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Tone Heard In DC9/MD8n PA Announcements  
User currently offlineWesternDC6B From United States of America, joined Mar 2013, 129 posts, RR: 0
Posted (1 year 3 months 3 weeks 5 days 19 hours ago) and read 1932 times:
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When traveling on DC9s and, I think on MD8-whatevers, there is this quite loud tone, or howl heard during the announcements when on the ground. It sounds like "rrrr Ladies and gentlemen rrrr welcome aboard Delta Flight 89 rrrr" and so on. I am not able to discern the pitch, but it sounds higher than the 400cps* of the on-board AC power. First, what is it, and second, why has no one found a way to isolate this annoyance? In many cases it overpowers what the stew or the pilot is trying to say.

Any ideas?

* "cps", or, cycles per second, is Hertz if you are over 55 years of age.   


Be kind to animals - Take a grizzly to lunch today.
6 replies: All unread, jump to last
 
User currently offlineSkydrol From Canada, joined Oct 2003, 972 posts, RR: 10
Reply 1, posted (1 year 3 months 3 weeks 5 days 15 hours ago) and read 1874 times:

I would expect it is 3 Phase, 400 Hz. This does not sound like a 400 Hz tone, or 1,200 Hz; polyphase is different. If you have a tone generator program, and generate one tone at 400 Hz and two others with the phase shifted 120° and 240° from the first tone, you will get this unique sound which can be heard on commercial airplane audio systems with poor shielding or grounds.



LD4



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User currently offlineWesternDC6B From United States of America, joined Mar 2013, 129 posts, RR: 0
Reply 2, posted (1 year 3 months 3 weeks 5 days 5 hours ago) and read 1807 times:
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I more or less expected it was a shielding problem. But when it comes to engineering electronics, shielding is pretty much at "Electronics 101" level. I wonder why this obvious fault was left in place.


Be kind to animals - Take a grizzly to lunch today.
User currently offlineDashTrash From United States of America, joined Aug 2006, 1532 posts, RR: 2
Reply 3, posted (1 year 3 months 3 weeks 5 days 4 hours ago) and read 1788 times:

Rotary inverters (whatever a rotary inverter is vs a not-rotary inverter). I flew an old Sabreliner that made the same noise. It was constantly heard in the headsets as well, more pronounced when keying the mic. It can also be heard on the receiving end of radio transmissions.

User currently offlineFlyHossD From United States of America, joined Nov 2009, 895 posts, RR: 2
Reply 4, posted (1 year 3 months 3 weeks 5 days 4 hours ago) and read 1786 times:

Quoting DashTrash (Reply 3):
Rotary inverters (whatever a rotary inverter is vs a not-rotary inverter). I flew an old Sabreliner that made the same noise. It was constantly heard in the headsets as well, more pronounced when keying the mic. It can also be heard on the receiving end of radio transmissions.

At a former carrier, we had the same problem, the hum in the headsets was nearly intolerable and was often written up. After a couple of weeks of this, maintenance changed one of the inverters and the hum was gone. The change was significant and very welcome.

I've heard the noise the OP mentioned and I believe it's an inverter hum, too. Why not fix it? Well, has anyone written it up (made an entry into the logbook)? Second, is it broken? Or just annoying?



My statements do not represent my former employer or my current employer and are my opinions only.
User currently offlineWesternDC6B From United States of America, joined Mar 2013, 129 posts, RR: 0
Reply 5, posted (1 year 3 months 3 weeks 4 days 17 hours ago) and read 1695 times:
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Quoting FlyHossD (Reply 4):
Second, is it broken? Or just annoying?

Or, a third choice: reduction in safety. If a stew or pilot with a voice that is not "full" is making an announcement, this tone could drown out something critical in the case of, say, evacuating the cabin, or bracing for a bad landing or some such.



Be kind to animals - Take a grizzly to lunch today.
User currently offlineprebennorholm From Denmark, joined Mar 2000, 6461 posts, RR: 54
Reply 6, posted (1 year 3 months 3 weeks 3 days 16 hours ago) and read 1530 times:

This sound has puzzled me for forty years by now. On SK DC-9 and MD-80 planes.

I have seen people put their fingers into the ears during safety demonstration and other announcements in order to avoid this very annoying sound.

Add in language problems: SK announcements are in English, plus one Scandinavian language depending on the nationality of the cabin crew. I do understand Norwegian and Swedish just as well as most Englishmen will understand a Scotsman, but with that noise added in, and imperfect English, plus a foreign Scandinavian language, then I don't understand anything - even when it happens to me as a Dane on a domestic flight within Danish borders.

Well, it doesn't matter since I know every word they are telling me. Until the day when they want to tell me something important, which luckily never happened to me.

It was even worse back in the 70'es and 80'es when many FAs thought that they appeared more professionally when they talked like Chuck Yeager, the greatest hero of all aviators, whom nobody understood outside some remote valley in West Virginia.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
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