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I'm doing a work on the acoustic emission of a leak. I am using a 1.80 m cylindrical tube with water, and the leak is simulated by means of a tap that opens after I have applied negative pressure inside the tube.

I am taking this spectrogram as a test on a tube filled with water and simulating a leak. I notice that in the spectrogram there are some horizontal lines visible at some specific frequencies throughout the spectrogram, and I have no idea how to interpret those lines at some specific frequencies.

The sharp difference in the middle of the image is when the tap is turned on.

enter image description here

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    $\begingroup$ I'd recommend reading en.wikipedia.org/wiki/Spectrogram and then, if you still have questions, asking a more specific question. $\endgroup$ – MBaz Oct 6 at 14:48
  • $\begingroup$ Hello! Welcome here. As MBaz says, you really forgot to ask a specific question, and that makes it hard to impossible to really help you as much as we could. Could you really try and edit your question to contain a clear question, and a bit of specific background on the challenges you're facing answering it? Between us engineers: I think this is very interesting problem, but I don't know exactly what the kind of thing you want to learn from your recording, and you sadly don't mention that. $\endgroup$ – Marcus Müller Oct 6 at 15:17
  • $\begingroup$ I apologize for the confusion, as I am not fluent in English that may be why I was not able to explain very well. I am taking this spectrogram as a test on a tube filled with water and simulating a leak. I notice that in the spectrogram there are some horizontal lines visible at some specific frequencies throughout the spectrogram, and I have no idea how to interpret those lines at some specific frequencies. $\endgroup$ – m-ella Oct 6 at 22:40
  • $\begingroup$ My perception is that the question is more about the physics rather than the spectrogam (?). As you probably know already, the spectrogram shows the frequency content of a signal versus time. But, in the context of a leaking pipe these lines denote harmonics that the pipes allow to propagate. You can probably relate those harmonics back to the dimensions of the tube and/or the fluid. It sounds like what you are doing is a typical recording of the impulse response. Is it possible to provide some details in the question about what you are working with and what you are after? $\endgroup$ – A_A Oct 7 at 12:55
  • $\begingroup$ My study is about the acoustic detection of leaks in fuel tanks. My idea is then to record the audio of a leak and characterize the spectrogram obtained. For this I am varying the sizes of the holes and the amount of water inside the tube, observing what conclusions I can draw from the spectrogram obtained. The figure I sent is the spectrogram of the response to a leak of 1mm in diameter with the tube empty. $\endgroup$ – m-ella Oct 7 at 17:31
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I don't know, but my guess on this is: just like air flowing through a flute, water flowing through a tube causes vibrations, tones. The strong tone with several overtones is pretty consistent with this being similar to an air instrument.

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