Why the 400 hz whine on aircraft PA systems?

I understand that for reasons of efficiency/weight, commercial aircraft derive their electrical power from small alternators spinning at very high RPM. What I don’t understand is why these aircraft don’t do a better job of filtering that sound out of the PA system; the whine is sometimes nearly as loud as the crew member’s voice.

Can anyone 'splain? I don’t get 60 Hz coming through my home stereo, and my car stereo sounds clean despite the alternator spinning at a wide range of speeds. So why do aircraft manufacturers permit the 400-Hz sound to come through on the PA system of a $100M+ aircraft?

To be honest, I’ve wondered why the screens the inflight movies are shown on use analog video, which means there’s signal and quality loss between the video source and the seatback screen. They also use analog audio. Combined, it can make for a nearly unwatchable movie - the airplane is noisy enough as it is.

Again, same argument - more than $100 million per aircraft, and apparently wiring it for movies costs $3 million or so.

My best guess is very conservative engineering. Instead of adding all the extra filters to hide the whine they made the circuitry as simple and rugged as possible. Also, some of that noise might actually be noise from the engines picked up by the microphone the flight attendant is holding.

A “60hz hum” on your TV, also called a ground loop, is very common. I had it on my last system, and try as I might, I could never get rid of it. In my system it manifested as a set of rolling bars on the TV.

Also, your car has a filter (condenser) for getting rid of the noise. It’s a little capacitor mounted under the hood somewhere to suck up the electrical noise. At least it was on older cars, I don’t know what newer cars do.

I can’t say I’ve noticed this but It could be ambient cockpit noise. I’ll check the predominant frequency of the cockpit hum today and get back to you.

It might just be feedback.

Because it would cost money, and it doesn’t bother their customers enough for them to be willing to pay more for planes that have it, nor to pay the increased operational cost of such planes.

It could be a number of factors. lead-acid Car batteries filter out some of the noise. This becomes noticeable if the battery dies. Not sure how big a battery that has to be in relation to the generator but car batteries have to start large engines. Aircraft batteries have to start a smaller engine which in turn starts the larger engines.

Aircraft are very long tubes of metal which make rather large ground loops.

Wiring is usually done in central pathways without shielding.