Air Vent Noise

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smoss
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Air Vent Noise

Post by smoss »

My 2007 DA40 XL front air vents have always been deafeningly loud when open, with a high pitch swooshing noise as the dominant sound--similar from both the right and left vents. The noise is almost like you'd think it would sound if the air was going through a very small orifice somewhere in the line. It completely goes away with closing the vents. Most other DA40's I have flown (all 2008 XLS's) have the same really loud vents, but a few days ago I flew in a 2008 DA40 XLS that had significantly quieter vents than mine! You still heard a mid frequency "wind noise", but no high frequency shwoosh/whine. It was much more pleasant than my vent noise. Any thoughts as the origin of the common loud vent noise, or ways to dampen it? I have seen behind my panel, and there are no obvious tight kinks or holes. There is a spot on the left vent hose where it goes behind the canopy hinge tube that gets a bit squished with closing the lid, but only to the point of being more oval than round. Could that do it??? I tried for the life of me to reroute that but there is really no other space where it goes that is any better on my plane. I was thinking of some sort of gun like silencer to add to the vent outlet, but even if I put something in front of the vent, like a hand or clipboard, without occluding the flow, it still is just as loud, as most of the sound seems to be coming from behind the outlet as opposed to being made from the outlet.
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Re: Air Vent Noise

Post by Gasser »

My headphones do a great job of getting the noise down but it's still noticeable. I thought a nice idea would be to make an internal muffler behind the dash if there was room. Basically a plastic cylinder with holes in it wrapped in some non flammable rigid Fiberglas covered in some fabric to keep any glass particles from getting in the air I breath. I used the same stuff to sound treat the walls in my home theater. The principal is the same as muffler construction. Sound goes through the holes and gets absorbed but there would be no reduction in air flow.

Problem is, getting something like this done wth the FAA would probably be impossible.
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Re: Air Vent Noise

Post by smoss »

muffler would be a nice idea, but there sure isn't much space back there at all. I was thinking external muffler, attach to vent on outside, but as mentioned before, it seems I hear a lot of noise coming from not only the duct hole itself, but the whole area, so not sure how much noise it would cut down. It wouldn't be pretty, but could try and test it by just taking a toilet paper roll or something and trying to rig up something crude. Obviously, heat is not an issue like in a real muffler.
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Re: Air Vent Noise

Post by rwtucker »

I have wondered this myself. The noise might be created by the venturi created by choking off the hose with the tie string followed by a rapid expansion into the vent. An easy way to test this is remove the vent and string and temporarily affix the vent hose directly through the vent opening without restricting it. I haven't got around to this yet. If someone does, let he rest of us sufferers know :scratch:

Overall the DA40 with a 3-blade seems only average in cockpit noise. I haven't flown a 2-blade. Then, not too many aircraft in this general size range come close to the quietness of the better C182s. Quieter air vents would help.
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Re: Air Vent Noise

Post by Steve »

Owners have been searching for a way to quiet the Diamond Star front panel vents for more than a decade. I myself have a couple ideas, but none have panned out yet. One involves stuffing a small section of the vent duct with open-cell foam. Trouble is finding material that is not flammable. Another is a "venturi muffler". I have one on the exhaust of my shop vac, and it is pretty effective. Problem would be finding (making) one out of aluminum and fitting it in behind or under the panel.

I have a sound level meter, so I'm ready to do some controlled testing, if anyone comes up with a good idea...

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Re: Air Vent Noise

Post by rwtucker »

Steve -- I'm having trouble remembering the exact configuration but, since you are ready to roll on this:

One fix might be to cut the air hose a foot or so before the vent and splice it into a hose that is about 25% or so large. Then (if memory serves) affix the larger hose over the entire vent valve, wrapping it around a thin foam or putty to fill in open spaces. This would reduce pressure and would eliminate some of the venturi effect at the point of connection.

Another thought is to spray coat the interior of the vent with an uneven surface that would randomize the path of the air across the surfaces. I haven't researched what type of paint might be safe in this context but something analogous to a sand impregnated paint. Sand blasting the interior surfaces with soft media would achieve a similar effect.

It might also be possible to take a file to appropriate sharp edges to soften the angles of "attack" as it were. If I recall correctly, the edge of the disk that closes the vent has an edge that is slightly tapered. We could test this by removing the disk (if it can be removed).

That's about as far as I've got.
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Re: Air Vent Noise

Post by smoss »

it sure does seem like it's the edge of the disc, or something like that causing most of the noise. I don't think that is removable, but maybe could get some foam tape near the edges that would effectively change the edge shape.
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Re: Air Vent Noise

Post by carym »

I would doubt that it is the vent itself, since the DA42 has the same vent and there is no vent noise in the DA42. It has to be some way that the vent tubing is routed to the vent that creates the noise. It may be interesting for someone to look at the difference in venting between the DA40 and the DA42 to figure out why the DA40 vent is so noisy.
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Re: Air Vent Noise

Post by smoss »

interesting the DA42 is quiet... Perhaps it really is the slight squishing of the scat tubing at places, or the turns it makes, that adds the noise. The interesting thing is that the high pitch noise does not seem directly flow rate dependent. As I close the vent, until it is almost completely closed, the high pitched noise is still there. Perhaps it is actually generated at the inlets on the sides of the plane, and transmitted down the tubing. If such is the case, different props may actually cause different noise as the slipstreams go to different spots. The plane I just flew with much quieter vents than mine had the 3 blade MT prop.
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Re: Air Vent Noise

Post by Steve »

My airplane has the 3 blade MT, and it is noisy! Sometimes I have difficulty hearing the radios if the vents are more than one notch open.

This discussion prompted me to look again, and I was able to find a source for a small quantity of heat resistant open cell foam. I will gather some baseline sound levels this weekend, and try the foam in various duct locations. I have to figure a way to measure how much the air flow is reduced (maybe one of those pocket anemometers held in front of the vent?).

Will keep you posted...

Steve
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