Use Google. I found this immediately. So, get yourself a permeability
meter and a permittivity meter and go to it.
resource for permeability and permittivity.... google or a library.
You can also look for an acoustic impedance of air.
Jim
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The characteristic impedance of free space, also called the Zo of free
space, is an expression of the relationship between the electric-field
and magnetic-field intensities in an electromagnetic field (EM field)
propagating through a vacuum. The Zo of free space, like characteristic
impedance in general, is expressed in ohms, and is theoretically
independent of wavelength. It is considered a physical constant.
Mathematically, the Zo of free space is equal to the square root of the
ratio of the permeability of free space (µo) in henrys per meter (H/m)
to the permittivity of free space (o) in farads per meter (F/m):
Zo = (µo/o)1/2
= [(1.257 x 10-6 H/m)/(8.85 x 10-12 F/m)]1/2
= 377 ohms (approximately)
The exact value of the Zo of free space is 120 pi ohms, where pi is the
ratio of the circumference of a circle to its diameter.
The Zo of dry air is similar to that of free space, because dry air has
little effect on permeability or permittivity. However, in environments
where the air contains seawater spray, excessive humidity, heavy
precipitation, or high concentrations of particulate matter, the Zo is
slightly reduced. Characteristic impedance is important to wireless
communications engineers involved in antenna design.
Also see Table of Physical Units and Constants.

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