One of the most commonly misunderstood concepts you’ll face when you start putting together a home theater or high-end listening room is the role absorption and diffusion play in how your room will ultimately sound – 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 those of us not in the know kind of grin and nod our heads while hoping that maybe someday we’ll figure exactly what the heck all of these smart people are talking about.

Remember back in college when you really didn’t put any effort into reading a chapter you were supposed to read because you knew you could just go to the end of the chapter and read the glossary so you could acquire the minimum level of knowledge needed to keep you GPA from tanking completely? Me too. (Ed. Note: For you kids today, that was before Google)

That’s why before we start our three part look at the roles absorption and diffusion play we’re going to define the terms we’ll be using.

 

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), or to express the ratio of a quantity to a fixed and appropriate reference.

Diffusion – The mixing of sound in a room, making it more uniform or homogeneous in total response.

Direct Sound – Sound that reaches the listener directly from the loudspeaker or source without being reflected first.

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 by the human brain 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 actually be stored by a room for occasional long periods of time as a result of the surfaces and dimensions found in a space.

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.

Check out this video from Acoustic Geometry on how sound works in a room. There are many manufacturers or high-end acoustic treatments and this isn't a direct endorsement of Acoustic Geometry products, but it is the best and easiest to understand video on the subject I've seen to date.

 

 

Jack Sharkey for KEF