My Coffee Sounds Funny - 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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