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Measuring a loudspeaker's diaphragm please help!

  1. Jun 25, 2011 #1
    Hi all,

    First of all, thank u for ur time and sorry about my english.
    I have measured a loudspeaker's Sound Pressure Level from a distance of 1 m. At the same time, i've measured, with a accelerometer, the displacement, velocity and acceleration of the loudspeaker's diaphragm. I've found that when the SPL is 60 dB and the frequency is 80 Hz (loudspeaker radiating a pure tone) the displacement is 0,034 mm, the velocity is 190 mm/s and the acceleration 9,76 mm/s^2.
    1) First wrong that i found is that, i know the magnitude of displacement is d=A, so d=0,034 mm, then the velocity is v=w·A=17,1 mm/s, and acceleration is a=w^2·A, so a=8,59 mm/s^2. The most different is about the velocity (17,1 mm/s so far from 190 mm/s).. any idea?
    2) i start slowing the SPL, and i find that the velocity and displacement decreasing also, however, the acceleration keep constant.. about 9,7 - 9,8 mm/s^2. When the SPL is 42 more or less, the acceleration start decreasing (at 42 dB is 2,1 mm/s^2) until almost zero at 39 dB.. is not a bit rare?

    Thank u very much.
  2. jcsd
  3. Jun 25, 2011 #2
    You might want to post this in the Engineering section, or find an acoustic website specializing in such things. Good luck though, perhaps someone will answer it.
  4. Jun 25, 2011 #3
    Thanks Dan, i'll do it.
  5. Jun 25, 2011 #4
    The engineering section on PF is rather small though. You're question is somewhat busy for physics folk, who like to see things boiled down to a little data and basic theory.
  6. Jun 25, 2011 #5
    ok, i see, so i'm going to post in another web. Thanks
  7. Jun 25, 2011 #6


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    Gold Member

    Question 1 . What type of speaker are are using?
    The reason I am asking is the displacement of 0.034 mm. 1 mm is about the thickness of a sheet of paper, and you are even less. This must be a very small speaker.

    Question 2. Why do you assume that velocity is w·A and acceleration is a=w^2·A. Is your accelerometer measuring maximun velocity and acceleration.

    Question 3. where is the accelerometer palced on the speaker?

    Your speaker gives out a pure tone at 80Hz and the sound waves travel as a sine wave.
    The speaker diaphram displacement is aslo a sine wave . Likewise the velocity and acceleration.
  8. Jun 25, 2011 #7
    1 mm? You mean 0.1 mm?
  9. Jun 25, 2011 #8
    Thanks for ur reply 256bits. I'm measuring a low-frequency loudspeaker.

    0,034 mm is a rare value, i know.. and that occurs at 60 dB.. i think i'm doing something wrong when i measure with the accelerometer.

    i was a bit confused about maximum values.. i see is a sinusoidal wave. i should integrate the displacement to get the velocity and so on with acceleration.. is ok? well, really, with my accelerometer is possible to get the three parameters but, there is something wrong.. starting with a displacement of 0,034 mm..

    i place the accelerometer close the dust cap, in the cone..

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