Yesterday at work, I heard what sounded like a DC-3 flying overhead. Since I’d recently added some port forwarding of my ADS-B receiver, I was able to call it up on my phone. I tapped on the plane icon. There was no FA information, but there was a tail number N467DM, and from that I was able to determine the plane was a Cessna 421. What a nice-sounding plane.
I’m sure part of why it sounded like a DC-3 was simply the twin engines, but there was something more. It was really nice to hear.
The flat 6 Continental engine isn’t particularly nice sounding in my opinion. There are a few things that might account for the difference in sound from the 421 however - it is turbo charged, which will have the effect of muting and evening out the somewhat farty sound of the engine, but it also has reduction gearing to bring the propeller speed down. This will reduce the frequency of the noise from the propeller significantly. The 421 is also twin engined, and if the engines weren’t synchronised the sound from each will interfere with that from the other and produce additional low frequency sounds.
It’s probably a combination of all those factors, and doppler shift (assuming it was flying away from you) lowering the frequency further.
Thanks for that good information about the engines. The plane was at 5100 feet when it flew over about about 60-70 degrees up from the horizon. So I saw it both coming and going with associated doppler changes. None of the other planes flying overhead had the same sound, though, so doppler effect at best just modulated whatever was already there.