Even a slight difference in RPM can create uncomfortable noise, oscillations and vibrations. The noise can be uncomfortable for the flight/cabin crew and the passengers. You can realize that engine RPMs are not synchronized during the flight if you hear a strange intermittent beat. Continuous exposure to vibrations may also induce early fatigue to the airframe.

The RPM indicator only allows the pilot to set the RPM roughly, but discrepancies in engines’ speed can be hard to adjust. To overcome this problem, some aircraft are fitted with an instrument called a synchroscope. The synchroscope is an instrument that indicates whether two or more rotating devices, such as engines, are synchronized and is, therefore, used to synchronize the speeds of engines/propellers to a RPM value by comparing the deduced frequency of alternators driven by engines/propellers. In a synchroscope, one engine is identified as the master, and the others are the slaves. The instrument indicates on a dial by means of a pointer, the variation in RPM relative to the master engine => the frequencies of the "slaves" are compared with the frequency of the "master" engine. These differences in frequency will create torque which, consequently, acts on the synchroscope indicator needle.