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What Makes The "Echoey" Humming Sound?  
User currently offlineSoxfan From United States of America, joined Mar 2008, 863 posts, RR: 0
Posted (3 years 9 months 5 days 8 hours ago) and read 2974 times:

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?


Pilot: "Request push, which way should we face?" JFK Ground: "You better face the front, sir, or you'll scare the pax!"
11 replies: All unread, jump to last
 
User currently offlineflybaurlax From United States of America, joined Oct 2008, 638 posts, RR: 0
Reply 1, posted (3 years 9 months 5 days 7 hours ago) and read 2945 times:

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.


Boilerup! Go Purdue!
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 2, posted (3 years 9 months 5 days 5 hours ago) and read 2899 times:

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.


User currently offlineHAL From United States of America, joined Jan 2002, 2555 posts, RR: 53
Reply 3, posted (3 years 9 months 5 days 5 hours ago) and read 2880 times:

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



One smooth landing is skill. Two in a row is luck. Three in a row and someone is lying.
User currently offlineDocLightning From United States of America, joined Nov 2005, 19365 posts, RR: 58
Reply 4, posted (3 years 9 months 5 days 5 hours ago) and read 2878 times:

Not to digress too much OT, but why do AC use such a high oscillation in their electrical systems?

User currently offlineGo3Team From United States of America, joined Mar 2004, 3267 posts, RR: 16
Reply 5, posted (3 years 9 months 5 days 4 hours ago) and read 2875 times:

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.



Yay Pudding!
User currently offlineKELPkid From United States of America, joined Nov 2005, 6343 posts, RR: 3
Reply 6, posted (3 years 9 months 5 days 4 hours ago) and read 2864 times:

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.

  



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineflybaurlax From United States of America, joined Oct 2008, 638 posts, RR: 0
Reply 7, posted (3 years 9 months 4 days 5 hours ago) and read 2616 times:

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.


Boilerup! Go Purdue!
User currently offlineMir From United States of America, joined Jan 2004, 21488 posts, RR: 56
Reply 8, posted (3 years 9 months 4 days 5 hours ago) and read 2612 times:

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



7 billion, one nation, imagination...it's a beautiful day
User currently offlineSoxfan From United States of America, joined Mar 2008, 863 posts, RR: 0
Reply 9, posted (3 years 9 months 3 days 17 hours ago) and read 2486 times:

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!"
User currently onlineShamrock137 From United States of America, joined Jul 2005, 137 posts, RR: 0
Reply 10, posted (3 years 9 months 3 days 9 hours ago) and read 2389 times:

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.



Time to spare? Go by air!
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 11, posted (3 years 9 months 3 days 9 hours ago) and read 2378 times:

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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