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I How does sound intensity affect the current produced by a mic?

  1. Jun 7, 2018 #1

    Daniel2244

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    I measure sound (dB) from a speaker at 0cm and 100cm using a diaphragm microphone.
    0cm-100dB, 100cm-30dB
    diaphragm microphone cinsists of a diaphram, coil and a permanent magnet. the coil is attached to the diaphragm, when a sound wave hits the diaphragm it causes to move back and forth which also causes the coil to move back and forth. This coild moved along a pernmanent maget which produced a magnetic flied, as the coil cuts the magnetic field a current is induced which travels to an amplifier.

    So at 0cm the sound intensisty (100dB) is larger than the sound in tensity at 100cm (30dB) away from the speaker becuase the sound waves have less distance to travel therefore losing less energy? Additionally, becuase the sound is larger it causes the diaphragm to fluctuate faster, moving the coil through the magnetic field more which inducing a larger current than it would at 100cm away? (Not sure if my thinking is correct :/)
     
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  3. Jun 7, 2018 #2

    sophiecentaur

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    Air is not a particularly lossy medium to sound waves - as can be experienced when you hear someone whispering down a speaking tube in a Science Park. The sound energy is not absorbed significantly by the air. The main reason for the sound level to drop with distance is that the sound spreads out over an ever increasing area and, hence, the sound energy flux that is intercepted by a microphone gets smaller. Over large distances in open space (3D space) the sound energy flux follows the inverse square law in the same way that light, radio waves and gravity does. The ISL falls down when you get close to the source. A loudspeaker is not a point source and you need to be a few wavelengths away, at least, for the ISL to be followed closely. Your experiment is very 'near field' and you cannot predict easily the sound levels so close to the speaker.
    The output from the microphone is pretty well proportional to the level of the incident sound.
     
    Last edited: Jun 8, 2018
  4. Jun 7, 2018 #3

    Daniel2244

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    Thanks, now I understand why the sound intesisty dropped. However, do you know if a high sound intensisty increases the movement of the coil through the magnetic field producing a larger current?
     
  5. Jun 7, 2018 #4

    sophiecentaur

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    I already said that the output of the microphone is proportional to the incident level. The greater the amplitude of motion (and hence the average speed) of a coil through a magnetic field, the greater will be the induced emf, which will cause more current to flow through the amplifier input stage.
    There are many different designs of microphone but they all (if they are any good!) follow this rule. A half decent microphone needs to have a linear response and not limit its output at high sound levels.
     
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