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Physics question for biologists

  1. Jun 15, 2010 #1
    Hello, independently of each other, two of my physics professors brought up the fact in class that they did not know that when you send you a high frequency sound at a high intensity, but with a frequency too high to hear (or a frequency young people can hear but that you already lost any sensitivity to), if it has any effect on your ears, i.e. if it can still do harm (just like sounds you can hear at high intensity). I'm not sure: on one hand: you can't hear it, so it seems weird it can wreck damage to your hearing; on the other hand, if it's a frequency you used to be able to hear, something must still be responding in your ear (although not sending a senseful signal), and maybe it can have a bad effect?

    Does anyone have an idea? I tried to google it, but found the choice of words hard.

    Thank you,
    mr. vodka
  2. jcsd
  3. Jun 15, 2010 #2

    Andy Resnick

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    High levels of ultrasound radiation can have deleterious effects:

    http://brneurosci.org/noise.html [Broken]
    'Ultrasonic frequencies (sounds above the range of human hearing) can also damage hearing due to the generation of subharmonics. Most experts recommend a limit of 110 dB for sound at 25 kHz [5,6]. The non-linear interaction of ultrasound with matter (such as when the energy is scattered at an air-water interface, or in the ear itself) creates subharmonics that fall within the range of human hearing. These subharmonics are perceived as squeaking sounds. Earmuffs are more effective at blocking ultrasound than low-frequency sound. Diagnostic ultrasounds for pregnancy do not pose a hearing risk.'

    (one of the above references):

    http://www.hc-sc.gc.ca/ewh-semt/pubs/radiation/safety-code_24-securite/guidelines-principes-eng.php [Broken]
    Last edited by a moderator: May 4, 2017
  4. Jun 15, 2010 #3
    I see, interesting. But in that case, the reason it can still do harm -they imply- is because due to some effect it 'gets' into the audible frequency zone; I quote: "[...] creates subharmonics that fall within the range of human hearing. These subharmonics are perceived as squeaking sounds."

    So if you hear nothing at all, can it still do harm?
  5. Jun 15, 2010 #4
    Sound is a mechanical phenomenon in a medium (air, water or more) and can pulverize more than your ears even if you are not able to hear it. Yes, it would take a lot of energy directed in a small area to have that effect, and the damage would not be to the cilliae in the cochlea, but probably general damage as vibrations in the bone and fluid of the inner ear tore apart tissues OR heated them.

    Infrasound at high intensities is definitely more deleterious, but again, you may not be deafened in the classic sense, but a broader and more... dangerous sense. Pump enough base at someone, and they'll be too dead to be deaf, with collapsed lungs and organ damage.

    Basically, you're going to liquefy your Organ of Cordi before you suffer normal hearing loss, but deaf or dead is still deaf or dead, right? Think of the case of the Pistol Shrimp.
  6. Jun 15, 2010 #5
    Your eyes can't detect UV light, but it still gives you a sunburn. If enough energy is being transferred to your body in any way then there will always be the potential for injury.
  7. Jun 15, 2010 #6
    I thank you all for your answers and what you guys are saying makes sense.

    But take the following situation: if one of our professors wants to do an experiment where he sends out a high frequency sound and gradually lowers it, so each of us experiences at a certain frequency a moment when one starts to hear the sound. When we hear the sound, we notice it has quite a high intensity and we have the tendency to cover our ears. It hurts our ears to listen to it. Now is the question: well it doesn't hurt our ears when we don't hear it, but can it still do harm specifically to the ears? (and not in the melting your ear channel kind of way :p) Strictly speaking, your ear drum is still vibrating, isn't it? And the hammer is maybe also still banging? So I mean, there is some activity still going on there, even though it's strictly (probably?) not vibrating any hears that makes us hear it (or maybe they are but due to some nerve damage the signals don't get across or something). Can it have an ill effect?
  8. Jun 15, 2010 #7
    In the case of sound, the damage from those intensities are causes by damage to the "little hairs" within your inner ear. Once the frequency drops out of their capacity to vibrate to the 'tune' that avenue of mechanical damage is lost. However, pump enough energy into the same sound, and the damage occurs through other means to the same regions. The case of uV light is good, but UV light packs a lot of energy compared to a laboratory source of ultrasonics. You could deafen a dog that way, but for you it would take a LOT of energy delivered to a small area. Take an ultrasound, crank it up to make and shove it into someone ear and you are going to hurt them, but not in the way that a loud concert would. Generally, energies in ultrasound required to do you harm are extreme, but low frequency, as your body is far more effective at capturing and resonating with that wavelength. Think of it as the difference between the shockwave of an explosion, versus a laser.
  9. Jun 16, 2010 #8
    Hm I see, interesting! :) Yeah I've heard low frequencies are underestimated because at parties you generally are not aware of the damage they do and just notice high intensity high frequency sounds (although you're not speaking about anything in the hearing range, but I suppose the characteristics don't change in that low frequency sounds, even when not audible, can do more harm than high frequency sound at the same intensity). It's something you wouldn't expect, but I suppose it's just because our body happens to be more receptive to low frequency sounds (audible or not), as you say.

    I'm happy I also got the answer on my main question which is that once you don't hear a sound, it can't cause further damage in a similar manner. Thanks :)
  10. Jun 16, 2010 #9
    Well technically sound is just something that living organisms perceive. It's not really that experience that can do harm do us, it's the vibrations that cause it. The vibrations that vary in frequency above and below our perception to "hear" them can still do damage to us if the amplitude is strong enough.
  11. Jun 16, 2010 #10
    My pleasure, and you're right, low frequencies cause resonance in more of our body than higher frequencies, so more energy is "dumped" into us, than in the other case. In air, sound, the vibrations as zewpals says, spread and lose energy rapidly, but in water for instance, sound is a viable weapon at range!
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