Hot chocolate effect (Frank Crawford 1982)

Click on youtube video: the Allassonic Effect:

• frequency of sound heard from tapping the bottom of the cup of hot cocoa is a function of
  1. speed of sound;
  2. wavelength.
• upon initial stirring of cocao, sound is transported via bubbles (gas) in the liquie, thus reducing the speed of sound and lowering the frequency;
• as bubbles clear, sound travels faster in the liquid and the frequency increases.