Above is a picture of Stephanie measuring the speed of sound, and, more importantly, moving along a new frame of reference (at the speed of light or sound), to shear the spacetime continuum so radio waves or sound waves sit still for direct observation by the naked eye.
In the picture, the S.W.I.M. moves back and forth over a 1.5 metre distance while emitting a 5000 cycles/per second tone, so we're seeing a 1.5 metre-long window into the sound wave that the microphone Stephanie is holding is hearing.
Over that 1.5 metre distance there are 21 "wiggles" (cycles) of that 5000 cycles per second sound wave.
Therefore each of the "wiggles" (cycles) of the sound wave is 1.5 metres / 21 cycles = .07 metres per cycle (i.e. 70cm/cycle).
Therefore Stephanie measures the speed of sound to be: 5000 cycles/second * .07 metres/cycle = 350 metres per second.
The theoretical speed of sound is calculated by the room temperature using the well-known formula: (331 + .6 * temperature/deg.C ) metres/second = 347 metres/second.
Therefore her experimental error is (350-347)/347 = 1%.
So she's measured the speed of sound to within 1% accuracy.
And, more importantly, she's made sound waves sit still by looking at them in a coordinate frame of reference that makes them comprehensible.
Link to images of Stephanine measuring the speed-of-light: