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.

Clipping Example in Spectrogram

Original Sine Sweep

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 Audio frequency sweep spectrogram artifacts... hints? but there does not appear to be an answer.

Raw Waveform:

enter image description here

  • $\begingroup$ Test the equipment: both generator and receiver, in an anechoic chamber to be sure it behaves as designed. Then you can evaluate the room. $\endgroup$
    – Solar Mike
    Commented Feb 27, 2023 at 6:24
  • $\begingroup$ @SolarMike The room is already as anechoic as we can get it (lots of acoustic treatment). $\endgroup$
    – Mason Wang
    Commented Feb 27, 2023 at 6:50
  • 1
    $\begingroup$ Are the ghosts, harmonics of the tone being played ? Can you share a plot of the waveform transmitted and received ? Perhaps from 6.0s to 6.1s ? Even better: share the digital data (say from 6.0s to 6.5s) ? Clipping is not the only non linear phenomena which can generate harmonics. $\endgroup$
    – AJN
    Commented Feb 27, 2023 at 12:11
  • $\begingroup$ drive.google.com/drive/folders/… $\endgroup$
    – Mason Wang
    Commented Feb 27, 2023 at 18:21
  • 1
    $\begingroup$ I shared a link above to the clipped wav file, and a .npy file of the audio data from 6.0 6.5 seconds. Thanks for your help, let me know if I can provide it in a different format. I added a picture of the raw waveform to the post. $\endgroup$
    – Mason Wang
    Commented Feb 27, 2023 at 18:22

1 Answer 1


The data provide by OP in the file testsweepsm573.wav is fed to GNU Octave. A slice of data taken at 6s is fed to the fft function. The result shows a pure sinusoid. The image is attached below.

signal and its spectrum

We can notice the second harmonic at 6000Hz when the spectrum is plotted in logscale (dB). Perhaps Audacity is using log scale to render the image.

So the ghost does exist, but appears to be of really small magnitude.

  • $\begingroup$ Audacity Settings, Spectrogram, Preferences. $\endgroup$
    – AJN
    Commented Feb 28, 2023 at 13:26
  • $\begingroup$ This is absolutely normal and corresponds to the first order harmonic distortion product. The appearance of such products is normal and a fortunate and welcomed "byproduct" of the method $\endgroup$
    – ZaellixA
    Commented Mar 2, 2023 at 16:39
  • $\begingroup$ -40 dB first harmonic, it'd be interestng to see wht the higher harmonics are at say around t=4. How many bits is the DA converter? $\endgroup$ Commented Mar 31, 2023 at 0:19

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