When you pour an aerated drink into a glass or some soapy water into a bucket, you get foam. If you listen carefully, you will hear a characteristic hissing sound from the foam (See video).
I noticed that this sound rises in pitch as the foam fades away. This is a spectrogram of the hissing sound using Audacity. The brighter parts are louder, you have time on the horizontal axis and frequency on the vertical axis.
The bright part till the 11-second mark is the sound of water falling into the bucket. The dark part from 11 seconds to 17 seconds is the hissing sound from the foam. You can see that the lower frequencies mostly die out after the 13-second mark.
The experiment was repeated and the pattern remained the same. The low frequencies die out after about 2 seconds of hissing. Note that this is roughly the same time taken for the foam to disappear in the video.
Why does the pitch rise when the foam becomes thinner ? Does this have something to do with the size of the bubbles remaining in the foam? (larger bubbles pop faster, smaller bubbles last longer)