Soxfan
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What Makes The "Echoey" Humming Sound?

Thu Oct 21, 2010 1:59 am

Sometimes when listening to ATC, I hear a high-pitched, echoey "humming" sound when a pilot speaks to the tower. I've heard this in the passenger cabin from time-to-time as well. It's like a middle "G" on the piano scale. Does anyone know what I'm talking about, and what causes that sound?
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flybaurlax
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RE: What Makes The "Echoey" Humming Sound?

Thu Oct 21, 2010 2:56 am

Yeah I know what you're talking about. I noticed it's always with UA transmissions, and not always with others. I can even hear it while listening to channel 9, when the crew of the plane I'm on transmits. I, however, have no idea what causes it. I've always wondered, too. Thanks for asking. I'm sure we'll get some cool responses.
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tdscanuck
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RE: What Makes The "Echoey" Humming Sound?

Thu Oct 21, 2010 4:45 am

Quoting Soxfan (Thread starter):
It's like a middle "G" on the piano scale. Does anyone know what I'm talking about, and what causes that sound?

A on most pianos is 440 Hz. Aircraft AC power is 400 Hz. What you're most likely hearing is electro-magnetic interference from the aircraft power system being picked up somewhere in the VHF system.

Tom.
 
HAL
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RE: What Makes The "Echoey" Humming Sound?

Thu Oct 21, 2010 5:25 am

Quoting tdscanuck (Reply 2):
A on most pianos is 440 Hz. Aircraft AC power is 400 Hz. What you're most likely hearing is electro-magnetic interference from the aircraft power system being picked up somewhere in the VHF system.

Yes, that's it. Almost all current aircraft use both AC and DC for various purposes onboard. The AC is normally set at 400 Hertz, vs. the much slower 60 Hertz current we get in our homes. If there's poor insulation in the radios (or some of the wiring) you'll hear that 400 cycle hum in the radio transmissions.

HAL
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DocLightning
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RE: What Makes The "Echoey" Humming Sound?

Thu Oct 21, 2010 5:34 am

Not to digress too much OT, but why do AC use such a high oscillation in their electrical systems?
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Go3Team
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RE: What Makes The "Echoey" Humming Sound?

Thu Oct 21, 2010 5:45 am

Quoting DocLightning (Reply 4):
Not to digress too much OT, but why do AC use such a high oscillation in their electrical systems?

If I remember right (from what I read here) it has to do with weight. 400 Hz Generators, Transformers weigh less.
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KELPkid
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RE: What Makes The "Echoey" Humming Sound?

Thu Oct 21, 2010 6:22 am

Used to be a familiar sound in aviation, but you don't hear the 400 Hz hum much anymore (better avionics, noise filtration, etc.).

Quoting Go3Team (Reply 5):
If I remember right (from what I read here) it has to do with weight. 400 Hz Generators, Transformers weigh less.

  
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flybaurlax
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RE: What Makes The "Echoey" Humming Sound?

Fri Oct 22, 2010 5:18 am

Cool thanks for answering it. It definitely makes sense. Would the fact that I always hear it on UA equipment is because they're older? Although this wouldn't necessarily make sense for the Airbus they fly. I could be wrong though.
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Mir
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RE: What Makes The "Echoey" Humming Sound?

Fri Oct 22, 2010 5:30 am

Quoting flybaurlax (Reply 7):
Would the fact that I always hear it on UA equipment is because they're older? Although this wouldn't necessarily make sense for the Airbus they fly.

FWIW, I notice it far more on 757/767 equipment than I do on Airbus equipment.

I've heard older Learjets that sound like they've got sirens going off in the background - that's kind of freaky at first.

-Mir
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Soxfan
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RE: What Makes The "Echoey" Humming Sound?

Fri Oct 22, 2010 5:03 pm

Thanks for the responses! It's not necessarily an annoying sound, in a sense almost pleasant unless it's loud or blocks out a voice/announcement.

Quoting KELPkid (Reply 6):
Used to be a familiar sound in aviation, but you don't hear the 400 Hz hum much anymore (better avionics, noise filtration, etc.).

I'm assuming, though, that it's still present on newer aircraft because of the power systems, but just not as noticeable? I feel like I've heard it sometimes when the FA or pilot makes an in-cabin announcement, but I could be wrong...
Pilot: "Request push, which way should we face?" JFK Ground: "You better face the front, sir, or you'll scare the pax!"
 
shamrock137
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RE: What Makes The "Echoey" Humming Sound?

Sat Oct 23, 2010 12:41 am

Quoting Mir (Reply 8):
I've heard older Learjets that sound like they've got sirens going off in the background

Sometimes this can be caused by improper grounding of the strobe lights. As the capacitors charge, it can become audible in the comm system. DC-9's are also famous for having this type of sound at start up as the engines ignite.
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tdscanuck
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RE: What Makes The "Echoey" Humming Sound?

Sat Oct 23, 2010 1:11 am

Quoting Soxfan (Reply 9):
I'm assuming, though, that it's still present on newer aircraft because of the power systems, but just not as noticeable?

EMI shielding requirements have gone up a lot since the early days. Digital systems are also much less succeptable to this type of interference so there's a lot less systems capable of picking up the hum in an audible way in the first place.

Tom.

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