"Mystery Tone" often heard over the air communications?

[Squawk 7700]

Hi All,

Attached is a composite of 4 short clips of both VHF and HF comms with the "Mystery Tone" in question.

The clip starts off with United 36 followed by United 41, then Dynasty 3. I selected these clips since you can clearly hear the "tone".

For the last track, I isolated the "tone" and looped it for 5 seconds.

There is 3 seconds of silence between the tracks.

I've heard this for years. Is there a technical name for it? What creates it?

Thanks!

Curious Ken  

[delta092b]

I was trying to hear the "tone" but then realised that you are likely referred to the noise heard when the pilots speak.

Not sure exactly but it's likely just the type of radio, combined with background noise. Whenever I hear that that majority of the time it's a 767/757

747's have a slightly different background noise. 777's hardly have any.

I think it's just the background cabin noise/radio rather than a tone but I stand to be corrected

[AviatorJud]

That is a 400Hz tone which is caused by the aircraft electrical system.  All modern airliners use 400Hz AC power and the tone comes out in the comm systems just like you hear 60hz tone in your household lights, speakers and tv.(in the US)  How loud the sound is depends on the aircraft and its systems, but it's always there.  If you think about every time you've been a passenger, you hear the 400Hz hum inside the cabin as well.

[sykocus]

Wind noise. The pilots have the window cracked to get some ventilation in the cockpit.

[JesseFSD]

That is a 400Hz tone which is caused by the aircraft electrical system.  All modern airliners use 400Hz AC power and the tone comes out in the comm systems just like you hear 60hz tone in your household lights, speakers and tv.(in the US)  How loud the sound is depends on the aircraft and its systems, but it's always there.  If you think about every time you've been a passenger, you hear the 400Hz hum inside the cabin as well.

Ahh, all these years I have been a passenger I have always wondered what that was when you board the plane. Now i know. interesting. Thanks!!

[Lost around KLAW]

If you think about every time you've been a passenger, you hear the 400Hz hum inside the cabin as well.

Oh yea... I always thought it was noise from the ramp equipment.  I guess I tone it out shortly after boarding because I never notice it once we get moving.

[Fryy]

Wind noise. The pilots have the window cracked to get some ventilation in the cockpit.

im pretty sure if the window(s) were cracked at "flight level" altitudes, their might be some pressurization issues..

[snipper_cr]

I trained for my instrument up in the Chicago area so I spent a lot of time on Chicago Approach. Every time I heard that in my little archer, I would think "I cant wait for the day when I speak with that 'hum' in the background" because I always heard it on big airliners.

That day still hasn't come and doesn't seem to be coming any closer...

[Squawk 7700]

That is a 400Hz tone which is caused by the aircraft electrical system.  All modern airliners use 400Hz AC power and the tone comes out in the comm systems just like you hear 60hz tone in your household lights, speakers and tv.(in the US)  How loud the sound is depends on the aircraft and its systems, but it's always there.  If you think about every time you've been a passenger, you hear the 400Hz hum inside the cabin as well.

Thanks for the 400Hz insight to the sound. Mystery solved. I'm doing a little sound design for an audio project and this "tone" puzzled me for years. I too heard this while parked at the gate as a passenger.

The closest I got by ear was playing the G note on a keyboard synthesizer at 392Hz.
 
Here is a link to 400Hz electrical systems: http://www.aerospaceweb.org/question/electronics/q0219.shtml
and attached is a short 10 second clip of a 400Hz square wave tone I created in Audacity.

From aviation to musical scales... LiveATC rocks!!! Thanks guys!  
 

[AviatorJud]

That is a 400Hz tone which is caused by the aircraft electrical system.  All modern airliners use 400Hz AC power and the tone comes out in the comm systems just like you hear 60hz tone in your household lights, speakers and tv.(in the US)  How loud the sound is depends on the aircraft and its systems, but it's always there.  If you think about every time you've been a passenger, you hear the 400Hz hum inside the cabin as well.

Thanks for the 400Hz insight to the sound. Mystery solved. I'm doing a little sound design for an audio project and this "tone" puzzled me for years. I too heard this while parked at the gate as a passenger.

The closest I got by ear was playing the G note on a keyboard synthesizer at 392Hz.
 
Here is a link to 400Hz electrical systems: http://www.aerospaceweb.org/question/electronics/q0219.shtml
and attached is a short 10 second clip of a 400Hz square wave tone I created in Audacity.

From aviation to musical scales... LiveATC rocks!!! Thanks guys!  
 

Nice work! 

[laylow]

It seems that MD-80s are loudest when it comes to this tone on the radio.  At least that's what I've observed over hours of listening at KSAN.

[AviatorJud]

It seems that MD-80s are loudest when it comes to this tone on the radio.  At least that's what I've observed over hours of listening at KSAN.

Do you work at KSAN?

[SkyWestRJPilot]

Well, I mostly hear it on MD-80 Series aircraft, (Allegiant Air)
As well as over the PA on the 80

Just aging aircraft radios cause it no big deal 

[laylow]

It seems that MD-80s are loudest when it comes to this tone on the radio.  At least that's what I've observed over hours of listening at KSAN.

Do you work at KSAN?

No.  I work nearby, and often listen in my office, or on my lunch.

[AviatorJud]

It seems that MD-80s are loudest when it comes to this tone on the radio.  At least that's what I've observed over hours of listening at KSAN.

Do you work at KSAN?

No.  I work nearby, and often listen in my office, or on my lunch.

Cool, just curious.  I used to work there as a mech for AWA/USAir.  Pretty sweet airport.