Originally Posted by
MadMojoMonkey
OK, now that's a really cool link, but it's not what I thought you were describing. I thought what you were saying is that if you stir the espresso, the tone drops when it is stirred, then rises as it stills. I thought you were saying this is wholly repeatable, in that you could then re-stir the same espresso and tap it again and hear the same phenomenon.
In your link, they're describing the change in the liquid's Bulk Modulus caused by nucleation. In other words... The liquid becomes less "springy" as the bubbles form. Then it returns to its normal "springiness" as the bubbles rise to the surface and pop. Since the liquid is carrying the vibrations, the springiness will affect how quickly they travel through the liquid, which in turn affects the frequency of the vibration.
However, this phenomenon would not be repeatable. Once the bubbles have worked their way out of the fluid, the fluid would need to be "recharged" with more trapped gasses to recreate the initial results.
If the notion of gasses trapped in a liquid seems odd, think of a fish tank with an air pump keeping a constant stream of bubbles in the tank. This is to re-supply the water with oxygen that is not trapped as molecular water (H2O) for the fish to "breath". Think about this while sipping a beer and notice the bubbles nucleate from the liquid as it trickles down your throat. ;)