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Room acoustic issues 101
by ct_acoustic_frontierJuly 06, 2022
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
There isn't really a clear transition from one region to the next, and the frequency across which the transition occurs also varies depending on room size. Room modes occur to higher frequencies in small rooms relative to large rooms. See the Transition Frequency article for more information.
The three regions each have different acoustical characteristics, which helps with analysis and solutions to room acoustic issues.
"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.
Much confusion still exists about what a room correction product does, what problems it can (and cannot) solve and therefore its 'place' in a modern high quality sound reproduction system. Part of the challenge of understanding room correction is that it requires a reasonable level of understanding of sound quality, acoustic science, acoustic measurement and psychoacoustics (how humans perceive sound). The majority of the articles I have read online or in print magazines do not cover the fundamentals in enough depth to allow the curious and committed reader a chance to understand room correction on anything more than a cursory level. By the end of this article I hope that you will have learnt enough to judge for yourself what room correction can and cannot do and how best to apply it in the context of a world class music or home theater system.