Regardless of the source, all sound waves exhibit certain characteristics when they change mediums. Sound waves are actually vibrations which travel from molecule to molecule through most substances. For instance, sound waves can easily travel through means such as air, water, drywall, metal and concrete, an inherent capability that presents the primary challenge in many sound reduction and soundproofing endeavors.
If you are in an enclosed room and yell your name, a portion of the sound waves you create will attempt to pass through molecules of the walls, ceiling and floor of the room, while the balance of the wave reflects from these surfaces and remains within the room. Sound transmission refers to the passing of a portion of the energy through the surfaces enclosing the room, while sound reflection refers to the behavior of the energy which reflects and remains within the room.
Sound reflection, or the sound energy that reflected back into the room as you yelled your name, can be further classified by a measurement of the time lapse between the end of the sound’s introduction and its reentry back into the room. Signals with such a time lapse lasting less than 0.1 second are reverberations, while signals exhibiting a time lapse greater than 0.1 second constitute echoes. To understand the distinction between reverberations and echoes, imagine once again yelling your name within an enclosed room. Since you are in close proximity to the walls, ceiling and floor of the room, the sound energy produced reflects quickly back to your ears. Due to the human ear’s inability to distinguish sound signals as recurrent as 0.1 seconds apart, reverberations are interpreted as one lingering sound.
Now suppose you are standing in a giant canyon, enclosed by walls one hundred feet away on either side, and you yell your name with the same intensity as you did in the small room. The longer distance to the walls of the canyon causes sound energy to take longer to reflect off of a wall and return to your ear. With a lapse greater than 0.1 second passing before the reflected sound energy once again reaches your ear, you are able to interpret two separate sound signals, a scenario which exemplifies the defining characteristics of an echo.
The time it takes for a reverberation to weaken by 60 or more decibels and become inaudible is a metric known as reverberation time (RT). Each room has a fixed RT value, which is influenced by such variables as the size and shape of the room, characteristics of the surface textures and the intensity of the original sound energy. With reverberations lasting more than 1.5 – 2 seconds, the human ear can no longer accurately interpret individual sounds, and background noise becomes an issue. For this reason, targeting the capture of sound reflections, eliminating background noise and maintaining RT values below two seconds are common goals across many sound reduction and soundproofing projects.