ESDU Challenge: Sound Propagation

I feel like I am at risk for being tricked by what seems like an answer that is a bit too obvious... however, I am still going to say:

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False.

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Sound propagation in the near field is going to be strongly influences by variations in temperature and speed of gasses through which it is traveling. At a distance these effects very small and washed out at signals overlap.

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So, how is my guess, is it in the near field... or way out in the far field?

I would agree. It's hard to determine exactly what is meant by near field and far field, but the speed of sound is determined by Hook's Law, where:

c = (E / ρ)^1/2

Where:

E = Bulk Modulus of Elasticity for the gas

ρ = The density of the gas

Sound will normally drop 6 dB for every doubling of distance, but that depends on the medium by which sound must propagate through and we can probably state that the density of the air is not the same at the nearfield as it is in the farfield due to temperature and gas type differences.

Also, the speed of sound is lower in a medium that is turbulent than it is for undisturbed medium of the same type.

Again, I don't know what the actual definition of near and farfield is, but jet exhaust near the nozzle has has smaller eddies compared to the exhaust at a larger distance where the eddies are much larger in magnitude so that the effect is higher frequencies nearer the nozzle and much lower frequencies produced further away.

So I think your gut answer is correct and you get a GA for it. :-)

My answer:-

The terms are not defined... it is a meaningless question.

Jet receiver

Jet? jet of what? jet engine?
Receiver? WTF?

Or do I have to read something that isn't linked to?

In general terms sound will propagate how it will propagate... if you put something in the near or far fields it will effect how it propagates. You can't measure it without effecting it.

Del

False.

http://www.lic.wisc.edu/glifwc/Polymet/SDEIS/references/Ray%202010.pdf

Sound, and sound propagation (in air) is incredibly complex.

GENERALLY, sound in the near field is unaffected by reflected energy.

We have not defined the near field adequately. The size of the near "field" is relative to the frequency of the sound being sampled or the size of the sound source, depending on things I don't remember. "Jet" leaves much to speculation.

Sound in the far field is much different in character.

So, we all agree.

There is a lot to be defined here! I'm assuming (dangerous thing to do) that the 'Receiver' is some form of detector, and that that detector is very small compared to the size of the 'Near field'. A good detector should have virtually no influence on the energy being detected.

According to those assumptions, the propagation is essentially the same whether the detector is present or not.

I have done some work regarding reducing the noise emitted by jet engines, and so that is what comes to mind. As you say, 'jet' could mean LOTS of other things...

Not to mention that if it is an actual aircraft jet engine the exhaust is also supersonic at the nozzle.

IHS ESDU provides validated information, insight and...
The question doesn't indicate much proficiency in either