EASA / GCAA Part-66 · Module 17 · Propeller · 17.4
Objective: two propellers turning at almost the same speed produce a slow throbbing beat that passengers hear immediately. A synchroniser removes it by matching the RPM. A synchrophaser goes further and holds the blades at a chosen phase angle so the pressure pulses partly cancel. See and hear the difference.
On the numbers: the blade passage and beat frequencies are exact. The cabin noise figure is a two-source interference model, which is the right principle but a simplification: on a real aircraft the best phase angle depends on the fuselage, the engine positions and where the microphone is, so it is found by flight test and is rarely a neat 180 degrees.
The beat is the problem. Each propeller pushes a pressure pulse past the fuselage every time a blade goes by, at the blade passage frequency, N times the shaft speed. If the two engines are even a few RPM apart, the two pulse trains drift slowly in and out of step, and the sum swells and fades at the difference between them. That slow throb is the beat frequency, and because it is only a few hertz it is far more noticeable and more fatiguing to passengers than the much louder steady tone underneath it.
A synchroniser removes the beat by picking one engine as the master and trimming the other's governor until the two shaft speeds match exactly. Once the difference is zero, the beat frequency is zero and the throb disappears. Note what it has not done: both propellers are still making the same noise, and the steady tone is unchanged. It has only stopped that tone from swelling and fading.
A synchrophaser goes one step further. Having matched the speeds, it also holds a chosen angular offset between the two propellers, so that a blade on one side passes the fuselage at a chosen moment relative to a blade on the other. Choose that phase angle well and the two pressure pulses partly cancel where the passengers sit, which drops the cabin noise itself rather than just steadying it. It is a comfort system, not a safety system: it is normally inhibited for take-off and landing, and it must be switched off before shutting down or feathering an engine so the slave governor is not chasing a propeller that has stopped.