Three Acoustical Issues that Room Correction Can’t Correct
Over the last 20 years, the increasing availability of cheap and powerful digital signal processing (DSP) hardware has enabled many audio companies to introduce “digital room correction” (sometimes abbreviated to DRC) devices. The first company to release a product in this field was SigTech, who were followed by a number of other companies – most notably TacT, who can probably be credited with introducing room correction to the two-channel audiophile market. In the last few years products like the DEQX have gone beyond mere room correction into full speaker management, incorporating driver correction and digital crossovers.
Despite these trends, 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. This article introduces three fundamental acoustical issues that exist in most listening rooms and home theaters that room correction cannot – for want of a better word -correct. These acoustical issues can only be addressed through good design, appropriate use of acoustic treatment, and appropriate system setup techniques (e.g. speaker placement).
Issue 1: Speaker Boundary Interference (SBIR)
Speaker boundary interference response (SBIR for short) is a little-known and poorly-understood issue that is responsible for deep dips or suck-outs in bass response below the transition frequency. SBIR is caused by the destructive interaction of the direct sound wave from the speaker and the reflected, indirect sound wave from a nearby boundary. If the difference between the direct and indirect path lengths (where path length is the distance the sound has to travel) is equal to half a wavelength, then the two sound waves will combine destructively and a notch in the frequency response will occur. The frequency that interference occurs at can be calculated through application of the wavelength formula as follows: cancellation frequency = speed of sound / (2 * path length difference). All of the boundaries in a small room can cause interference and audible suck-outs – the ceiling, front wall, back wall, side walls and floor.
Using a positive correction filter (for example, a 6 dB boost at 79 Hz) to attempt to remove this cancellation will not be effective, since the increased strength of the direct wave will be met in turn by the same increase in strength of the indirect wave. Reducing the magnitude of the cancellation depth can only be addressed through use of absorption at the reflection point on the boundary in question, which will reduce the magnitude of the indirect wavefront and therefore the amount of cancellation. If the magnitude of the indirect wavefront can be reduced by 50% then the cancellation expected at the null will also be reduced by 50%. The frequency that SBIR occurs at can also be varied by movement of the speaker and listening positions. Crossing over a speaker system to a subwoofer at 80 Hz, positioning the mains more than four feet from any boundary and placing the subs in corners will effectively remove SBIR effects, since there will be no difference in the direct and indirect path lengths for the subs, and the cancellation frequency for the mains will be below the crossover to the sub.
Issue 2: Strong Early Reflections
When we listen to music in a small room what we hear is a combination of the direct sound from the speaker and the multiplicity of reflections from the surfaces of our room. As Benade (From Instrument to Ear in a Room, Journal of the Audio Engineering Society, 1985) states:
“The auditory system combines the information contained a set of reduplicated sound sequences (i.e. the direct sound and its reflections) and hears them as if they were a single entity. The singly perceived composite entity represents the accumulated information about the acoustical features (tone color, articulation, etc) shared by the set of signals. It is heard as though all the later arrivals were piled upon the first one without any delay.”
In small rooms the contribution of reflections to the sound we hear can be as much as 60%. What we hear from a sound quality perspective in a small room is therefore determined as much by the reflected sound as the direct sound. The reflected sounds arriving at the ear are known to cause perceived changes in tonal color and increase the width and spaciousness of the soundstage. Because of these effects, the generally accepted target amongst acousticians is that all reflections be 10 dB lower in magnitude relative to the direct sound. Note that a sound at minus 10 dB is perceived as being around half as loud.
It can be stated categorically that a room correction product cannot reduce the percentage of sound reflected from a boundary. If a correction filter of -3 dB is applied, then it is true that the level of the direct and reflected sound will both be reduced by -3 dB, but the important thing is that the ratio of direct to reflected sound will still be the same. The level of reflections can only be controlled through appropriate use of absorption or diffusion or by selecting speakers designed to control directivity (i.e. waveguides, dipoles, horns). Some examples of these speakers are those from the companies Emerald Physics, GedLee and Gradient, which are all designed to control dispersion and reduce the amount of sound energy reflecting from adjacent boundaries.
Issue 3: Long decay times
Decay time is more widely termed reverberation time and is typically defined by the “RT60″ measurement, or the time taken for the sound level to decay by 60 dB. From a purely scientific perspective the use of RT60 in small rooms is not valid since the science and reasoning is based on the assumption of a “statistically diffuse” soundfield, which is not a characteristic of a small room. From a practical perspective, however, RT60 is a good indicator of whether reverberation time is in the ballpark for high-quality sound reproduction. An experienced acoustic consultant or sound engineer will be able to detect an overly live room by merely speaking or clapping their hands and listening to the decay of the sound field. The target RT60 is different for two channel audio and home theater. Two channel audio needs a room with longer decay times to create the illusion of envelopment, since there are no surround speakers to assist in this regard. Excessive reverberation, even when obvious echoes cannot be detected, audibly reduces a system’s retrieval of critical acoustic cues such as the decay character of a recording studio. In addition low-level detail, micro-dynamics and tonal color all suffer if sound takes too long to decay within a room.
The decay time of the room can only be brought to the level needed for high quality sound reproduction through appropriate decorative choices, furnishings and the use of acoustic treatment such as absorbers. As Art Noxon, founder of Acoustic Sciences Corporation states “If you have no acoustic treatments in the room and you have a DSP processor, what happens to the articulation in the room? You’re still injecting energy into the room. You still have reverberation time. You still have the lack of intelligibility that you had before…DSP doesn’t address the decay rate factor of rooms, and neither does equalization. There is no electronic sound absorber for sale.” (TAS Roundtable, Room Acoustics: Audio’s Final Frontier, The Absolute Sound, 2004).
I hope you have found this article educational and that you now have a good understanding of the three acoustical problems a room correction product cannot fix. Room correction is not a magical cure-all that can rid us of the sound quality ills given to us by our rooms. It cannot solve phase interaction issues such as speaker boundary interference that cause audible bass suck-outs. Nor can it solve long reverberation times and strong early reflections that degrade imaging, sound staging and clarity. These issues can only be solved by good acoustic design, treatment and system setup techniques.
What do you think about the role of room correction in a modern high quality sound reproduction system? Did you realize that room correction can’t fix all the acoustical issues of our listening rooms?
Nyal Mellor, Acoustic Frontiers