High pitched whine

[NineEighteen]

Hello,

I’m a humble PPL(A) and I like to listen to ATC sometimes. I find it fascinating and strangely calming.

Anyway, I noticed that some of the American airlines, United especially, seem to have a high pitched whine accompany the voice via RT. I wonder if there is a technical reason for this?

[aloominumtoob]

Crew discussing Ts & Cs?

[ZOOKER]

It could be just the particular type of radio kit used by he airline. Historically, many a/c types had distinctive 'transmission sounds'. The VC10s were easy to spot, so were Vanguards. The BAC1-11 500 series had a distinctive 'squeak' at the end of each Tx.

Pan Am had a distinctive sound too, irrespective of a/c type.

I wonder if the crews are using microphones/speakers instead of headsets, and it's ambient noise?

[Hotel Tango]

I wonder if the crews are using microphones/speakers instead of headsets, and it's ambient noise?

Good point. During my many rides on the FD of different American airlines (before 911) I can't remember ever seeing them use headsets.

[chevvron]

Back in the '70s, Aeroflot RTF had strange background noises, usually the interpreter shouting at the pilot telling him what he just said was wrong.

[eckhard]

Could be a 400Hz background noise from the AC system?

[Pugilistic Animus]

maybe turbomachinery or if not that then it's usually the pilots

[Mooncrest]

Britannia Airways 737-200s and BA/BAF Viscount 800s each had their own distinctive background TX tone. It may have been something to do with APU or GPU use during a short turn round. Probably ! Back in the day, listening in on a FM rather than AM radio might have emphasised the sound.

[Duchess_Driver]

I can always spot a Bell 206 Jet Ranger / 206L Long Ranger by the sound of the RT transmission.

[HEATHROW DIRECTOR]

I think eckhard is on the ball...

[EEngr]

Originally Posted by eckhard

Could be a 400Hz background noise from the AC system?

Quite possibly. Often, power supplies and other equipment produce odd numbered harmonics of the power frequency. 1200 Hz, 2000 Hz, etc. And the audio equipment doesn't filter them perfectly.

[Uplinker]

I’ve noticed a tone consisting of harmonics of the two 400Hz unlocked engine generators on the intercom and RT transmissions of the 737’s I now fly, which the Airbus did not have.

I should imagine that the more modern audio circuits on the Airbus have better design, filtering, de-coupling, earthing and much higher common mode rejection ratio than the aged Boeings. The latter is the ability of an audio amplifier to reject unwanted noise and interference on it’s input. De-coupling, filtering and better earthing reduce the effects of noise, interference and ripple present on the power supply lines to the amplifier.

Also, lots of the Airbus wiring loom is contained in screened cables which prevents interference between parts of the electrical system - I don’t know if Boeing’s wiring is.