User avatar
WesternDC6B
Topic Author
Posts: 293
Joined: Thu Mar 14, 2013 3:05 pm

Tone Heard In DC9/MD8n PA Announcements

Thu Jun 06, 2013 11:14 pm

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.
 
Skydrol
Posts: 1053
Joined: Fri Oct 03, 2003 12:01 pm

RE: Tone Heard In DC9/MD8n PA Announcements

Fri Jun 07, 2013 3:32 am

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
∙ ---{--« ∙ ----{--« ∙ --{-« ∙ ---{--« ∙ --{--« ∙ --{-« ∙ ----{--« ∙
 
User avatar
WesternDC6B
Topic Author
Posts: 293
Joined: Thu Mar 14, 2013 3:05 pm

RE: Tone Heard In DC9/MD8n PA Announcements

Fri Jun 07, 2013 1:09 pm

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.
 
DashTrash
Posts: 1266
Joined: Mon Aug 21, 2006 8:44 am

RE: Tone Heard In DC9/MD8n PA Announcements

Fri Jun 07, 2013 2:21 pm

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.
 
FlyHossD
Posts: 1274
Joined: Mon Nov 02, 2009 3:45 pm

RE: Tone Heard In DC9/MD8n PA Announcements

Fri Jun 07, 2013 2:35 pm

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 avatar
WesternDC6B
Topic Author
Posts: 293
Joined: Thu Mar 14, 2013 3:05 pm

RE: Tone Heard In DC9/MD8n PA Announcements

Sat Jun 08, 2013 12:51 am

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.
 
prebennorholm
Posts: 6447
Joined: Tue Mar 21, 2000 6:25 am

RE: Tone Heard In DC9/MD8n PA Announcements

Sun Jun 09, 2013 2:23 am

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

Who is online

Users browsing this forum: Baidu [Spider], flight152, zaphod42 and 15 guests

Popular Searches On Airliners.net

Top Photos of Last:   24 Hours  •  48 Hours  •  7 Days  •  30 Days  •  180 Days  •  365 Days  •  All Time

Military Aircraft Every type from fighters to helicopters from air forces around the globe

Classic Airliners Props and jets from the good old days

Flight Decks Views from inside the cockpit

Aircraft Cabins Passenger cabin shots showing seat arrangements as well as cargo aircraft interior

Cargo Aircraft Pictures of great freighter aircraft

Government Aircraft Aircraft flying government officials

Helicopters Our large helicopter section. Both military and civil versions

Blimps / Airships Everything from the Goodyear blimp to the Zeppelin

Night Photos Beautiful shots taken while the sun is below the horizon

Accidents Accident, incident and crash related photos

Air to Air Photos taken by airborne photographers of airborne aircraft

Special Paint Schemes Aircraft painted in beautiful and original liveries

Airport Overviews Airport overviews from the air or ground

Tails and Winglets Tail and Winglet closeups with beautiful airline logos