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.