Sound absorption is a concept that my local Chili's restaurant architect failed to consider when designing the interior.
Echo and reverberation occur when sound is reflected off a surface. Sound absorption occurs when sound waves strike a surface which absorbs the waves. Sound absorption diminishes sound energy. The sound energy is converted to heat on the surface of the absorbing material. Sound absorption is literally the changing of sound energy to heat. This then lowers the sound volume in the environment.
Walking into the Chili's restaurant, the first thing I usually notice is the volume. Even sitting in a booth, in an aisle of booths, it's abnormally loud. One day, as I was chewing on a chicken sandwich, I looked up. Above me was the answer.
Have you ever yelled in an empty gymnasium? Have you ever yelled in a cave? Have you ever talked into a toilet bowl? These are all environments with highly reflective surfaces. You hear a mass of echo and reverberation because there is very little, if any, sound absorbing surface.
The restaurant has an open ceiling – that is to say, you can see the roof trusses, air flow piping, and electrical pipes above you. In an attempt to give the room a larger feel, they leave that all open. The problem is there is nothing up there is absorbing any sound. Everything is a reflective surface – metal, steel, concrete, etc.
I visited a busy pizza place a few days later. It was crowded – but it was noticeable quieter. The difference is in the ceiling and on the walls near the ceiling are sound absorbing panels.
Sound absorption is an important concept for those of us in the audio production field because it enables us to control reverberation. A room with too much reverberation can result in speech that is hard to understand or audio that just plan sounds bad. Reverberation can destroy the quality of sound in a room if it's too great. For this reason, sound absorbing panels have been created.
Sound absorbing panels are made of highly fibrous / particled material. When a sound wave strikes the material the fibers vibrate. This vibration causes tiny amounts of heat due to the friction. Thus, absorption occurs through energy to heat conversion. The thickness of the material also has a great impact on the ability to absorb sound waves.
The use of absorption panels and proper placement are a science and best left to those skilled in their usage.
Absorption is a term that's used often enough that any sound tech should have some knowledge of what it is and how it affects the sound in a room.