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Room acoustic issues 101
by Nyal MellorApril 17, 2016
The characteristic acoustic regions of rooms
Different parts of the frequency spectrum have different room acoustic issues or distortions. Acoustic distortions can be defined as sound quality degrading issues introduced by the interaction of a sound source such as a speaker and the acoustic environment it is placed in. At Acoustic Frontiers we believe there are three main regions that are relevant to the types of rooms used in residential settings for high performance audio reproduction:
"Sparse" room mode region
"Dense" room mode region
Speaker off axis region
"Sparse" room mode region
In this region the response typically exhibits wild swings and dips, with 15dB or more of SPL variation being common. The response is dominated by room modes that are spaced far enough apart from each other that they do not interact. For example there might be a room modes at 25Hz, 45Hz and 60Hz. Each one of these is far enough away from the next that a noticeable dip in the response occurs between each one. The room modes in this region are also the most powerful of all room modes, being the major axial and tangential modes. This area is hard to improve through acoustic treatment, since the wavelengths of sound are very long. Typical commercial bass traps, with a few exceptions like the RPG Modex Plate and Edge, become increasingly ineffective under 100Hz. Our approach to dealing with this region involves:
Room dimension optimization, to space the room modes as consistently apart as possible
Structural optimization (yes, the wall, ceiling and to a lesser extent the floor can act as "bass traps")
Speaker / listener position optimization, to change the interaction with the room modes
Multiple subwoofer arrays, to drive room modes destructively
Equalization, to flatten the response and reduce ringing in the time domain
This region is still dominated by room resonances but the individual modes are spaced close enough together in frequency that they interact. It's generally much easier to deal with issues in this frequency range than in the "sparse" region. Our approach to this area includes:
In this region the response at the listening position in two channel listening spaces and home theaters is dominated by the off axis response of the speaker. On the face of it, this seems "crazy"...surely the direct sound dominates? In recording studios or other nearfield listening situations the direct sound does dominate, but as Harman research shows the frequency response at the listening position above the room’s transition frequency can be predicted from the anechoic measurements of the speaker by combining the data together as follows: 14% direct sound + 44% early reflected sound + 44% late reflected sound.
Speaker off axis response - we've written a comprehensive set of articles on speaker directivity that includes an examination of coaxial, cone / dome, constant directivity and some other designs
Early reflections- this article looks at the acoustics of "single bounce" reflections and the Energy Time Curve, the acoustic measurement most commonly used to examine these phenomena
Late reflections - this article looks the sound that arrives at your ear after multiple bounces. It's more frequently known as reverberation, and often characterized by an acoustic measurement known as RT60.
Other acoustic distortions
There are other types of room acoustic issues that can occur at any frequency.
“We built this home theater so I could watch all of my favorite movies. It’s just a better experience overall. I wanted it to look better than anything possible, other than what you can see at the studio." Travis C.