ultra sound transmission through compressed air

[Capernicus]

Ultra sound in your usual air u breath at 14psi, is about 1 db/m absorption, for 50khz, so I read.

I have a hypothesis, but I cant get any answers on the internet->
If I compress the air,  does this raise the frequency/ drop the absorption - whilst keeping the velocity of the sound the same?

If I lower the temperature its true,  the sound stops absorbing when the air is cold, but that increases the velocity, is the same true for when the pressure is risen high enough - but the velocity stays the same?

[T3sl4co1l]

https://en.wikipedia.org/wiki/Speed_of_sound

Pressure has very little effect on velocity.  For sure, frequency is not changing, unless something else is in relative motion, or other particular nonlinear effects.  It is rather rare to change an absolute frequency, even with nonlinear interactions.  But the spacial frequency i.e. wavelength, definitely.

Absorption will generally be different, because density and therefore mechanical impedance matching will be different.

Tim

[Capernicus]

So the absorption is lower then at higher pressures?

If the velocity doesnt increase, it means winner winner chicken dinner when it comes to making an ultrasonic delay line with compressed air as the medium,  because absorption lowered, but the velocity stayed the same.  that means u get higher frequencies into it at the same delay distance,  superior.

Just has to be able to handle 1000psi or something inside it, so needs to be made out of something tough like metal.
Plastic can handle up to 200- or so - psi - so maybe plastic can also have a go, if its at least 1 millimetre thick.

Im just looking at reflection now,  because every time u bounce u absorb as well,   so u need to take that into account too.

[TimFox]

Stokes' Law for sound attenuation in Newtonian fluids (including air) can be found in the usual source:  https://en.wikipedia.org/wiki/Stokes%27s_law_of_sound_attenuation
In the first equation, the attenuation rate a is inversely proportional to the mass density of the fluid, which is directly proportional to the air pressure (at fixed temperature).
A(d) = A_o exp(-ad), where A is the sound amplitude, d is distance in m, and a is the rate in m^-1.
As the pressure increases, the density increases proportionately, so the rate a decreases.  Therefore, the attenuation at a given distance decreases.

[TimFox]

By the way, a steel vessel pressurized to 1000 psig (pronounced "pigs eye" in American usage) will require safety-rated electrical feedthroughs to connect your delay line to useful circuitry.
They are available, for example  https://www.conaxtechnologies.com/wp-content/themes/conax/pdf/5001-38-39.pdf
You may need to check with your local authorities about boiler inspection regulations.
American safety regulations on pressure vessels kick in at 15 psig.  https://www.osha.gov/pressure-vessels/standards

[Capernicus]

I C NOW!    So I needed to look up something called "thermodynamics!"

And it means something cool.

If you pump something up, it increases temperature,  but then if u refridgerate it, you can fill it up with MORE air, to get the pressure back! 

And then that drops attenuation over distance for sonic transmission!!!!   (so cool it down AND pump it up.)

But its dangerous...  You have to support the pressure WITHOUT refridgeration, because then if it heats up because the fridge deactivated it might expode!

[newbrain]

So the absorption is lower then at higher pressures?.

Ah, uBeam could have a chance then!
On Jupiter...