Removing noise from wav file in matlab

Question

Very similar, practically the same question was asked here: Remove noise from wav file, MATLAB

however I still don't know what to do. Here is the sound file that I am dealing with: https://uploadfiles.io/pstrt

i need to remove the noise from it. I know how to apply a variety of filters, the only thing I don't know is where does the noise actually occur. Following the steps from the question mentioned above I have the following spectrum:

The person that answered the mentioned question was able to deduce where is the noise from the spectrum. I have no idea what gives it away. Could someone explain it to me? Thank you.

Answer 1

There is no foolproof way how to distinguish signal from noise just by looking at the spectrum unless you have some knowledge of the signal a-priori. The person that answered to the mentioned post listened to the sound, concluded that the speech had a higher pitch than the noise, calulated the FFT and started to tinker around with what he guessed was noise and listed to the modified result. An iterative process if you like. Often the stronger tones in your spectrum are your signal. In your application this would be around 900 Hz. Also, tones that are at multiples of the frequency you believe is your main signal component are often distortion. Distortion means that you have some nonlinearity in your device and that creates tones at muliples of your signal (in your case maybe 1800 Hz, 2700 Hz ...). Hope this helps