Don't ask me where, but I'm sure I remember seeing a Luftwaffe pilot interviewed and he said engines were deliberately desynchronised (on nearing the target?) in an effort to add a further psychological dimension to bombing raids - similar to the thinking behind the Stuka siren.
It certainly had an effect on my father who grew up a city which was a major target during the Blitz; "A horrible laughter" he once described it as. It made it very hard (to a teenager's ears) to tell how many aircraft there were, and until the sound had totally gone, whether there were more approaching.
Edited at add, there is no reason why you wouldn't get a so-called 'beat frequency' from two aircraft, each with their own (twin?) props perfectly synchronised, but with both aircraft at slightly different RPM, (the frequency of the 'beat' being exactly equal to the difference in RPM.)
Last edited by Smoketoomuch; 26th Jun 2004 at 21:44.