RT high tone carrier wave on UAL?

[fox niner]

During the last couple of years while flying, I noticed that all transmissions of United airlines have a distinct carrier wave. Some sort of high monotonous tone in the background. (About 150 hertz).
Why is that? And why only with that airline?
I must say that in the airspace that i work in, all ual flights are wide bodies. (Europe)
If someone knows, i would be delighted.

[applecrumble]

They use the hand mic above 10,000 feet. This is the background noise you are hearing.

[EcamSurprise]

I know what you mean but it isn't just UAL.

Always presumed it was a 757/767 thing.

[triploss]

FWIW same thing can be heard for announcements in the cabin.

[MurphyWasRight]

high monotonous tone in the background. (About 150 hertz)

150 Hz is actually fairly low, my (pure guess) is that it could be 400Hertz which would be equivalent to 50/60Hz hum in domestic power sound gear.

For reference middle C on piano is about 262 Hz.

Google 'middle c' and you will find a link to a site that will play it and other notes in case you don't have perfect pitch :)

And as to why UAL more than others, perhaps others don't use hand mike above 10,000 and it is more likely to pick up the power 'hum', again pure speculation.

[CaptainMongo]

SOP - boom mikes required below 18,000'. "Most" crews won't go on speaker/hand mike until cruise. Flying over the pond, virtually all crews will be on speaker/handmike.

[KingAir1978]

I thought it was the humming of the chemtrails machine.