-This thought experiment proposes an acoustic definition of absolute motion and simultaneity utilizing the Michelson-Morley interferometer formula and the violation of Galilean invariance by any type of waves:

T = [L /(c-v)] + [L /(c+v)] = [2Lc] / (c²-v²)

-On a windless day an archetypical flatbed train car (no walls, fully open to the air / medium) of length, L, is in motion with the velocity, v. This velocity, v, can be solved for by rearranging the above formula and adding the total time, T, from two synchronized clocks. Also by a comparison of the times on the two clocks, simultaneity can be determined.

-Since the poles are at rest relative to each other (if not, that would be a bit of a train wreck) but each is in motion (in tandem, at the same velocity) relative to the air, this dynamic of the experiment can lead to a means of determining the motion of the train car relative to the air.

-At either end of the train there are vertical poles (approx. 1m tall). Also at the midpoint of the train car is another pole of the same height. Mounted at top of the end poles is a light activated sensor electrically connected to a sound emitter directed back towards the central location. At the central location (on a dais perhaps) are a light emitter, two sound receivers (microphones), two synchronized clocks, and a single observer (seated).

-Now at the speed of a typical train car (v << c) there should be no measurable or observable Relativistic effects; but the speed of sound waves could be easily measured by typical clocks. Thus, the time of travel for the returning sound signals are the only times that can be measured by the clocks. The experiment then begins.

-From the central location a split beam light emitter sends a light pulse to the fore and aft pole sensors. This light signal is effectively instantaneous in terms of travel time through the air, and by logical extension, each light signal arrives at either end pole simultaneously. Thus the fore and aft sound emitters are activated, each sending a sound pulse back towards the central location.

-If the time for each returning sound signal is measured on each clock, then the times can be added to get a total time, T. Then the velocity of the train car relative to the still air can be solved for. If the times are the same then the signals will arrive at the central location simultaneously, and the train car is at rest relative to the air. If the times are different then the train car is in motion in the direction of the lesser time. Thus this experiment finds the magnitude and direction of the train car velocity or absolute motion relative to the air (medium).

T = [.5L] / (c-v) + [.5L] / (c+v) = 2[.5L]c / (c² - v²)

- (Times and distances to and from the central location).

Source: Michelson-Morley experiment, speed of speed, c