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I have been doing an experiment regarding sound. I use an audio generator to produce a certain frequency of sound at a certain loudness. A microphone connected to my computer will record the sound and I find out the dBs of the sound using a computer software called Audacity.

The problem is, when I change the frequency of the sound without changing the volume (there are 2 knobs on the audio generator, one for frequency and one for volume), the volume of the sound (measured in dBs in Audacity) somehow changes.

For example, 100Hz gives me -30dB. But 300Hz gives -50.0dB. Can anyone explain this and possibly solve the problem for me? (many online sources and all that I have been learning states that frequency does not affect amplitude)

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  • $\begingroup$ Unless you performed your test in an anechoic chamber, then your room probably resonates at some frequencies and damps others. That can have a huge effect on the outcome of your experiment. $\endgroup$ – Solomon Slow Oct 25 '17 at 16:32
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Your microphone (probably) isn't equally sensitive to all frequencies.

I suspect that deliberately by design an audio microphone + amplifier is designed to amplify certain frequencies more than others to match the response of the human ear.

ps. as James comments: The speakers are probably a large source of differences.

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    $\begingroup$ The speaker's frequency response probably isn't flat either. And the room probably resonates at some frequencies and damps others. $\endgroup$ – Solomon Slow Oct 25 '17 at 15:59
  • $\begingroup$ @jameslarge, I was thinking that a frequency generator should be designed to produce a constant power - but of course speakers have huge THD $\endgroup$ – Martin Beckett Oct 25 '17 at 16:01
  • $\begingroup$ Apparently the speaker I use only has distortion of 2% at frequencies > 1kHz. So I'm thinking along the line of James's answer. $\endgroup$ – gerald ek Oct 28 '17 at 1:47

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