[Charles]

Charles asked the Naked Scientists:
   
Hi, I'd like to know whether the moisture in the fog can have a dampening effect for sounds?

It always feels quieter on a foggy night than a clear night, and I wonder if it's due to ambient sounds being dampened.

Love your show - great science, British accent, and bad puns - what more could one ask for?

Best, Charles

What do you think?

[Soul Surfer]

Yes it does the dampenig effect is particularly effective for higher frequencies and ultrasound. The mass of the particles absorbs and scatters the sound energy. Many years ago tried to patent a fog detector using a cheap pair of ultrasonic transducers and some very basic signal processing.  The idea was to design a device that would automatically turn on motorway fog warning lights in poor visibility but it did not work very well and no one was really interested in paying for it.

[thedoc]

Listen to the answer to this question on our podcast.

[chris]

One point I'd like to have made when answering this question on the show (but didn't owing to lack of time) is that this is also the reason why fog horns use low frequencies.

High pitched sounds have a high frequency, which in turns means they have a short wavelength (because velocity = f x lambda). This means that to transmit a high frequency wave lots of air has to be moved back and forth, and very quickly. Consequently the losses / damping effects are high, so the sound doesn't travel very far.

But low pitched notes, with a low frequency and a big wavelength, are moving the air less fast and less often and therefore the losses are reduced. Consequently low-pitched notes as less affected by fog and will also travel further. Hence they are more ideal for a fog horn.

This also explains (at least in part) why you can hear the bass from a passing car's stereo much more than the treble.

Chris

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[Bill]

Have you ever been in fog?  sounds travel extremely well in fog, better than clear air by far.