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B "A sharp sound is perceived as being more intense than low"

  1. Sep 24, 2018 #1
    Hi everyone,

    Apologies for what may be a relatively obvious question for you but I'm struggling to understand a concept that was discussed in my introductory Physics class. I would really appreciate any help you could offer.

    During an explanation of sound, we were told today that a sharp sound tends to be more intense than a low sound. Rather than just assuming so, I'm trying to understand why.

    If I'm not mistaken, pitch (if a sound is sharp or low) is related to the wavelength (a high pitch and therefore sharp will have a longer frequency than a low pitched, low wavelength sound). Loudness is related to amplitude.

    I kind of get lost from here trying to draw this link back to why we perceive sharp sounds as being more intense. Am I on the right track or completely left-field?

    Many thanks in advance!
     
  2. jcsd
  3. Sep 24, 2018 #2

    anorlunda

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    Your question sounds more like biology than physics. Our perception of sound and the measurement of intensity by an instrument are not always equal.
     
  4. Sep 24, 2018 #3
    Thanks! It is in the context of a Physics course but tailored to medical students.

    The question posed was: "the level of intensity given (e.g. 90 dB) is perceived in the same manner, regardless of frequency. True or False."
     
  5. Sep 24, 2018 #4

    anorlunda

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    Isn't perception with our ears biological?
     
  6. Sep 25, 2018 #5

    sophiecentaur

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    I think it would be quite in order to discuss the physics of the mechanisms of hearing - right up to our 'experience' of the sensation. For a start, the ear response has a well defined lower frequency roll off (as opposed to cut off) and a peak at around 3kHz. This link shows the variation of hearing threshold with frequency. It plots the hearing threshold, which is the most objective measure, I would think although that says nothing about the linearity of response with stimulus level.
    The "why" part of the OP has two answers to it. The structure of the hearing system accounts for its frequency response and the peak it has 'chosen' must be a compromise between the engineering of the system and the requirement for best survival of the species. Nature / evolution seldom provides us with unnecessary capabilities. If the peak of the sensitivity curve includes the sounds cracking twigs and the cries of babies then that could be a factor to affect our survival. So I would say that we are more sensitive to that mid / high range of frequencies because we need to be. (But that's not actual Physics)
    The hearing mechanism is pretty small and the reason that it has low sensitivity at low frequencies is largely that it is increasingly difficult to match sound energy in the air to a small receiver that's immersed in fluid. (In the way that good quality bass loudspeakers are usually large) The Ossicle bones in the middle ear act as a series of levers which 'transform' the impedance of the vibrations of the ear drum to the inner ear. But any sort of impedance matching mechanism will have a limited frequency response . Also, the sensors in the cochlea have a range of lengths which also imposes limits on the frequency range (only listening for stuff that gets through the ossicles, no doubt).

    The level of sound in modern life is so much higher than it was when we evolved our hearing and we seem to have no natural protection mechanism against sustained loud sounds - we are all going deaf, as a result. We have no equivalent of the eyes' iris to regulate the admitted sound power. The message from our ears: "this sound is too loud" doesn't seem to register with our behaviour.
     
  7. Sep 25, 2018 #6

    anorlunda

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    It is more than frequency response, more than thresholds. It is perception in the brain. Just as we have optical illusions, there should be analogies to audible illusions.

    The only objective way I can think of to test the assertion that sharp sounds are perceived as louder (than non-sharp sounds of the same intensity and frequency), would be a hearing test analogous to tests given to people needing hearing aids. The result of the test would be a family of curves, objective intensity versus perceived intensity, versus "sharpness" of the sound.
     
  8. Sep 25, 2018 #7

    sophiecentaur

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    I agree that, as with all sensations, the perception is largely subjective. Nonetheless, the mechanics of the middle ear and the sizes of cavities and components impose some objective parameters that would be worth discussing. There are a lot of parallels with colour perception here and, in that field, the sensitivity curves of the three sensors have been measured and that seems to be an acceptable topic for discussion on PF. So I think it should be justifiable to discuss hearing at a similar level.
    I have had hearing aid tests and the NHS version seems just to plot threshold level over a range of frequencies. The aid is programmed to try to level up my top end loss and that's all. This link describes results of an extended version of that, which is the sort of thing you are suggesting. The OP may find that interesting but it's just data and doesn't suggest a reason. I would suggest that, perhaps we could say that the basic Physics of the transfer of vibrations from outside air to cochlea is what dominates the basic band pass characteristic and that the subtle differences are 'software' based. But I wonder about the contribution of the cochlea. Each of the hairs has its own variable amplification and uses positive feedback to optimise sensitivity and signal to noise ratio. This wiki article mentions it*.
    The way the curve changes around the mid range (most interesting?) spectrum gives a flat response when the sound levels are very (too) high. That could suggest some self protection mechanism.

    *The otoacoustic emissions mentioned in the article can be used in measurements of the hearing of babies without them needing to take part actively in the test.
     
    Last edited: Sep 25, 2018
  9. Sep 25, 2018 #8

    anorlunda

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    It comes down to the meaning of "sharp." In my view, a sustained sharp sound is a step function in intensity (regardless of frequency). Soft sound is a ramp change. If not sustained, it is an impulse compared to a gradual rise/fall. In that view, frequency response is irrelevant unless we are thinking of the Fourier harmonics.

    What is your interpretation of "sharp", @Mahavir? @sophiecentaur?
     
  10. Sep 26, 2018 #9

    sophiecentaur

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    I interpreted the term as meaning full of high frequency components (probably not actually harmonics because it would be dissonant at the same time. I didn't assume an impulsive sound but the same comments would probably apply.
     
  11. Sep 26, 2018 #10
    Your or your prof's nomenclature sucks too much for an answer. Could you unconflate physics, perception and musical terms, and try again ?
     
  12. Sep 27, 2018 #11

    sophiecentaur

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    +1
    It's what you get when you try to make something too easy and underestimate students' abilities and general knowledge.
     
  13. Sep 27, 2018 #12
  14. Sep 27, 2018 #13

    tech99

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    I sharp sound is presumably one that has high frequency energy and is short lived. A low sound is presumably one that has low frequency energy and is long lived.
    The audibility of different frequencies is shown by the Fletcher-Munson equal loudness curves, which show maximum response at rather high frequencies, around 3 kHz. I also wondered if the ear has a automatic gain control type of action, which would mean it is caught out by a sudden sound, but I think not.
     
  15. Sep 27, 2018 #14

    Tom.G

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    Well mine certainly do, to an extent. High intensity, sudden onset, mid- to high- frequencies cause some unknown muscles in my ears to contract, significantly decreasing the perceived sound level; after a quite short but discernible response time. I can sometimes intentionally contract these muscles, but not reliably or for extended periods. When done intentionally there does not seem to be any change in frequency response.

    Yeah, weird. I was surprised too.

    Cheers,
    Tom
     
  16. Sep 28, 2018 #15

    sophiecentaur

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    Yes. As the Wiki article says, there is a positive feedback mechanism with a short control delay. We (animals in general) need our hearing to be as sensitive as possible when there's a danger of a predator or enemy being there. AGC is essential for this.
    +1 I sometimes notice a sort of 'click' after a loud percussive sound which I put down to the AGC relaxing.
     
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