Slow Sound

This is an educational project started while guiding our advanced undergaduate lab. The work is in preparation for submission to AJP. It is a common task to find resonances in an acoustic cavity. This system is analogous to atomic system and it is natural to look for phenomena that are well known for atomic systems. This is especially true given that it is often hard to demonstrate or have a hands-on experiment for students showing he behaviour of atomic systems. The analogies with sound can sometimes be a helpful replacement.

The analogy we are looking for here is based on the fact the close to a resonance the phase often rapidly changes. Such a phase shift has been used in atomic system interacting with light to dramatically slow pulses of light. Does a similar phenomenom occur for sound close to cavity resonances?

The answer is yes. The figure shows a measurement and calucation of the time evolution of a sound pulse. The delay of 12 ms for the used distance between the detector and speaker of 22 cm gives an effective "speed" of sound of about 20 m/s. Much less than the usual 340 m/s.

Theory graph vs experiment graph of slow sound