The hot chocolate effect, also known as the allassonic effect, is a phenomenon of wave mechanics first documented in 1982 by Frank Crawford, where the pitch heard from tapping a cup of hot liquid rises after the addition of a soluble powder. It was first observed in the making of hot chocolate or instant coffee, but also occurs in other situations such as adding salt to supersaturatedhot water or cold beer. Recent research has found many more substances which create the effect, even in initially non-supersaturated liquids. It can be observed by pouring hot milk into a mug, stirring in chocolate powder, and tapping the bottom of the mug with a spoon while the milk is still in motion. The pitch of the taps will increase progressively with no relation to the speed or force of tapping. Subsequent stirring of the same solution will gradually decrease the pitch again, followed by another increase. This process can be repeated a number of times, until equilibrium has been reached. Upon initial stirring, entrained gas bubbles reduce the speed of sound in the liquid, lowering the frequency. As the bubbles clear, sound travels faster in the liquid and the frequency increases.
The phenomenon is explained by the effect of bubble density on the speed of sound in the liquid. The note heard is the frequency of a standing wave where a quarter wavelength is the distance between the base of the mug and the liquid surface. This frequency f is equal to the speed v of the wavedivided by four times the height of the water column h: The speed of sound v in a homogeneous liquid or gas is dependent on the fluid's mass density and Adiabatic Bulk modulus, according to the Newton-Laplace formula: Water is approximately 800 times denser than air, and air is approximately 15,000 times more compressible than water. When water is filled with air bubbles, however, the fluid's density is very close to the density of water, but the compressibility will be the compressibility of air. This greatly reduces the speed of sound in the liquid. Wavelength is constant for a given volume of fluid, therefore the frequency of the sound will decrease as long as gas bubbles are present. Different rates of bubble formation will generate different acoustic profiles, allowing differentiation of the added solutes.
