My Coffee Sounds Funny

A number of people have noticed that if you stir instant coffee into a cup of hot water and tap the side of the cup with a spoon, you will notice that the pitch (frequency) of the sound produced by the tapping changes greatly as the coffee is added and as you stir. Why does this happen?

• A. Air trapped in the instant coffee power is released as you stir it. The speed of sound is lower in an air-water mixture than in pure water, resulting in a changing sound frequency as the air escapes from the water during stirring.
• B. The instant coffee lowers the temperature of the hot water, lowering the speed of sound. Additional stirring also lowers the temperature resulting in changing sound frequencies produced by the tapping.
• C. The instant coffee forms a temporary thin film on the top of the water which reflects some of the sound and changes its frequency by superposition effects. As this film layer dissolves with additional stirring, the pitch returns to normal.
• D. This is one of the dumbest questions yet. Like, who cares?

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Answer

My Coffee Sounds Funny - Answer = A

The air trapped in the instant coffee is initially mixed with the water forming an air-water mixture. Since the speed of sound is lower in this mixture than in pure water (or coffee), the resonance frequency in the coffee cup produced by the tapping is lowered. As the air bubbles escape with additional stirring the resonance frequency rises (since the velocity of the sound increases). (Some information from The Flying Circus of Physics by Jearl Walker)

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