Sound Reflection and Absorption

Some basics

Like light waves, sound waves will reflect off various surfaces and interfaces.

Sound, like light, obeys the law of reflection, which states that a reflected ray emerges from the reflecting surface at the same angle to the surface normal as the incident ray, but on the opposing side of the surface normal in the plane formed by the incident and reflected rays. This type of reflection is called "specular" and requires a smooth surface.

Another type of reflection is called Lambertian reflection, such as what you see with light and a matte surface. The apparent brightness of a Lambertian surface to an observer is the same regardless of the observer's angle of view or the angle of incidence of the light. There is an analogue with sound.

A smooth surface is one where the dimensions of the reflecting surface are small compared to the wavelength of the sound. In the case of the ROVL, the sound wavelength is roughly 60mm in water (it will vary somewhat depending on the properties of the water). So, for example, a surface composed of a smooth layer of grains of sand will be reasonably specular, while a surface composed of a smooth layer of bowling balls will be reasonably Lambertian.

Few surfaces are either ideally specular or ideally Lambertian for sound, with most surfaces exhibiting properties of both to varying degrees.

Sound waves are also absorbed when they strike surfaces. After absorption, the sound waves may be conducted, re-transmitted, diffused, dissipated, or all of the above. Different interfaces and wavelengths lead to different mixtures of absorption and reflection.

Lambertian reflections spread the reflected energy in many directions, reducing energy in any one direction. Lambertian reflections are better for the ROVL because they have much less ability to affect the received signal we are looking for -- the ROVL is designed for direct transmission, not reflected transmission.

For our purposes, "surfaces" include both physical transitions from one material to another, and interfaces where the impedance of the material changes enough to cause reflections and absorptions. Here a some examples of "surfaces" that we need to think about when operating the ROVL system:

  • The floor of a body of water

  • The air/water interface at the surface of a body of water (the presence and size of waves will affect the specularity of the surface)

  • The sides of a pool

  • A buoy in the water

  • The air/steel/water interfaces at the hull of a boat or ship

  • The sides of a pier

  • A boundary between warmer and cooler water, or more and less saline water

  • The air/metal interface in the tank of a scuba diver

  • The swim bladder in a fish

  • The air surface in the electronics and battery enclosures of an ROV or UAV

  • The air bubbles contained submerged vegetation

  • The water/polypropylene interface in the frame of an ROV

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