# Decibels vs. time for a passing plane

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### jid...@jidanni.org

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.)

### Scott Dorsey

Dec 6, 2013, 6:25:02 PM12/6/13
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Part of it depends on how you weight it.

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."

### jid...@jidanni.org

Dec 6, 2013, 10:02:59 PM12/6/13
>>>>> "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!

### jid...@jidanni.org

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

### Scott Dorsey

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> 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!

Partly. Again, depends on the plane. Go to your local GA aircraft and
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/

If you're in Taiwan you're missing all of the really fun planes like Tupolevs
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.