6

Context:

We all know that by galvanizing a plate with a silver coating, this plate will have a very high reflection coefficient and act as a mirror for EM radiation (not for all $\lambda$ ranges of course) with specular reflection. This is nicely elaborated by John Rennie in a recent post here.

Now I would imagine that one could conceive a mirror for any type of wave, be it mechanical or of EM nature. I would imagine that the same line of reasoning with Huygens principle as for a light mirror should be applicable to acoustic waves.

Questions:

  • Knowing that sound is a mechanical longitudinal wave, does this fact make the properties of a sound-mirror very different than that of a light-mirror?

  • How would one go about building a sound mirror (let's say for our own audible range)? Should the material used be very elastic or instead very hard? How should it be galvanized?

  • Last question: wikipedia's page on acoustic mirrors gives a few pictures of acoustic mirror used in WW1 made of concrete, they all seem to be concave and not flat, why do they have to be concave?

I have tried to make the questions as related as possible to one another, so that the post remains coherent and answerable. Note that I'm just trying to gain an understanding and don't intend to build an actual acoustic mirror myself, this should spare repliers from the possible in-depth technical details.

Community
  • 1
user929304
  • 4,635

2 Answers2

4

I am a retired oil industry geophysicist. Considering basic ray theory, I would not consider sound and light mirrors to be very different in principle.

Shiny reflects better for light; hard reflects better for sound. The shape of a sound mirror would be based on its purpose. A rock cliff is a great sound mirror - you say "hello" to it and it says "hello" right back.

Concave sound mirrors concentrate the sound at a focal point so it is louder, easier to hear.

The oil and gas industry images sound waves to create a picture of subsurface structure and other physical attributes. This is called seismic data.

Seiscat
  • 76
3

Most solids reflect sound, in particular, painted concrete block will reflect about 97% if the incident sound striking it. There is a difference in complexity between an optical image and sound. When we try to reflect an undistorted image, we want to reflect not only one ray, but light from different locations in space and preserve this structure in the reflected image. For sound there is no real equivalent of the "image", we usually do not attempt to preserve the spatial structure of the sound. You might want to concentrate the sound though, but this is more similar to an antenna than to a mirror (like in a parabolic acoustic mirror)

  • Here is an article on the parabolic acoustic mirrors at Dungeness (UK) built in the 1920’s and 1930’s to detect enemy aircraft. http://www.davesnowdon.com/blog/sound-mirrors-lade-pit-dungeness – Neil Apr 11 '16 at 10:08