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In a message dated 3/11/2003 12:06:48 AM Central Standard Time, Newlan2dl@aol.com writes:
1. Did you stick the dB meter all around the plane to see where the other
noise leaks were? Like how is the back of the firewall at the penetrations?
Dan,
I did move it around within sight - A 320 is pretty small. I don't remember anything significant showing up in those readings but I think it was generally noisest above the longeron in the cockpit. There were no "open" firewall penetrations - bulkhead fittings and cable fittings were used - the two wire bundle passages are liberally filled with silicone. Not only is the back of the firewall covered (heavy green stuff/dead aluminum), but so is the face of the header tank to cut off any resonance in that area. More important is the nose wheel well with its large interior surface. While that area is somewhat closed off from the engine compartment by a wee door, the cockpit side surface is covered with 1/4 in dense foam/dead aluminum.
2. That's interesting about the baggage compartment. It was one area I
suspected and was going to test. So did you try adding insulation there to
the bulkhead or just test it with and without it being installed? And how
well does it fit around the edges? A LOT of noise can escape through a VERY
small gap.
Yeah, I think the tailcone is a noise box. The only cockpit air exit is thru the elevator pushrod tunnel with the air entering from forward of the instrument panel and under the seats. The bulkhead and tunnel is upholstered. I don't think much noise comes thru the bulkhead, but I have not taken any data on that. Hmmmm, interesting. I would not want to place much heavy sound insulation aft, but the aft side of the bulkhead is a possibility.
3. Did you check around the canopy for air coming under the frame, around
the hinges, etc? A lot of noise is aerodynamic and is from boudary layer
buffeting. It is often the dominant noise factor in large jets.
The canopy side walls are extended to below the longeron (inside and out) to stop weather entering (outside) and canopy bulge from external low pressure (inside) and the side and front seals appear to be completely effective. This is a topic all by itself for "forward hinged canopy" planes. Anyhow, when the canopy "bulges", it grows slightly shorter thus pulling it away from the rollover a bit. This has proved more difficult to easily seal perfectly. I do have a very small air leak at this point, but it is not a significant contributor to noise.
4. When you did your testing, was the noise meter set to dB "A" scale or
"C" scale?
I used the "slow" scale. I don't remember whether that is A or C.
5. Excellent observation on the air vents. What was the open and closed
vent readings?
I didn't save the test information and the airplane is getting some high speed stealth paint slathered on so I can't get new data for a few weeks. I think is was just a Db or two. The airstream noise is like a rushing noise - you can hear it but it is not, um, painful. Of course, my ears have not been calibrated and they are old and somewhat useless for detail.
6. Do you know what the weight of your insulation kit was?
I think about 15-18 pounds. I would have rather traded some of that weight for the weight of a thicker canopy.
Maybe we need to analyse the noise spectrum to determine what best needs attenuation.
Scott Krueger
N92EX
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