Received Sound Of Radial Aircraft Engines In Flight

[4legsgood]

When I was a boy, I often listened to the sound of 40+ German bombers with radial engines passing overhead. The received sound to the ear of all these engines gave a synchronized two tone high and low pitch in unison. As all these engines were not started up in unified synchronization how does this phenomenon occur? Why did I not hear a mixed and combined noise of all the sounds emitted? Is it something to do with air pressure waves and the amalgamation of all sounds into just two pitches? I did, in later life in the RAF hear several aircraft with radial engines warming up together and this two pitch tone was not present at ground level.

[heathfield]

The German bomber crews were ordered to adjust their engines so that they ran unsynchronised over their target.. The resulting sound was supposed to cause apprehension in the civil population. The bomber crews hated doing this - they said it was more annoying to the crew of the aircraft than to the people on the ground!

[Teddio]

Not all the German bomber aircraft had radial engines, for instance the Heinkel-111 had an inverted V-12 engine.
A two-engined piston aircraft will rarely have both engines running at the same frequency. If there is a difference in frequency then "beats" or throbbing sounds are heard. This is caused by the constructive and destructive interference of the sound waves entering your ear from both engines.
If one engine has a frequency of 47 Hz and the other engine has a frequency of 45 Hz then the "beat" frequency will be 47 - 45 = 2 Hz.
For pure sine waves the theory is simple but of course the sound from a multi-cylinder aero engine is more complex.

[Clanad]

Actually, the answer is much simpler...

The engines themselves are never in synchronization... meaning the revolutions of the crank shaft could never be in unison. However, the propellers (regardless of type of engine even turbine powered propeller driven ones) are easily controlled from the flight deck.

The pilot can remove the "beat" one sometimes hears manually by moving one of the prop control levesr very slightly forward or aft, which changes the "pitch" or angle of the propeller blades and, correspondingly changes how fast the blades are actually turning. He listens to the sound and "syncs up" the props until there's a smooth sound and no beat...

This can be done automatically if the aircraft is so equipped...

[4legsgood]

Thanks Teddio for a more scientific answer. This I can understand but how did 40+ aircraft (80+ engines) result in my hearing a syncronized 2 beat frequency from them all. Could all aircraft act in unison on given frequences to produce this phenomenon as Heathfield states or did I just hear the sound waves from one (the closest) aircraft.
We always knew which were the German raiders from this sound and as he points out, it really was demoralizing.

[jim360]

I'd probably need to hear a clip of what you are describing to have an idea of why it might be that way. Were there not two separate types of German bomber in most raids?

[Teddio]

4 legs: I can't explain why you would hear a "synchronized two beat frequency". Each bomber would likely emit its own set of beats with a slightly different frequency to the rest. Perhaps your hearing was selective and you picked out a steady rhythm from the cacophony?
To deliberately set the two engines out of synchronization would play havoc with the German crews and make for a most uncomfortable bombing run.

[ludwig]

// ..would play havoc with the German crews and make for a most uncomfortable bombing run.//

My great uncle Frank lived in Bethnal Green and he reckoned every German bombing run was uncomfortable.

[jomifl]

Ludwig, my family alsolived near there and were forced to move to another part of the country because they too found the bombing runs uncomfortable.That is why I'm a Lancastrian and not a Cockney.:-)

[4legsgood]

Many thanks to all for helping to enlighten me on this subject which I have pondered on since I was a young lad. I lived on the Surrey/Sussex border in open countryside and was never in any danger of being bombed - a couple of near misses from V1's and a crashed Dornier though.
We could always tell from the sound pattern of the raiders direction as to which part of the West Country/South Wales were "in for it tonight". Usually confirmed on the radio news the next day.