Sound Clock.

A sound clock could measure time with respect to observers at rest in inertial reference frames S′ and S. A straight rigid rod of fixed length D could be oriented lengthwise, parallel to the x′-axis of S′ and the x-axis of S. The rod would have a sound emitter at one end and a sound receiver at its other end. This apparatus could emit a sound pulse which would propagate at the constant velocity c through some medium, from the emitter to the receiver. Each emission and reception event would represent a tick of the clock. The times between ticks would be ∆t′ in S′ and ∆t in S, as measured by a typical clock. If S′ and the rod were at rest relative to S and the medium, then: ∆t′ = ∆t = D /c. If S′ and the rod were in motion at the constant velocity v (v < c) relative to S and the medium, parallel to the rod's length, then: ∆t′ = ∆t = D /(c ± v).