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I am collecting data for a project, where I am measuring the room impulse response by playing a sine sweep from a speaker (QSC K8 1KW Active Loudspeaker) and measuring it from the other side of the room using a microphone array. Even though the sine sweep is very clean (bottom image), there appear to be these "ghost sweeps" in the recordings from the microphone.

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If I look at the recording in Audacity it is not clipping. I tried using other microphone systems and have concluded that the clipping must be occurring while the sound is playing, or before that in the signal chain. I have tried playing the sound in different ways (from Audacity, from Python) and the clipping is still occurring. So, I have concluded that the clipping must be occurring somewhere in one of these steps:

  1. When the signal is sent to the audio interface (MOTU 8M)
  2. When the audio interface is sending the signal to the speaker
  3. When the speaker is playing the audio.
  4. Power source to the speaker?

I have tried using a different speaker (an ADAM studio monitor) still with clipping occurring. The signal is being sent to the speaker via

  1. A TRS to XLR cable
  2. and XLR to XLR cable. and I have tried replacing cable 2 (I do not have a replacement for cable 1).

I have also tried lowering the volume of the signal sent to the speaker. This alleviates the artifacts, but even at very low volumes there are still some ghost sweeps! I get a bad signal-to-noise at such volumes.

I have read something about "impedance matching" but I don't really understand. I have seen https://engineering.stackexchange.com/questions/16101/audio-frequency-sweep-spectrogram-artifacts-hints but there does not appear to be an answer.

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    I don't like OP's assumption of clipping. If they were seeing real clipping, those harmonic bands would be much brighter. This just looks like garden variety harmonic distortion that can be introduced almost anywhere in the signal path. It might even be in the source sound.
    – Data Processing
    Feb 27 at 17:41
  • I see. I tried routing the output directly into the input and got some distortion, but it was minor compared to what happens if I record the speaker.
    – Mason Wang
    Feb 27 at 18:58

1 Answer 1

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Those are not extra sweeps or clipping.

There is really no way to produce a pure sine wave out of a speaker so those extra lines are harmonics being produced by the speaker. Check out this PDF pg.8,12 from Stanford’s Computer Research in Music and Acoustics to see they have the same lines and point out that the speaker generates the harmonic series

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