Doppler Turntable

Doppler Shift - Interference - Beats

What it shows:
Doppler shifting of sound to higher frequencies occurs when a source is moving towards the observer, and shifted to lower frequencies when the source is moving away. Here two sources emitting the same frequency when stationary rotate on a turntable. With one source moving towards you and one away, the Doppler shifted waves interfere to create beats.

How it works:
Two 1.5W 8Ω speakers are mounted at opposite ends of a 1.5m long wooden beam (see figure 1). The signal generator, based on an Intersil 8038 waveform generator chip and an LM386 audio power amp (see figure 2), sits in the central control box. It has adjustable frequency and volume, but sends the same magnitude of each to both speakers. The beam is itself mounted on a turntable, and can be set to spin in a horizontal circle.

figure 1. Speaker arrangement (side view)

Setting it up:
Secure the turntable platform [p details of turntable construction somewhere else] with a C-clamp to a cart away from obstacles. Choose a volume setting in keeping with the hall size. A frequency of [p] Hz will give good beating at around [p] rpm, but you will need to play with the rotation speed for a given frequency or vice-versa to obtain good beats.

Comments:
As mentioned in Setting it up, you need the space to swing your speakers, so take into account which demos accompany this one onto the floor. Also beware of reflections off other nearby objects Rating **

figure 2. Circuit diagram for frequency generator.