Bizarre Intercom Problem

[Nick]

WOW! I have the exact same problem on my 77 M5-235C, I like the others assumed it was the mike picking up the pressure difference around the leaky doors,

[Andy Young]

Ran into this once in a C182. It seemed that the air flow from the vent would break squelch and make noise when the boom mike was put in the stream of air flowing from the vent. It was the airflow which did it, rather than a person audible sound from the airflow. So, we figured that there was some sort of whistle being created at the boom mike as the air flowed past the boom mike.

Perhaps the ANR on your headset is particularly good.

Thanks. It happens with both ANR and non-ANR headsets. I've considered the whistle theory, and agree that it still might be that. Just have to figure out what it is that's whistling....

[RobHill]

Ran into this once in a C182. It seemed that the air flow from the vent would break squelch and make noise when the boom mike was put in the stream of air flowing from the vent. It was the airflow which did it, rather than a person audible sound from the airflow. So, we figured that there was some sort of whistle being created at the boom mike as the air flowed past the boom mike.

Perhaps the ANR on your headset is particularly good.

Thanks. It happens with both ANR and non-ANR headsets. I've considered the whistle theory, and agree that it still might be that. Just have to figure out what it is that's whistling....

In the 182, the whistle was at the boom mike itself. Think blowing over the top of a coke bottle. Can the headsets be configured so that the boom mike for the passenger is on the left side of their face?

[Andy Young]

Ran into this once in a C182. It seemed that the air flow from the vent would break squelch and make noise when the boom mike was put in the stream of air flowing from the vent. It was the airflow which did it, rather than a person audible sound from the airflow. So, we figured that there was some sort of whistle being created at the boom mike as the air flowed past the boom mike.

Perhaps the ANR on your headset is particularly good.

Thanks. It happens with both ANR and non-ANR headsets. I've considered the whistle theory, and agree that it still might be that. Just have to figure out what it is that's whistling....

In the 182, the whistle was at the boom mike itself. Think blowing over the top of a coke bottle. Can the headsets be configured so that the boom mike for the passenger is on the left side of their face?

That's how it's set up now. Or do you mean coming off the right ear cup and across to the left? I'll try that.

[51598Rob]

Three letters....N S A. Shhhhh

[captnkirk]

If you have access to an ipad/iphone there are apps that measures sound levels I've used them in the cockpit a good bit, you may find that there is some "sound" in that area that is effecting the anr .Kind of a long shot but the levels where all over the place in my m4 and in the airbus. My sennheisers work better in some airbus's than others I assume for that reason . Good luck finding those gay electrons---- you know the ones that blow circuit breakers.

[andy]

Sounds like you've done a thorough job of troubleshooting, Andy. The only thing that I can suggest is that you try to narrow down whether the noise source is electrical or mechanical. You'd be able to feel any air flow strong enough to cause the mic to break squelch. If you can't feel any air flow near the mic in the right seat, then I'd say the noise is electrical.

The fact that you get the noise no matter which jack you plug into when sitting in the copilot seat, indicates that the noise is due to a noise source close to that position but not close to other positions. Something in or behind the right panel would be my guess. I take it that nothing is connected to a cigarette lighter plug in the panel. As Jeremy said, voltage converters that charge iPhones or iPads use switching transistors that feed noise back into the electrical system. Anything with a switching type power regulator is a potential electrical noise source, so I would look for something in or behind the right side panel that might be doing that. The right side strobe power pack is a possibility, but it won't produce noise if it's not turned on.

[Andy Young]

Thanks for all the suggestions, folks. I went out today and did another set of tests, based largely on suggestions submitted by you all. The short version is that it seems to NOT be electrical; it seems to be either air movement (which seems unlikely, since I can barely feel any) or a weird airframe noise, like a whistle or something. I thought it might be the antenna on the right wing root, but I removed it and taped over the hole, and found no change. I was able to adjust one of my headsets (the Telex) to mostly eliminate the problem, and the Zulu is fine, but the third one is unusable in that position. I think that's a result of differences in mic impedance, but that's not the true root of the problem. Something's still weird there. Open to any new suggestions of where to look for the source of the noise. The root fairings look fine.


A few additional observations (this section is repeated below as well):

1. The problem is speed dependent. Speeding up makes it worse, slowing down makes it better.

2. With the problem mostly fixed via one of the methods detailed below, the problem area can be definitively narrowed down to the front upper corner of the copilot window.

3. The level of background noise while speaking is dependent on mic proximity to that problem area. Closer, and the background noise is worse; further away, and it drops to normal levels.

4. Using my Zulu 1 headset in the copilot seat shows no problem, unless I stick the mic right up into the upper left corner of the window.

Here's the full report from today's tests, for those with the tolerance to read on:


Below is a list of checks/tests performed, with results:

1. Inspected rear mic jacks (no problems found).
2. Disconnected radio speaker (in headliner), panel light rheostats, and the avionics cooling fan. Looked under the right side of the panel to see if there were any other likely culprits for electrical noise, but found none that had not been previously tested by turning off. With all these off, there was no change in the problem.

The following tests were performed using the old Telex (non-ANR) headset:

1. Shielded mic with aluminum foil (the idea being to block any stray electrical signals). With the foil over the mic muff, it helped some; with the muff removed and the foil directly over the mic, it helped more, but did not completely eliminate the problem. With another layer of foil, the problem was gone. Of course, the foil also blocks air movement and noise signals, so the mic could not pick up voice, and the problem could still have been sound or air movement. I tried putting one small hole in the foil to allow enough air for voice to work, and the problem came back full-force.

2. I removed the foil and shielded the mic with duct tape. This worked better than the foil, completely eliminating the problem, thus eliminating the electrical interference theory. I was going to try a hole in the tape for voice, but peeling the tape back enough even to just uncover the hole for adjusting gain brought the problem back.

3. I adjusted the mic gain. This and a mic muff eliminated the problem. NOTE: Not sure this is a gain exactly, as changing it only changes the background noise sentsitity, the does not seem to change the sensitivity to voice.

The following tests were conducted with the Flightcom (non-ANR) Denali headset:

1. As soon as it's plugged in. It's even noisier than the Telex was, and adding a mic muff makes no difference. It also has no mic gain adjustment.

2. One layer of tape to mostly eliminates the problem. Two eliminates it completely.

Additional observations:

1. The noise is speed dependent. Speeding up makes it worse, slowing down makes it better.

2. With the problem mostly fixed via one of the above methods, the problem area can be definitively narrowed down to the front upper corner of the copilot window.

3. The level of background noise while speaking is dependent on mic proximity to that problem area. Closer, and the background noise is worse; further away, and it drops to normal levels.

4. Using my Zulu 1 headset in the copilot seat shows no problem, unless I stick the mic right up into the upper left corner of the window.

Conclusions thus far:

1. The problem is noise or airflow, not electrical.
I do not feel any significant airflow at the copilot door or window. A little, but much less than when the vent is open or the door is off.

2. It seem possible that there is some weird noise from the airframe (maybe a whistle or something) that is setting off the mic. I hVe already tried removing the OAT, to no avail, so I thought the problem might be the comm antenna on the right wing root. I removed it and taped over the hole, but it didn't change it at all.

At this point, any weird theory is welcomed, so fire away!

Thanks
Andy Young

[gbarrier]

Man, you've done a lot of work. Have you tried duct taping the door seam in the front and upper areas?

[andy]

Since you can't feel any significant air flow from the upper right corner of the copilot window, it sounds like a mechanical whistle that makes it through the background noise when you are near the source. The whistle might be some part protruding into the air stream close to that point since it gets worse with more airspeed. You might have already done this, but try moving the headset (and your head) around that area and see where the noise is strongest. Then try streamlining whatever is outside the aircraft nearest that point with duct tape. Obviously, that's not a permanent fix, but it might help identify the source so you can deal with it.

Have you eliminated the Plexiglas window vent as a possible source? Most of them don't seal perfectly.

[Andy Young]

Man, you've done a lot of work. Have you tried duct taping the door seam in the front and upper areas?

Yeah, I tried that in the first round, but it didn't change anything.

[Andy Young]

Since you can't feel any significant air flow from the upper right corner of the copilot window, it sounds like a mechanical whistle that makes it through the background noise when you are near the source. The whistle might be some part protruding into the air stream close to that point since it gets worse with more airspeed. You might have already done this, but try moving the headset (and your head) around that area and see where the noise is strongest. Then try streamlining whatever is outside the aircraft nearest that point with duct tape. Obviously, that's not a permanent fix, but it might help identify the source so you can deal with it.

Have you eliminated the Plexiglas window vent as a possible source? Most of them don't seal perfectly.

I've isolated the noisiest place to the forward upper corner of the window. Maybe I'll duct tape the entire door seam, window seam, vent, and maybe even the root fairing-to-airframe and wing seams to see if that eliminates the noise. If so, I can then work backwards, taking tape off a little at a time to see what makes it come back.

[Andy Young]

Did another round of tests today, and I think Ive got it nailed. The long version is below. The short version is that it is indeed a wind noise, even though there is no air felt leaking into the cabin in the area that exhibits the problem. So noise (whistle or whatever) instead of actual wind hitting the mic. Also, it's several areas causing the problem. My simple model of taping it all up and taking pieces off one at a time to see which one made the difference didn't cut it; I had to go back and see which COMBINATIONS of taped areas fixed the problem. At the end of it all, it turns out that I needed to tape up the following areas to eliminate the noise:

1. Windshield-to-pillar area
2. Door front to pillar area
3. Window-to-door top
4. Window-to-door front

For those with too much time on their hands or a similarly geeky brain to mine, here's the long version. NO obligation to read on.

I taped up all potential noise-producing areas around the right front door, as listed below:

-Windshield to airframe pillar
-Door front edge (upper part) to airframe pillar
-Door top edge to airframe
-Window to door frame, at front, top, and bottom
-Wing root fairing to wing
-Wing root fairing to airframe above door
-Door opening handle slot

With this all taped up, I did flight tests using the Flightcom Denali headset (which is by far the most sensitive one) with a mic muff. The squelch was set to the 11:00 position, which is a little high for comfortable use with my headset (10:00 is best). The squelch needed to be at 11:00 with that headset, as it was too sensitive to be useful for diagnostic purposes otherwise.

Set up as above, with all the tape on, the problem was solved. I then started pulling pieces off one at a time to see what made a difference and what didn't (steps below):

1. Pulled the tape covering the windshield-to-pillar area: noise back, with mic within about two inches of the upper front corner of the side window.
2. Pulled the tape sealing the front of the door (upper part) to the airframe: noise back and horrible, with mic within a foot of the door. It's anywhere (even back by the yoke).
3. Pulled all the remaining tape, and reinstalled tape removed in steps 1 and 2 (windshield and door front): noise still there within about eight inches of window upper front.
4. Added tape back at the window front: noisy within four inches of window upper front, plus for eight inches back along the top edge of the window.
5. Added tape back to the top area of the window: noise gone, except for very intermittent squelch breaks when mic held very close to the window upper front corner.
6. Removed windshield front: maybe a little worse intermittent noise, with sometimes steady noise within two inches of window upper front corner.
7. Removed door front tape: noise horrible, back to where it was Step 2.

Note that all noise more or less stops when slowed down below about 100 mph airspeed.

Conclusions:

Problem can be mitigated in the short term by taping:

1. Windshield-to-pillar area
2. Door front to pillar area
3. Window-to-door top
4. Window-to-door front

Long-term solution:
1. Use sealer to smooth and streamline windshield to pillar interface.
2. Improve streamlining and sealing of door to pillar. Perhaps re-shaping door is in order
3. Improve sealing of window top and front in frame.

Of course, as discovered above, this whole thing is mostly a non-issue with my headset, and indeed the Telex headset seems to be ok now too, with it re-adjusted. So if I had a set of identical headsets, I should be able to just set the squelch to the right level and be done with it. The problem is exacerbated by using headsets of varying mic sensitivity.

[RobHill]

Sounds like the noise triggering the VOX may be above the humanly audible range. Maybe life was more predictable back when we pushed a button to communicate on the radio or intercom.

[Andy Young]

Just thought I'd make a final posting on this, in case anyone is ever battling a similar problem and searching here.

Any this point, the problem is finally completely solved.

I used sealer to smooth and streamline the windshield to pillar interface on the right side. This eliminated some of the problem.

I then replaced the seal material for the front and upper sections of the door and the window, but this brought no improvement.

I then re-shaped the door skin slightly at the front upper section of the door so it is now flush with the windshield pillar when the door is closed. This eliminated more of the problem, but still not all of it.

Lastly, I replaced the window with one that I made that fits much closer to the door frame and skin at the top forward corner The old window, although flush along the front edge of the window frame, left a 1/4" gap between the window frame and the door frame, as the window frame is not tight to the door frame in this area. It doesn't stick out to the side; it's just further back, if that makes sense. The new window glass extends 1/4" forward of the window frame at the top, covering this gap, and the problem is entirely eliminated.

I must emphasize that none of this was done to prevent air incursion into the cabin; I could never feel any air coming through these areas. I even tried holding up a piece of paper to detect airflow, and found none. Best I can guess is that it was a whistle or something that the mic was picking up.