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Dec 5, 2013, 11:40:01 PM12/5/13

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Given a airplane at speed 600 knots, whose track at its closest point

passes by one at 15 kilometers away (elevation angle not important,)

what are the formulas that would describe the shape of the decibel

readings over time? (which are something like 25 25 40 40 40 38 36 34 30

25 25, with 25 dbA being the background.)

passes by one at 15 kilometers away (elevation angle not important,)

what are the formulas that would describe the shape of the decibel

readings over time? (which are something like 25 25 40 40 40 38 36 34 30

25 25, with 25 dbA being the background.)

Dec 6, 2013, 6:25:02 PM12/6/13

to

If the airplane was a uniform isotropic sound source and it went by, the

level of sound would be inversely proportional to the square of the

distance, and you could figure out the distance with a little bit of

trig by drawing a right triangle whose base is the distance of the plane

from the space directly above you, whose height is the altitude, and whose

hypotenuse is the distance.

BUT... and where this gets interesting.... is that although some sounds

from an aircraft (like the noise caused by turbulence over the body) can

be considered an isotropic source, not all of them can be.

Jet engine noise is much louder behind the plane than in front of it,

louder in front of it than directly below it, and the frequency spectrum

varies a lot with angle too.

Props are different.... and recprocating engines and turboprops have

different engine noise characteristics even though the prop noise is

more or less the same.

If you go to ntrs.nasa.gov and put in ANOPP, you can see some reports on

typical examples for modern jets.

--scott

--

"C'est un Nagra. C'est suisse, et tres, tres precis."

Dec 6, 2013, 10:02:59 PM12/6/13

to the_ph...@askthephysicist.com

>>>>> "SD" == Scott Dorsey <klu...@panix.com> writes:

SD> Part of it depends on how you weight it.

SD> If the airplane was a uniform isotropic sound source and it went by, the

SD> level of sound would be inversely proportional to the square of the

SD> distance, and you could figure out the distance with a little bit of

SD> trig by drawing a right triangle whose base is the distance of the plane

SD> from the space directly above you, whose height is the altitude, and whose

SD> hypotenuse is the distance.

Yes the hypotenuse is 15 km.

SD> BUT... and where this gets interesting.... is that although some sounds

SD> from an aircraft (like the noise caused by turbulence over the body) can

SD> be considered an isotropic source, not all of them can be.

SD> Jet engine noise is much louder behind the plane than in front of it,

SD> louder in front of it than directly below it, and the frequency spectrum

SD> varies a lot with angle too.

Ah ha, so the sudden onset, and then gradual decline, that I hear on the

ground is not due to some speed of sound / Doppler style thing, but the

plane itself!

SD> Props are different.... and recprocating engines and turboprops have

SD> different engine noise characteristics even though the prop noise is

SD> more or less the same.

Too few props here for me to comment. I'm talking about the jets going

by me on

http://jidanni.org/comm/air/m750/trail.html

http://jidanni.org/comm/air/m750/

SD> If you go to ntrs.nasa.gov and put in ANOPP, you can see some reports on

SD> typical examples for modern jets.

Wow there's a lot there. Thanks!

SD> Part of it depends on how you weight it.

SD> If the airplane was a uniform isotropic sound source and it went by, the

SD> level of sound would be inversely proportional to the square of the

SD> distance, and you could figure out the distance with a little bit of

SD> trig by drawing a right triangle whose base is the distance of the plane

SD> from the space directly above you, whose height is the altitude, and whose

SD> hypotenuse is the distance.

Yes the hypotenuse is 15 km.

SD> BUT... and where this gets interesting.... is that although some sounds

SD> from an aircraft (like the noise caused by turbulence over the body) can

SD> be considered an isotropic source, not all of them can be.

SD> Jet engine noise is much louder behind the plane than in front of it,

SD> louder in front of it than directly below it, and the frequency spectrum

SD> varies a lot with angle too.

Ah ha, so the sudden onset, and then gradual decline, that I hear on the

ground is not due to some speed of sound / Doppler style thing, but the

plane itself!

SD> Props are different.... and recprocating engines and turboprops have

SD> different engine noise characteristics even though the prop noise is

SD> more or less the same.

Too few props here for me to comment. I'm talking about the jets going

by me on

http://jidanni.org/comm/air/m750/trail.html

http://jidanni.org/comm/air/m750/

SD> If you go to ntrs.nasa.gov and put in ANOPP, you can see some reports on

SD> typical examples for modern jets.

Wow there's a lot there. Thanks!

Dec 7, 2013, 4:24:10 PM12/7/13

to

OK I got the answer added to

http://www.askthephysicist.com/ask_phys_q&a.html !

So indeed the delayed/sudden is all due to the front of the plane being

quieter not matter if in the sky or on the ground!

http://www.askthephysicist.com/ask_phys_q&a.html !

So indeed the delayed/sudden is all due to the front of the plane being

quieter not matter if in the sky or on the ground!

Dec 9, 2013, 10:06:05 AM12/9/13

to

In article <87bo0tq...@jidanni.org>, <jid...@jidanni.org> wrote:

>>>>>> "SD" == Scott Dorsey <klu...@panix.com> writes:

>

>SD> Part of it depends on how you weight it.

>

>SD> If the airplane was a uniform isotropic sound source and it went by, the

>SD> level of sound would be inversely proportional to the square of the

>SD> distance, and you could figure out the distance with a little bit of

>SD> trig by drawing a right triangle whose base is the distance of the plane

>SD> from the space directly above you, whose height is the altitude, and whose

>SD> hypotenuse is the distance.

>

>Yes the hypotenuse is 15 km.

At one point. But as the airplane moves, that changes.
>>>>>> "SD" == Scott Dorsey <klu...@panix.com> writes:

>

>SD> Part of it depends on how you weight it.

>

>SD> If the airplane was a uniform isotropic sound source and it went by, the

>SD> level of sound would be inversely proportional to the square of the

>SD> distance, and you could figure out the distance with a little bit of

>SD> trig by drawing a right triangle whose base is the distance of the plane

>SD> from the space directly above you, whose height is the altitude, and whose

>SD> hypotenuse is the distance.

>

>Yes the hypotenuse is 15 km.

>SD> BUT... and where this gets interesting.... is that although some sounds

>SD> from an aircraft (like the noise caused by turbulence over the body) can

>SD> be considered an isotropic source, not all of them can be.

>

>SD> Jet engine noise is much louder behind the plane than in front of it,

>SD> louder in front of it than directly below it, and the frequency spectrum

>SD> varies a lot with angle too.

>

>Ah ha, so the sudden onset, and then gradual decline, that I hear on the

>ground is not due to some speed of sound / Doppler style thing, but the

>plane itself!

listen to a Cessna coming over.... you will hear a very different onset of

sound because the radiation pattern is different.

>Too few props here for me to comment. I'm talking about the jets going

>by me on

>http://jidanni.org/comm/air/m750/trail.html

>http://jidanni.org/comm/air/m750/

and 707s. The JT3D engines in the 707 have an enormous amount of high

frequency noise that is due to the turbine acting as a big siren, and that

noise all comes out the rear. It's just awful, you need hearing protection

for your hearing protection. But then, 45 degrees away, the noise is almost

tolerable. They don't allow them to be used commercially in the US anymore

because of the noise concerns but there are still a few out there.

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