noisy or partially distorted signal?
    Receiving all the bits faithfully only proves that the signal was not too badly distorted.
    I think you should do you experiment with analog signal to prove the point.

    I am not saying that the air introduces significant distortion, in normal conditions, for usual distances.
  7. Oct 25, 2011 #6
    A better analogy would be to compare the spring which you grab and yank with air with winds and turbulence. So both will be macroscopic distortion of the medium.

    Otherwise, the atoms in a stationary, not yanked spring move too in all kind of directions and with various frequencies. Some of these will be the same as the frequency of the transmitted sound. And the speed of the collective motion waves is the same as the speed of the transmitted sound, in the acoustic range of frequency.
    Maybe is more amazing that we hear "clear" sound through solids than through air.
  8. Oct 25, 2011 #7


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    When you brush air with your hand, the frequency component of that sweep is very low. The amount of energy coupled into the air is proportional to the frequency, for a given displacement. In other words, if you drive a speaker cone with 10 Watts of a 20Hz signal, and then again 10 Watts of a pure 200Hz signal, the displacement of the cone will be a tenth of the displacement of the 20Hz signal. (Actually it might not be a linear term, I'll have to go check that.) That's why low frequency speakers need drivers that have a long travel, because you really need to push the air back and forth by a big displacement at low frequencies to generate a longitudinal rarefaction wave of a certain power that can propagate well.

    Unfortunately, modern electronics has provided the means for the miscreants of my neighbourhood to pump these large, low frequency rarefaction waves into the environment that they can travel far enough to then resonate the cavities of my house.

    Now that we picture these waves of low and high pressure being pushed back and forth longitudinally, you can see that a pulse of air will push into the air ahead of it at the speed that the component molecules are moving at - the speed of sound. Similarly, the rarefaction causes the air ahead of it to rarefy, also as fast as the air is moving - by the speed of sound.

    In terms of 'clarity', this hints also about resonance. The ear is a 'clever' shape that permits a wide-band of resonances to stimulate the senses in the ear. This resonance is like the miscreants' loud music - if they hit the right frequency then it'll cause audible resonances in a room of a house possibly a long way away that'd not otherwise have been heard. Similarly, the ear orders the resonances of rarefaction waves directed into it by the pinna, meaning small displacements are amplified so that the sensitive parts of the ear are stimulated by the resonances. However, resonance itself means that you are sampling the characteristics of several waves which in part is why harmonic and resonant sounds sound more 'fulfilling' than crisp, precise sounds. e.g. people report preferring LPs and tube amplifiers than CDs and silicon amplifiers - even though the latter are more 'faithful' reproductions, the ear likes to be stimulated by these longer resonances that are easier to hear, although actually less 'clear'.

    There should also be an effect which I cannot really claim to have heard. This is that different frequencies will propagate at different speeds. Though the air molecules will push back and forth into and out of the pressure waves, it takes a bit longer for them to fill up the 'larger', lower frequency rarefactions. If I have it right, the higher frequencies should propagate quicker. This might be a 'medium related' effect too, e.g. more humidity, or whathaveyou. If you watch someone kick a ball from a long way off, the lower frequencies get blurred out to the end of the noise, so it tends to sound more muffled as the high and low frequencies get very slightly out of step with the noise of the ball-kick. Like I say, I can't actually claim to have really noticed that effect, but I think it must happen to some degree.
  9. Oct 25, 2011 #8
    Whereas the attenuation dependence on frequency is quite significant, the dispersion (dependence of speed on frequency) is very low, at least in the audible range. At ultrasound frequencies may become visible, even though still small, I believe.
    I did not find any numerical values yet.

    Regarding absorption, there is a nice tool here:
    Even for dry air it shows significant increase with frequency.
    I did not check the values against some textbook or paper.
  10. Oct 25, 2011 #9


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    Precisely the reason I pointed this out.

    We were talking about an objective measurement of the clarity of a signal. A signal received with zero checksum errors means the distortion is below some arbitrary objective level.
    Last edited: Oct 25, 2011
  11. Oct 25, 2011 #10
    @cmb, you raise a further point with your 'miscreants' comment that also puzzles me, namely the propagation of not just low frequency waves that set a room resonating but the transmission of much higher frequencies that somehow pass right through a window. Even in a window with double insulation one can still hear the frequency range of a human voice and higher which means that these pressure waves travelling through the very elastic medium of the air somehow transmit themselves to the very very rigid glass, (rigid compared to a speaker cone) then through another elastic air gap to another piece of rigid glass back to a large expanse of air into my eardrum.

    That I can make out any sounds at all other than a muffled din after the above, is also astounding to me.

    The reason I gave the example of a speaker cone and bass, voice and bells, which may be heard in well mixed music, was to get across what I meant by clarity. I did not mean the clarity that a phone gives in the sense of being able to understand the words, I mean the clarity in the sense of faithful reproduction of high resolution sound. It seems utterly incomprehensible to me that the unique timbres of a gong, human voice, and triangle, as well as the more subtle nuances of many sounds all together, can not only be produced live then transmitted through the elastic air, but further these sounds can be made to be reproduced, albeit with some studio work, by a couple of speaker cones and then these cones can set up pressure waves with the necessary complexity to excite the receptors in my ear.

    I mean all these pressure waves come in one lump all overlapping through the elastic air and through the huge and varied sounds of an orchestra the ear can pick out the most delicate of textures. How the mechanism of the ear works and the digital processing in the brain is separate to the fact that the information nevertheless needs to be transmitted first through the air.
  12. Oct 25, 2011 #11


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    I don't think sound through a window propagates as a wave. The velocity of sound in glass is around 4km/s, so a frequency of 20kHz (>highest audible) is a wavelength of 20cm. I think sound waves that encounter glass, which it will see as an impedance discontinuity, will fail to couple to it as a 'wave' at all (unless higher than several humdred kHz, I suppose?).

    Instead, the glass will respond as an elastic panel, as if it were a loudspeaker element. Therefore, I think what you hear is more a function of how the glass is suspended in its frame, than the nature of the glass. I would hazard a guess that the more rigid it is in the frame, then the more it will pass high frequencies, and the more damped it is (both mounting and whether, e.g. it is plastic laminated) the less low frequencies it will pass.

    These are just educated guesses, I am no expert on the acoustics of windows.....
  13. Oct 25, 2011 #12


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    Once above a few 100 Hz, the things you imagine to be very very rigid can become high Q structures that resonate with surprising amplitudes. In fact, the more rigid they are, the bigger the Q of any resonances they might actually have.

    If you have never seen things under vibration testing with a stroboscope (which you run slightly off-freqeuncy to the imposed vibration frequency, which gives the appearance of motion by phase displacement), you'd be amazed at what thick pieces of metal can do once you push them to resonance. Inches-thick pieces of solid metalwork can look like they are waving backwards and forwards as if they were just soft plastic!
  14. Oct 25, 2011 #13
    Sorry, I was a little unclear, I did not mean to give the impression that I thought the pressure waves travelled through the glass, but rather that even the considerable attenuation of the waves caused by the glass responding to the pressure waves by itself moving back and forth (like a speaker cone but with considerable restriction of movement) and therefore resetting up the original pressure waves as best as the glass can.

    Like, I am already amazed that this level of structure can travel through the elastic air to my ear, never mind the reinterpretation if you will, of the original pressure waves by not one but two separate layers of glass before continuing on it's way to my ear receptors. This is just an added level of amazement to me which your example reminded me of.

    The original question still relates simply to how can such astounding detail be transmitted by the randomly moving molecules of such an elastic medium as air.

    It's the level of 'faithfulness' that I find difficult to comprehend if I try to picture the physics of the moving molecules.
  15. Oct 25, 2011 #14
    This is probably the real crux of my question. Like you, these things do amaze me, and I wish to get to the underlying cause of the amazement. I suppose in a very real sense it's akin to the amazement of how so many different properties of materials are realised simply by rearrangements of pairs of up/down quarks and electrons.

    Is this property of elastic air to be able to faithfully transmit such high precision pressure waves a consequence simply of the extreme number and smallness of molecules? This is what I wonder. Is my amazement simply due to my inability to fully comprehend the tininess of individual atoms.
  16. Oct 25, 2011 #15


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    Judging that you are evoking 'feelings' into this, I guess the best thing I might suggest to you is to imagine the wavelengths involved. At ~1kHz for human speech, we're talking a wavelength of some 50cm in air.

    So, shooting at the hip to find an answer to address your curiosity, if we were having to listen to a wavelength of mm over kilometers, then I would agree that at first sight you might get to wonder how all those waves accurately travel that distance and still all stay in respective synch. But I think for the most of the time, we are listening to sounds that are units to no more than several dozen wavelengths away. I don't know if that helps, but maybe recognising this - that it's quite a big wavelength we're talking about - might make you feel better about why you get to hear the right bit of the audio signal at around about the right timing.
  17. Oct 26, 2011 #16
    But it seems to me that sound transmitted by water is not as clear as that by air. And sound wave is very different from electromagnetic wave.

    I think that sound could be easily transmitted just because it is easy to deform. Solid like diamond which is very difficult to deform apparently would be reluctant to transmit sound with the same amplitude.

    But if you are talking about the quality of the sound transmitted by stethoscope is much more clearer than that transmitted by air, since less energy among the wave is dissipated.
  18. Oct 26, 2011 #17


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    "Shooting from the hip" would be a little less messy... :smile:
  19. Oct 26, 2011 #18


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    No. Other way around. The harder the substance, the better it transmits sound.

    That's why a Newton's Cradle is made out of ultra hard steel ball bearings (the harder the better) rather than out of cotton balls. And why we make church bells out of brass instead of wood.
  20. Oct 26, 2011 #19
    Hence my puzzlement at the level of fine grain detail able to be transmitted though the elastic and insubstantial air molecules. Before every 8 year old had a mobile phone, in the days when they used two tin cans, the cans were joined by a string pulled as tight as possible, not a rubber band.
  21. Oct 26, 2011 #20


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    See my post above. This is about damping and the frequency spectrum being transmitted. There must be [one presumes!] some sort of 'relaxation' time for all materials, and if the frequency is above that rate then, whatever the material, it will transmit the wave. If not, it will relax before the wave has passed a given spot and damp locally. So the 'rigid' materials are 'rigid' precisely because they will push back against the force you push on them - they are 'efficient' at springing back at you, therefore providing you are above this 'relaxation' time then they will transmit the frequency 'efficiently'.

    However, that is not to say 'soft' things cannot transmit efficiently providing they, also, are pushed quicker than a 'relaxation' time but also are not too lossy and damping of the signal. Rubber bands are very lossy to compression, whereas compressible fluids may act anywhere up to 'near perfect' when they are compressed.

    (The notion of 'relaxation time' is just an illustration of mine and is not intended to be read as meaning this is part of 'acoustic theory'. If it is, well, that's co-incidence.)
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