# Why are high frequency sounds absorbed more than low frequency sound?

Atmospheric absorption seems to play a big role in bat echolocation as it apparently strongly limits perception range. I know absorption changes with frequency, but I don't understand why, and all the explanations I have found are either too general or too complicated for me.

Could anyone explain in simple terms (accessible to biology students) why this happens and/or know a good resource that could help?

## 3 Answers

We know that sound is a pressure pressure wave where air/water particles compress and dilute. Compression needs more energy than dilution generates energy. The reason for this is the friction between randomly moving air/water molecules (keyword: viscosity). This excess energy is transformed into heat and is lost. The kinetic energy of moving particles is increasing with frequency as at higher frequencies air/water particles are moving faster, so that for given viscosity the energy loss (Absorption) is increasing with frequency.

Other absorptions exist in water due to boric acid and magnesium sulphate.

I think you have this the wrong way around, based on your question? Atmospheric attenuation of sounds limits the distance over which high frequency sounds travel, so that they don't propagate as far as low frequency.

In the simplest terms, higher frequency (=short wavelength) sounds are subject to more scattering and absorption than low frequency (long wavelength) sounds through interference with air/water molecules.

This affects the overall signal being received at distance, plus alters the frequency characteristics of a sound, e.g. if a bat produces a broadband call covering a range of 100 kHz down to 20 kHz, then at a distance the highest frequencies will be lost and so only the signal between, say 80-20 kHz would be received.

• Yes - thanks for pointing my mistake! - I corrected the question accordingly Jul 7, 2022 at 13:02

I once heard this phenomenon compared to water waves hitting a jetty. Swells (equivalent of low Hz sounds) pass through the posts of the jetty relatively unabated, but surface ripples (equivalent of high Hz sounds) bounce off the posts and scatter. The analogy doesn't explain the underlying physics but I find it helpful for visualising what's happening.

I think of this analogy whenever neighbours are having a party.. you mainly hear the thumping bass because higher frequencies are being absorbed and scattered, lower frequencies pass through.