Sound levels and cold weather

[fatrol]

We conduct interior and exterior sound level tests for ambulances and emergency response vehicles. As you know it is winter in the great white north and the temperature is below 0 oC (32 oF). The applicable standard states the sound level tests must be conducted between 0 and 30 oC (32 and very hot). Winds must be below 13 km/hr (~8 mph).

Question: What is the effect on sound levels due to cold temperatures and excessive wind speeds.

I have an educated guess: Denser air, sound waves lose energy quicker. Harder for waves to achieve the appropriate amplitudes.

 

[25362]

Although I consider myself quite nescient on this subject, I think you are right, on colder weather, the impedance, i.e., the product of density and speed of sound [ρ] c, increases.

This is because with a drop in temperature, the increase in air density is proportionately greater than the simultaneous drop in sound speed, at the same atmospheric pressure. As for the wind see, for example,

http://www.squ1.com/sound/propagation.html

On the other hand, I understand that high intensity sounds (high "volumes") of, say, 100 dB, even having different frequencies (i.e., pitch) f=c/[λ], as may result from a change in air temperature, are perceived by the human ear as having the same volume.

[λ] is the sound wave length, ~0.64 m.

 

[fatrol]

Thanks. the website is very informative.

 

[chicopee]

I may be sticking my foot in my mouth but I would say that air temperature should not affect sound intensity but will affect the speed of sound. Wind speed and direction on the other hand should affect both.

 

[25362]

To chicopee, you are practically right.

One-cycle average wave intensity: [Î] = [½] [Δ]p_o²/([ρ] v)

Sound intensity is somewhat affected (attenuated) by molecular absorption, i.e., by air pressure, temperature and RH. RH (itself depending on temperature) affects the vibrational frequencies of the air molecules (principally oxygen) shifting them in the audible spectrum, introducing a lag in the acoustic waves and a slight reduction in their intensity.