> The whistle you hear when tuning an old AM radio is also moiré beats caused by the overlap of the local oscillator frequency and the incoming carrier. Also w

> The whistle you hear when tuning an old AM radio is also moiré beats caused by the overlap of the local oscillator frequency and the incoming carrier. Also when you receive Morse code on a radio using a Beat Frequency Oscillator, which is tuned close to but a little off from the IF frequency. It's also how a Theremin works, using the capacitance between your hand and the pitch and volume antennas to pull the frequency of an oscillator. If you'd like to play with a Theremin I might be selling mine - I haven't touched it for months...