Room Correction: A Primer

by Nyal Mellor July 01, 2010

Introduction

It has been known for many years that the room we listen to music in is a significant arbiter of the sound quality we enjoy. A number of different approaches have been tried over the last 40 years to remove the negative influence of the room on the quality of sound reproduction achieved from a given set of electronics. In the early days crude frequency response equalization was performed through application of analog parametric equalizers or graphic equalizers. These devices were typically not designed for high quality home audio reproduction and so gained a negative reputation amongst discerning audiophiles (whether stereo or multichannel) and home theater enthusiasts. Over the last 20 years the increasing availability of cheap and powerful digital signal processing (DSP) hardware has led audio companies to attempt to design and implement ‘digital room correction’ (sometimes abbreviated to DRC and also known as room equalization / EQ) 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. Audyssey, a company that primarily licenses their DSP code to other manufacturers such as Denon, have been responsible for the rapid penetration of room correction into the home theater market. It is probably fair to say that digital room correction is now more embedded in the home theater than it is in audiophile systems, a likely consequence of manufacturers trying to ‘out feature’ each other in the surround sound processor market.
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. 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.

The conclusion I have reached from my study and from its application in my client’s systems is that room correction can be a valuable tool to address certain acoustical problems, particularly those related to room mode resonances. There are many things 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 calibration (e.g. speaker/listener positioning) techniques.

The first task for you on this journey is to:

• Learn a vocabulary for describing sound quality through the five sound quality metrics: clarity, focus, envelopment, dynamics and response. Here.

• Understand the science behind how a room behaves at different frequencies; above and below what is termed the ‘transition frequency’ at 200Hz. Here.

• Relate acoustical measurements to the typical major issues a room introduces and how these impact the five sound quality metrics. Here.

It is important to take the time to study these topics, since the remainder of the discussion on room correction assumes familiarity with the terminology and language used.

How a room correction product works

The process used is, at a high level, the same for nearly all types of room correction products:

1. Play back a known signal and measure the magnitude versus frequency response of the system (i.e. equipment, speakers AND room).
2. Compare the measured magnitude versus frequency response to a target magnitude versus frequency response.
3. Create correction filters to match the measured to the target responses.
4. Apply the correction filters to the music signal.

An example will help to illustrate this. At 60Hz the measured response of a nominal system is 78dB. A target has been set of 75dB. The measured response is therefore +3dB louder than target, and a correction filter of -3dB is needed. An additional example is shown in the table below; all figures are in dB:

Frequency	Measured Response	Target Response	Difference	Correction Filter
60	78	75	+3	-3
1000	73	75	-2	+2

An illustration of what occurs in a room correction product; the measured frequency response has been compared to a target and a correction filter calculated.

Different products may have sub-processes within each of the four major steps or may implement the process differently:

• Audyssey digital room correction products, for example, use a swept sine signal during the measurement step. They require that the user takes multiple measurements at different spatial locations within the listening room. Correction filters are generated by finding a solution that will be effective for all locations.
• The Rives Audio PARC, which is a product using analog parametric equalizers, requires the user to generate and play a test signal such as pink noise, measure the results on a separate analyzer and calculate the required filters before manually setting up the product. There is no automated filter generation process.
• Velodyne subwoofers provide an automated pink noise based measurement and filter generation system using a measurement at a single location.
• Some products (e.g. TACT) allow the user to ‘draw’ their own frequency response target. A couple of others (e.g. Audyssey) provide a limited selection of choices. Many however use a simple flat frequency response as the target.

So whilst you can see that different manufacturers implement the process in their own unique way really we are looking at a single simple and understandable high level process.

The only exception to the use of magnitude versus frequency response in the measurement and comparison steps (1 and 2 above) is Meridian. The Meridian digital room correction system follows the same high level process (measure, compare, generate and then apply correction filters) but utilizes the reverberation time versus frequency response of the system. The system analyzes the deviation of the measured reverberation time from a target. Since the Meridian system only operates below 200Hz (i.e. below the transition frequency) it can take advantage of the minimum phase behavior of the room in this region, allowing it to correct reverberation time (or more correctly modal decay time) through magnitude based correction filters.

What room correction can fix

Let’s start with a statement: room correction products work by fixing problems in the frequency domain. Many of the issues with getting good sound in small rooms are time domain issues and therefore cannot be fixed (with one important exception) except through good acoustic design and correct usage of acoustic treatment.

Below the transition frequency room correction can fix:

• Bumps in the frequency response caused by modal resonances. This is a simple case of application of corrective filters to match a measured frequency response to a target. Room correction can be very effective at reducing the sound quality issues associated with room modes. Bass traps generally start to lose effectiveness at 100Hz and therefore a large number of them have to be used to have an impact on room modes at very low frequencies. Unfortunately it is exactly these frequencies below 100Hz where room modes are most problematic. This is because they are generally spaced quite far apart in frequency (therefore being more audibly prevalent). The region below 100Hz is often called the ‘sparsely populated modal region’. Generally speaking it is very difficult to effectively treat a room mode at 60Hz with acoustic treatment alone. In these circumstances room correction is the best answer. Room correction can also provide additional smoothing between 100Hz and the transition frequency (around 300Hz) and so is a compliment to bass traps in this frequency range. As Floyd Toole said in the excellent Home Theater Geeks podcast: ‘In any room that I have encountered I’d have to say that equalization at low frequencies always improve the bass quality’.
• Ringing in the time domain caused by modal resonances. As Floyd Toole states: “Room resonances at low frequencies behave as “minimum phase” phenomena, and so, if the amplitude vs. frequency characteristic is corrected, so also will the phase vs. frequency characteristic. If both amplitude and phase responses are fixed, then it must be true that the transient response must be fixed – i.e. the ringing, or overhang, must be eliminated” (Toole, The Acoustical Design Of Home Theaters, 1999). There has been some debate about this fact, but doubters please reference the paper which contains measured evidence of the efficacy of magnitude based equalization in reducing time domain ringing (see Fig 3, Pg 8 in particular, although Toole’s entire discussion of equalization in this paper is worthy of your time and attention).

So in conclusion we might say that room correction is great at fixing room mode related issues. There is one caveat, however, which is that it is important to understand that the correction filter generated applies equally to all places in a room. Room modes, by their very nature, are caused by standing waves which means that frequency response varies significantly across different points in space. Some seats in a room may exhibit a 10dB peak at a particular frequency whereas another seat may have a 5dB dip. If we apply a -10dB correction filter to solve the problem at seat 1 then we have made the problem at seat 2 much worse. For two channel rooms, where there is only one seat we care about, this is not an issue. But for home theaters, where we have multiple seats that we want to sound good, it is important first to reduce seat-to-seat frequency response variability by using bass traps and multiple subwoofers to destructively drive room modes.

Above the transition frequency room correction can fix:

• Variation in frequency response. It is important to understand that above the transition frequency most of the correction filters that would be generated by a room correction product are actually correcting the speaker’s power response (i.e. the frequency response at the listening position). This correction should therefore be termed speaker rather than room correction. Moreover, it should be left to the user’s discretion whether they want their speakers to be corrected or not (i.e. for the room correction product to generate and apply filers above the transition frequency). Correction will change the acoustic signature of the speaker in ways that the original designer did not intend or voice for and may therefore be undesirable.
  
  

What room correction cannot fix

Below the transition frequency room correction cannot fix:

• Speaker boundary interference, since these are caused by destructive interference of the direct and reflected wavefronts. Using a positive correction filter 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. The cancellation depth in dB can only be addressed through use of absorption, 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 in dB expected at the null will also be reduced by 50%. The frequency that SBIR occurs at can be varied by movement of the speaker and listening positions. Crossing over a speaker system to a subwoofer and placing these in corners will effectively remove SBIR effects, since there will be no difference in the direct and indirect path lengths.

Above the transition frequency room correction cannot fix:

• Strong early reflections. The target for this metric, which is that all reflections be 10dB lower in magnitude relative to the direct sound, can only be met through appropriate use of absorption or choosing speakers designed to control directivity (i.e. waveguides, dipoles, horns).

 

 

• Long decay times due to lack of acoustical absorption.
  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).

 
Conclusion

We hope you have found this article educational and that you now have a good understanding of the acoustical problems a room correction product can and cannot fix. Room correction can be a useful tool in combating typical problems found in a home listening environment, particularly those related to room mode resonances which cause audible bumps, resonances and other bass issues. It is not, however, 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 suckouts. Nor can it solve long reverberation times and strong early reflections which reduce our ability to locate a sonic image within a soundstage. These issues can only be solved by good acoustic design, treatment and calibration.

Employing the services of a suitably knowledgeable professional to measure your room, determine its unique problems and work with you to specify appropriate solutions is much more cost effective in terms of sound quality improvement per dollar spent than buying a room correction product off the shelf and blindly applying it in the hope of a miracle. If you cannot afford the services of a professional then the only real option is to learn the science of acoustics, buy yourself a measurement rig capable of taking measurements and experiment! In a future article we will outline a set of functional criteria for a room correction product and complete a survey of the products on the marketplace against these criteria. Stay tuned!

Did you realize that room correction can’t fix all the acoustical problems of your listening room? Let me know your thoughts using the ‘post a comment’ feature!

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