Date: Thu, 7 Nov 2002 15:26:40 -0600

Author: Paul Nord

Subject: Re: Hot Chocolate Effect Question


I'm looking for the video because it had a nice explanation and a
graphic showing the changes. There's a good web reference:

Basically, tiny bubbles in the liquid dramatically change the "bulk
modulus" and therefore the speed of sound in the liquid. As the
bubbles drift upwards and finally pop (or dissolve) the speed of sound
in the column goes back up. Notice that you can get the pitch to go
back down with a little bit of stirring.


On Thursday, November 7, 2002, at 03:19 PM, Eric Ayars wrote:

> I don't know if this is the effect you mean, but I've always wondered
> why it is that the pitch changes when you tap the inside of a mug.
> Here's what I do:
> Fill the mug with hot chocolate (or coffee, or cider, or whatever. Hot
> water works.)
> Tap the inside of the mug, and listen to the pitch. Keep tapping, and
> the pitch gradually rises.
> Stir. Tap again, and the sound will be at the original pitch.
> Repeat until bored, cold, or both.
> I don't need the videoclip: I can do it myself any time. I just need
> to know WHY! I was thinking it was bubbles forming on the inside of
> the mug, but I did it with hot water and there were no bubbles > visible.
> Eric
> --
> --------------------------------------------------------------------
> Dr. Eric Ayars
> Assistant Professor of Physics
> Walla Walla College
> (509) 527-2476