In all of these, bubbles crucially affect the performance of the system. As a bubble is formed or broken up, it suffers a severe distortion to its surface. As the surface recoils, the gas within the bubble gets compressed. Like any natural system with inertia and stiffness,the gas and surrounding liquid vibrate in response to this shock. Bubbles moving freely through liquid may also emit sound, probably as they are impacted by turbulent eddies.
Bubbles have a natural frequency that depends on their size.
This simple fact, first quantified in 1933, is the key to bubble acoustics. As the bubbles get larger, the pitch of the note gets lower. Place your mouse over each bubble to display its frequency. For any instrument system, a frequency is easier to measure than an amplitude - counting cycles is more reliable than reckoning size. This fundamental principle is behind the superiority of FM over AM radio and an enormous number of practical sensors.