Notices
Results 1 to 5 of 5

Thread: Displacement amplitude of sound wave at minimum audible intensity

  1. #1 Displacement amplitude of sound wave at minimum audible intensity 
    Forum Masters Degree thyristor's Avatar
    Join Date
    Feb 2008
    Location
    Sweden
    Posts
    542
    Hi!

    In my wave mechanics class, we derived that the time-average intensity of a sound wave, is given by
    ,
    where is the wave speed, is the density, is the displacement amplitude and is the angular frequency.

    The derivation seemed allright, but in the next sentence the lecturer stated the minimum audible intensity of sound is (a figure supported by other sources, and concluded from the above formula that (with , , and ). This value equates to about a tenth of the radius of a hydrogen atom, so surely there must be some flaw to the argument. I and my fellow students supposed that the problem lies in the formula being derived from a macroscopic point of view, where the air is treated as a continuous medium, rather than a discrete medium consisting of molecules (which effect must certainly be present at displacement amplitudes corresponding to the low intensity of .

    Would anybody care to shed some light on this issue?

    Thanks!


    Last edited by thyristor; November 30th, 2011 at 06:00 PM. Reason: Typo
    373 13231-mbm-13231 373
    Reply With Quote  
     

  2.  
     

  3. #2  
    Forum Professor
    Join Date
    Jul 2008
    Location
    New York State
    Posts
    1,125
    Your question is confusing. You have a distance s0 in units of W/m^2. ????????????


    Reply With Quote  
     

  4. #3  
    Forum Masters Degree thyristor's Avatar
    Join Date
    Feb 2008
    Location
    Sweden
    Posts
    542
    How silly of me; the unit was of course supposed to be . I have corrected it now. Thanks for pointing it out!
    However, my original question remains the same; what is the flaw of the argument?
    373 13231-mbm-13231 373
    Reply With Quote  
     

  5. #4  
    Forum Junior
    Join Date
    Sep 2011
    Location
    Manchester, UK
    Posts
    236
    Assuming your figures to be correct, it is quite possible that all the molecules in some small region might be displaced by a small amount relative to where they would have been in the absence of the sound wave. That the displacement is small in relation to the size of a molecule doesn't make such a scenario impossible. Are you proposing that such small displacements aren't possible, or that they are possible but would not give rise to audible sound?
    Reply With Quote  
     

  6. #5  
    Forum Masters Degree thyristor's Avatar
    Join Date
    Feb 2008
    Location
    Sweden
    Posts
    542
    I am proposing that such a small displacement would not be detectable by the cilia in the ear.
    373 13231-mbm-13231 373
    Reply With Quote  
     

Similar Threads

  1. sound wave vacuum possiable?
    By harmtz in forum Physics
    Replies: 5
    Last Post: May 20th, 2010, 12:03 AM
  2. About sound wave
    By ~James~ in forum Physics
    Replies: 6
    Last Post: March 20th, 2010, 11:25 PM
  3. Longitudinal Waves and Amplitude
    By Gex in forum Physics
    Replies: 2
    Last Post: February 7th, 2010, 04:32 AM
  4. Sound Intensity
    By helpmeplease13 in forum Physics
    Replies: 1
    Last Post: February 23rd, 2008, 06:12 PM
  5. Running multiple sound amplifiers off one sound card output?
    By MacGyver1968 in forum Computer Science
    Replies: 0
    Last Post: May 13th, 2007, 10:59 AM
Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •