One of the most common misconceptions you’ll face when you start putting together a home theater or high-end listening room is the role absorption and diffusion play – regardless of the equipment you put in it. And just like any other technology or specialized field, important terms get thrown around by people in the know while the rest of us grin and nod our heads as we hope that someday we’ll figure exactly what the heck all those smart people are talking about.

 

In the next few weeks we’re going to explore the roles absorption and diffusion play when preparing a theater or listening room, and we’ll discuss what to do and what not to do. But before we get started, here’s a look at some important terms and definitions.

 

Glossary of Terms for Absorption & Diffusion

Ambience. The degree to which sound appears to come from multiple directions.

Anechoic. Free from echo. Generally applied to a specially designed room that is free from all reflections, reverb and echo.

Bass Trap. A sound energy absorber that is tuned to a specific frequency or range of frequencies. 

Decibel (dB). A dimensionless unit used to compare the ratio of two quantities, in this case Sound Pressure Level (SPL).

Diffusion. The mixing of sound in a room, with the aim of making the total audio response uniform.

Direct Sound. Sound that reaches the listener directly from the loudspeaker or source without first being reflected off of another surface.

Early Sound or Early Reflection. Sound that reaches the listener less than 50ms after the direct sound. The human brain will generally differentiate Early Reflections and Direct Sound as being two different sounds through the precedence effect.

Echo. A single reflection of a primary sound, generally processed as being a separate and distinct sound from the primary.

Free Field. A space free from reflections of any kind. Generally, an outdoor space or a completely reflection free room as found in an anechoic chamber.   

Initial Time Delay (ITD). The time difference between the arrival of the direct sound and the first reflection to the listener.

Live-End, Dead-End (LEDE). A listening room philosophy that sets the front half of the room as absorptive and the back half (relative to the listener) of the room as diffusive.

Reverb. Sound energy that builds up and gradually decays over time. Reverberant sound can be stored by a room for long periods of time as a result of the surfaces and dimensions in the room.

Reverberant Sound. Sound energy that reaches the ear after being reflected a large number of times. As a listener moves away from the sound source (in either time or distance) the sound energy level reaches a steady value known as the reverberant level.

Reverberation Time. The time required for a Reverberant Sound to decay by 60dB

Sabin. Unit of measuring absorption of sound. One Sabin is equal to one square foot of open window.

Sound Pressure Level (SPL). Extremely small periodic air pressure variations that our ears respond to. The minimum pressure fluctuation a human ear can respond to is 1 billionth (10-9) of atmospheric pressure.

Sweet Spot. The listening position where the best spatial imaging is present.