During rotation, the two series of holes match the same number of times as there are holes in every revolution; each time this happens there is at the same time the emission of a jet of air from all the holes in the rotating disk and a subsequent temporary interruption. These periodic emissions and interruptions of the flow of air through the holes in the rotating disk become more and more frequent and, after a short time, by effect of the succession of compressions, they produce in the surrounding air, each followed by a weak rarefaction and an interval of silence, the siren emits a sound that becomes more and more acute.
The frequency of the sound emitted is given by the number of compressions per second and thus by the product of the number of holes in the rotating disk times the number of revolutions it makes in one second. A revolution counter, directly connected to the shaft of the disk, with the addition of a stopwatch, allows determination of the value.
The siren also makes it possible to verify the fact that for all sounds that a practiced ear judges as being of the same pitch (sounds in unison) the number of vibrations per second is the same, no matter what their origin may be.