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Oxford1G
24th Apr 2002, 17:27
Hi

If you were to fly over a noise monitoring point at say 1000ft agl, and the sound from your engines was 100db, then what would be the level in db, that the noise monitoring point would record?

What would the effect of flying at 1100ft, or 1200ft, above the noise monitoring point have on the db reading on the ground?

What would be the best technique, to reduce noise levels, thrust reduction, or increased altitude with no thrust reduction?

Do sound waves behave much like radio and light waves in this respect?

25th Apr 2002, 11:11
I would say yes. Since sound is, as em-waves, an energy form it would distribute itself evenly along the surface of the wavefront. So the signal strength (or noise level) would decrease considerably with growing distance. Additionally, the damping effect of the travelling medium (air) would work much better on sound, wouldn't it?

Another point to consider, however, is that noise disturbance is a very subjective impression and not necessarily based on the dB-level arriving...

googolplex
28th Apr 2002, 10:53
Ignoring the effects of atmospheric absorption, your measured sound pressure level (SPL) would fall by roughly 6dB per doubling of distance (or increase by 6dB for every halving of distance) due to the effects of spherical spreading.

Using the following equation;

SPL2 = SPL1 + 20*log10(D1/D2)

(where SPL1 is the measured noise level at distance D1, etc)

if your SPL at, say, 1000ft was 100dB, then at 2000ft it would be 94dB. At 1200ft it would be 98.4dB.

Oxford1G
29th Apr 2002, 09:35
Thanks, that was just what i was looking for.:)

Mark 1
29th Apr 2002, 13:39
There is one other important consideration besides sperical spreading (which is 20*Log10(d1/d2) as stated). That is atmospheric attenuation which varies with frequency and ambient conditions (temp and humidity).

Jet mixing noise which tends to dominate take-off levels is predominantly low frequency and not largely affected, but noise from fans and turbines is higher in frequency and quite considerably affected by this type of absorbtion mechanism.

Power cutback after takeoff can only normally be done from a minimum of 7-800' and you must use enough power (if I remember correctly) to maintain a 4% climb and/or level flight after an engine failure, whichever is greater. This is more useful for 3-4 engine aircraft than twins.

The optimum point for cutback is usually just before reaching the monitoring point but will vary from type to type. It isn't a very popular technique with the airlines.

Another aside - If you are measuring EPNdB (the certification noise unit), it is corrected for the duration of the noise event, and the higher you are the longer the duration correction will be.