overhead jet noise: amplitude variablilty

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Les Schaffer

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Jun 21, 1999, 3:00:00 AM6/21/99
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I have a curiousity question maybe someone has insight into.

When you listen to jets pass overhead, the amplitude of the jet roar
is slowly variable (modulated), sometimes louder, sometimes quieter. I
heard one passs last nite and was tempted to record it just to plot
the amplitude of the sound.

does anyone know what causes these variations? is it fluctuations in
gas ejected from the jets? that'd be a little hard to believe cause
(there is one passing overhead right now) the modulation frequency
sounds like maybe on the order of 0.1 to 1 sec.

cheers

les schaffer

--
____ Les Schaffer ___| --->> Engineering R&D <<---
Theoretical & Applied Mechanics | Designspring, Inc.
Center for Radiophysics & Space Research | http://www.designspring.com/
Cornell Univ. scha...@tam.cornell.edu | l...@designspring.com

Noral D. Stewart

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Jun 21, 1999, 3:00:00 AM6/21/99
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I believe it is largely related to fluctuations in the atmosphere
influencing the path between you and the plane. Some of these
fluctuations are related to the presence of the plane disturbing the
atmosphere.

Ken Plotkin

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Jun 22, 1999, 3:00:00 AM6/22/99
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On Mon, 21 Jun 1999 13:26:27 GMT, Les Schaffer <godz...@netmeg.net>
wrote:

[snip]


>does anyone know what causes these variations? is it fluctuations in
>gas ejected from the jets? that'd be a little hard to believe cause
>(there is one passing overhead right now) the modulation frequency
>sounds like maybe on the order of 0.1 to 1 sec.

[snip]

Primarily atmospheric effects. Scattering of sound by turbulence;
focusing/defocusing effects by large eddies; multiple path
propagation.

When you stand near a running jet engine (try 500 feet from an F-16 on
afterburner!) the sound is pretty steady. Some fluctuations, but not
like the stuff you're describing. You need bigger structures to match
those rates.


>Theoretical & Applied Mechanics | Designspring, Inc.
>Center for Radiophysics & Space Research | http://www.designspring.com/
>Cornell Univ. scha...@tam.cornell.edu | l...@designspring.com

Talk to some of your radiophysics colleagues. The pehnomena are
similar to those involved in scintillation of interstellar radio
signals. (Hope I said that last bit right.)

When I was a grad student at Cornell, studying scattering of sonic
booms by atmospheric phenomena, one year I had a roomate who was in
Astronomy doing the scintillation stuff. We were using many of the
same books, hacking the same equations.

Ken Plotkin


Les Schaffer

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Jun 22, 1999, 3:00:00 AM6/22/99
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Hey Ken:

>>>>> ">" == Ken Plotkin <kplo...@nospam.net> writes:

>> Primarily atmospheric effects. Scattering of sound by
>> turbulence; focusing/defocusing effects by large eddies;
>> multiple path propagation.

yeah. after i hit the send key this is what occured to me. its one of
those things i have wondered about ever since i was a kid, but only
today did it occur to me to even ask "why?"

>> When you stand near a running jet engine (try 500 feet from an
>> F-16 on afterburner!) the sound is pretty steady. Some
>> fluctuations, but not like the stuff you're describing. You
>> need bigger structures to match those rates.

i guess an ocean of air qualifies as big!

>> When I was a grad student at Cornell, studying scattering of
>> sonic booms by atmospheric phenomena, one year I had a roomate
>> who was in Astronomy doing the scintillation stuff. We were
>> using many of the same books, hacking the same equations.

i'd be real interested to read some of the literature on this stuff,
especially if it has some experimental verification. can you recommend
anything?

--
____ Les Schaffer ___| --->> Engineering R&D <<---

KKARMA

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Jun 22, 1999, 3:00:00 AM6/22/99
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Les Schaffer wrote:
>
> I have a curiousity question maybe someone has insight into.
>
> When you listen to jets pass overhead, the amplitude of the jet roar
> is slowly variable (modulated), sometimes louder, sometimes quieter. I
> heard one passs last nite and was tempted to record it just to plot
> the amplitude of the sound.
>
> does anyone know what causes these variations? is it fluctuations in
> gas ejected from the jets? that'd be a little hard to believe cause
> (there is one passing overhead right now) the modulation frequency
> sounds like maybe on the order of 0.1 to 1 sec.
>
> cheers
>
> les schaffer

>
> --
> ____ Les Schaffer ___| --->> Engineering R&D <<---
> Theoretical & Applied Mechanics | Designspring, Inc.
> Center for Radiophysics & Space Research | http://www.designspring.com/
> Cornell Univ. scha...@tam.cornell.edu | l...@designspring.com


Although the phenomenon is clearer with piston engines this is probably
the same thing. When two or more engines are running at almost the same
rpm:s their sound waves occasionally coincide and thus strengthen each
other, i.e. there is a "beat" in the sound at the frequency of the
difference of the rpm:s. You can hear this also by tuning two strings
close to each other.

Kai Karma
Sibelius Academy
Helsinki, Finland

Jon Mooney

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Jun 22, 1999, 3:00:00 AM6/22/99
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Les,

I believe what you are describing is a Lloyd Mirror effect, where ground
reflections interfere with the direct path and either add or subtract from
the signal depending on the path length differences and frequency. Since
the path length difference is continually changing, the effect continually
sweeps through the frequencies. When the plane is at a distance, the change
in path length is very slow and therefore the fluctuation in amplitude is
very slow. When the plane is at its closest point of approach, the change
in path length is fastest and the fluctuation in amplitude is fastest.

Best regards,

Jon W. Mooney, ASA

Les Schaffer

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Jun 22, 1999, 3:00:00 AM6/22/99
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>>>>> ">" == KKARMA <KKA...@siba.fi> writes:

>> Les Schaffer wrote:

>> When you listen to jets pass overhead, the amplitude of the jet
>> roar is slowly variable (modulated),

...


>> the modulation frequency sounds like maybe on the order of 0.1
>> to 1 sec.

>> Although the phenomenon is clearer with piston engines this is
>> probably the same thing. When two or more engines are running
>> at almost the same rpm:s their sound waves occasionally
>> coincide and thus strengthen each other,


here is why i think maybe not:

a.) there seems to be some randomness in the amplitude variations. in
my posts, i should have said "amplitude varies over a __characteristic
time scale__ of 0.1 to 1 sec".

b.) the intriguing aspect of this is as follows: you listen to a jet
pass overhead and track the sound for as long as you can. over time
the jet moves farther away and the sound level decreases, on average.

but i often find that towards the end of the "tracking period", you
can get pretty loud amplitudes again, which would be more suggestive
of the "atmosphereic twinkling" explanation, or the Lloyd Mirror
effect mentioned in the next post, though i dont grasp the basics of
this effect yet.

les

Angelo Campanella

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Jun 22, 1999, 3:00:00 AM6/22/99
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In article <376E5E...@ix.netcom.com> "Noral D. Stewart" <no...@ix.netcom.com> writes:
>> When you listen to jets pass overhead, the amplitude of the jet roar
>> is slowly variable (modulated), sometimes louder, sometimes quieter. I
>> heard one passs last nite and was tempted to record it just to plot
>> the amplitude of the sound.

This is most common in the day time. Local variations in density and velocity
will affect the porpagation direction of sound passing through them.

The most common daytime variation are the "thermals" (so valued by gliders)
comprising a rising bubble (actually a torus rising like a smoke ring) of
warm air. These bubbles, a few hundred meters in diameter form refrating
parcels through which the aircraft (point source) noise is obliged to travel
and perforce be bent in various direction from moment to moment.

The resulting modulation in amplitude and phase of the sound received at a
remote point on the ground is as expected.

Ang,


Ken Plotkin

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Jun 23, 1999, 3:00:00 AM6/23/99
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On Tue, 22 Jun 1999 02:09:13 GMT, Les Schaffer <godz...@netmeg.net>
wrote:


>i'd be real interested to read some of the literature on this stuff,
>especially if it has some experimental verification. can you recommend
>anything?

There is a lot of stuff out there. Much more for propagation near the
ground than from high altitudes downward. I'm most familiar with the
sonic boom part of the literature, which is a bit specialized (to say
the least).

However...

Take a look in the Journal of the Acoustical Society of America for a
review/tutorial paper on outdoor sound propagation by Tony Embleton,
somewhere from 2 to 5 years back.

Browse through some recent meeting programs of the Acoustical Society,
and look for Physical Acoustics sessions on outdoor sound propagation.
The abstracts will give you a feel for who's doing what, and you can
search from there.

If you like a classical approach to things, rather than the modern
computerized junk (ducking hard! ;-) ), two great books from the 60s
are Tatarski "Wave propagation in a Turbulent Medium" and Chernov
"Wave Propagation in a Random Medium". Both published first by McGraw
Hill, then in affordable editions by Dover.

Ken Plotkin


Les Schaffer

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Jun 23, 1999, 3:00:00 AM6/23/99
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>>>>> ">" == Ken Plotkin <kplo...@nospam.net> writes:

>> Take a look in the Journal of the Acoustical Society of America

...


>> Browse through some recent meeting programs of the Acoustical

...

thanks... will do some footwork...

>> If you like a classical approach to things,

wouldnt give it up for all the tea in china.... (whatever that
means).

>> rather than the modern computerized junk (ducking hard! ;-) ),

you wont get no trouble from me here, though i do love n. computation.


>> two great books from the 60s are Tatarski "Wave propagation in
>> a Turbulent Medium" and Chernov "Wave Propagation in a Random
>> Medium".

struck out at fatbrains.com and amazon.

just got off the phone with barnes and ignoble. they have a
Wave Propagation in a Random Medium by Tatarski from SPII (?)
publishers for like $35 and they show a Wave propagation and
Turbulent Media by a Roy Adams, buts its hard to get.

either of these close to what you had in mind. i ordered the Tatarski.

Ken Plotkin

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Jun 24, 1999, 3:00:00 AM6/24/99
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On Wed, 23 Jun 1999 02:47:50 GMT, Les Schaffer <godz...@netmeg.net>
wrote:


>just got off the phone with barnes and ignoble. they have a
>Wave Propagation in a Random Medium by Tatarski from SPII (?)
>publishers for like $35 and they show a Wave propagation and
>Turbulent Media by a Roy Adams, buts its hard to get.
>
>either of these close to what you had in mind. i ordered the Tatarski.

The Tatarski book sounds right. Might be an updated edition. Haven't
heard of the Adams book.

BTW - the Embleton article I mentioned is in the June or July 1996
issue of JASA. Mostly propagation near the ground, which is cool
because the ground is there for everything to bump into.

Ken Plotkin


jon

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Jun 27, 1999, 3:00:00 AM6/27/99
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> Les Schaffer wrote:
>
> > I have a curiousity question maybe someone has insight into.
> >
> > When you listen to jets pass overhead, the amplitude of the jet roar
> > is slowly variable (modulated), sometimes louder, sometimes quieter. I
> > heard one passs last nite and was tempted to record it just to plot
> > the amplitude of the sound.
> >
> > does anyone know what causes these variations? is it fluctuations in
> > gas ejected from the jets? that'd be a little hard to believe cause
> > (there is one passing overhead right now) the modulation frequency

> > sounds like maybe on the order of 0.1 to 1 sec.
> >
> > cheers
> >
> > les schaffer

> >
> > --
> > ____ Les Schaffer ___| --->> Engineering R&D <<---
> > Theoretical & Applied Mechanics | Designspring, Inc.
> > Center for Radiophysics & Space Research |
http://www.designspring.com/
> > Cornell Univ. scha...@tam.cornell.edu | l...@designspring.com
>
>
>
>
The effect you refer to is a (very) fast frequency selctive fading effect
arising from the interference from multipath arrivals from the jet. Due to
the refraction/diffraction of the sound field paths will arrive at slightly
different times. For a narrow band source ( whiney jet ?) then tonals will
dissapear down troughs in the effective transfer function of the channel.
As these troughs are time varying then the signal undergoes rapid magnitude
variation and extremely rapid phase variation. At longer ranges all paths
will have comparable Doppler shift associated them and so the variability
is probably less severe. At Shorter ranges ( CPA) there is significant
difference in the temporal scaling on each path.

I see very similar effects in broadband underwater acoustics, however in
air the effects are 5 times more severe !

Regards

Jon Davies


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