|Sound Tips 6|
|Home Theater Feature Articles Audio Related Articles|
|Written by Bob Hodas|
|Saturday, 01 February 2003|
AV Education on RHT
Sound Tips 6
By Bob Hodas
This month we are going to finish looking at my room. In Sound Tips 4 & 5, we addressed my speaker placement and acoustic treatments. We are now going to look at the final issue, the icing on the cake, equalization.
Now, I know that word, “equalization,” strikes fear and loathing into the hearts of many audiophiles, but hey, let’s get real. If you have a home theater, you should know that almost every commercial movie theater in existence uses EQ (equalization). Every dubbing stage (film mixing studio) uses EQ. All of the places doing high-end film audio remixes for DVD release, like Mi Casa Multimedia (which works with New Line on films such as “The Lord of The Rings” trilogy), uses EQ. In fact, the Mi Casa guys are so tweaky about the sound in their rooms that I have a lifelong gig several times a year making sure the rooms stay in tune.
So why, after all the trouble we went to, properly placing our speakers and treating the room, would we need EQ? Basically, most rooms we listen in are imperfect, even with acoustical treatment. There are rare cases where I set up rooms that did not require EQ after the physical work was finished, but those are one in a hundred. I do want be clear: I do not believe in the indiscriminate use of EQ. I’m against adding more electronics into the listening chain unless it improves significantly more issues than it creates. However, most rooms that were not designed for audio from the ground up have some problem or other that is not treatable with just acoustic fixes. These problems would include some situations that exist in my room: open walkways, asymmetrical walls, asymmetrical furniture placement, etc. Then there is the problem of dealing with bass in a fashion acceptable in most living spaces. Treating bass problems requires more space than most home owners are willing to sacrifice and there is the nagging issue of the aesthetics of a bass trap. A well-built minimum phase parametric equalizer can solve low-frequency problems cost effectively and with no aesthetic intrusion in a room.
Many subwoofer manufacturers have recognized the above facts and are now building EQ into their products. Even high-end companies like Meridian are putting EQ and room correction as features in their AV preamps. The age-old audiophile arguments against using EQ thankfully seem to be finally fading away.
There are two styles of equalizers currently utilized for room tuning. These are third octave (fixed frequency centers and bandwidth) and parametric (adjustable frequency centers and bandwidth). I maintain strong opinions on this subject, backed up by science and listening. I recommend minimum phase parametrics over third octave equalizers.
To date, most equalizers utilized for tuning rooms have been third octave models. At the time of their origin, people actually believed that humans could not hear bandwidth narrower than third octave, so those EQs were applied to room tuning in professional audio. We now know that human hearing is much more sophisticated. The fixed nature of the third octave equalizer is its primary limitation. In my opinion, third octave EQ is now "old school.” With third octave EQ, “You can’t get there from here.”
Room problems are not well-behaved phenomena that follow fixed patterns. A parametric equalizer allows you to dial in the exact center frequency to address the problem. Then it allows you to shape the curve (bandwidth) to give a proper fit solution. Third octave is simply hit or miss on the center frequencies and brute force with its fixed bandwidth. We end up equalizing more than necessary or often can't get to the problem at all with third octave equalizers. Parametric solutions just make sense. Figures 1 & Figures 2 demonstrate the point. (Note that the green equalizer curve displayed is the inverse of the equalization that is being applied. This simply makes it easy to see how it fits into the room curve.) Figure 1 shows the low end of a room curve with a third octave solution applied to it. Notice that you can't quite get to the problem and wind up affecting more frequencies than necessary, especially around 150Hz. Figure 2 is the same room with a parametric solution. The parametric exhibits a much better match.
In the past, it was difficult to design parametrics that did not introduce extraordinary phase shift, but today many designers have this situation solved. Some room equalizers of modern design even utilize minimum phase filters. Minimum phase frequency response anomalies occur when speakers are placed in proximity to boundaries such as walls, ceilings or soffets. Minimum phase filters allow for more latitude to boost or cut without hearing that excess phase shift. It has also been proven that they can restore the impulse response of a speaker affected by minimum phase problems.
Equalizable room/speaker interaction is a minimum phase, second order phenomenon, exhibiting constant bandwidth and linear frequency spacing. At best, third octave equalizers are constant percentage bandwidth with logarithmic frequency spacing. Figure 3 demonstrates the type of frequency response comb filtering caused by a one-millisecond echo. In practical terms, this is typical of a reflection from a coffee table sitting in front of the couch. Note that the combing is not logarithmically spaced (e.g., third octave), but rather a constant bandwidth of 1kHz. The frequency centers of the comb are an octave wide from 1kHz-2kHz and 1/10 octave wide from 10kHz-20kHz, meaning that a device with fixed third octave bandwidth cannot create a complement. They also fall on frequencies that often do not match the fixed ISO standard frequency selections of a third octave equalizer. Once again, parametrics give you the versatility to address the problem as it exists. Parametrics also allow you to utilize a single filter to address an overall trend in the response, while with third octave, several filters must interact to address this same issue. I am not advocating equalizing the coffee table reflection; this is simply a practical demonstration of the problem.
Part of the blame for using third octave equalizers for so long must be placed on the fact that the industry primarily used third octave analyzers. This made it a simple match-up for tuning and is quite understandable. But third octave analysis does not offer enough resolution to actually see the whole story. There are now a number of affordable analyzers on the market that have much higher resolution. My SIM System crunches the numbers at 1/48th-octave resolution. Look at Figure 4 and Figure 5. Figure 4 is a third octave mid-band shot of a room. Notice that there appears to be a hole from about 500-800Hz (highlighted area). This would lead one to believe that some boost centered at 630Hz on your third octave equalizer would fix the problem. In actuality, viewing the 1/24th octave resolution of Figure 5, we see that this is not a wide band hole but a series of tight combs that should probably not be equalized. The third octave analyzer averaged this section out in a broad stroke. This simplified but accurate picture is the rule rather than the exception. I strongly suggest that you look at your rooms with effective resolution if you want to correctly solve the problems.
While many people understand the need to "tune up," there is quite a bit of confusion about maintaining the system tuning. There are several contributory factors as to why a system needs to be checked periodically to stay in tune. Some acoustic factors are relatively easy to understand. If wall treatments or furniture have been added, then you have changed the linearity of the system response, as well as the room's reverb structure. The same would apply to construction, such as adding an equipment rack or anything that changes wall angles, room volume or mass.
Changing electronic components can also make a big difference in linearity. New amplifiers may handle your speaker impedance or wire capacitance differently and cause changes in linearity. I have witnessed changes of several dB in different parts of the frequency spectrum when clients switched between solid state and tube amps. Of course, changing crossovers applies as well. Filter order and the crossover point itself will have an effect on the system. Of course, with any of these changes, attention must be paid to overall system polarity.
Any time you change a speaker component, you should check the system response. I have seen fluctuation by as much as 3dB across the range of a speaker when an old element was replaced with a new one. Sensitivity will vary and many manufacturers do not have a tight grip on quality control in this department.
The above examples are fairly clearcut, as they are actual physical changes that are easy to identify. Now we enter into a gray area. The most asked question I hear is, "How often should I adjust the EQ in my system?" It is a physical fact of life that, as speakers age, their resonant frequencies change from fatigue. These resonances need to be adjusted over time, as the tuning will "drift" out. How long that takes depends on how often and loud a system is driven. Moderately-driven systems can often go years before changes in linearity occur. Some clients, like busy recording studios that operate their speakers 24 hours per day and listen often at loud volumes, may need attention after six months.
So getting away from all the tech talk, how did my room finally turn out? Figure 6 shows my little EQ rack. I am using five channels of the Meyer CP-10 & 10S EQs for the Meyer HD-1 speakers on L/C/R/LS/RS. I use the EQ built into the Revel B-15s for the LFE channel. Figure 7 shows the correction I applied to my right speaker. This is fairly typical of the correction performed in my room.
After equalization, I heard several significant improvements. The bass was much tighter and better defined. The frequency spectrum was more linear, which equates to no musical notes missing or outstanding; the musical spectrum was smoothly connected from top to bottom. Imaging improved as well, because now the speaker frequency responses were extremely well-matched. Not only was it easier to pick out individual instruments in the soundstage, but the center image was rock solid as well. No more voice that sounds three feet wide.
Between the use of parametric equalizers and high-resolution analysis systems, there exist good solutions to address room/speaker interface correction. These solutions can significantly upgrade the quality of the listening environment, leading to a more satisfying and involving listening experience.
Bob Hodas tunes many of the worlds finest recording studios and mastering labs as well as many of the best private home theater and music playback systems. His clients include: George Lucas, Abbey Road London, Sony Music Tokyo, Paul Stubblebine Mastering and many more http://www.bobhodas.com/clients.html.
Based in Berkeley California, Hodas travels the world to tune audio systems, designs acoustically excellent rooms and implements acoustical treatments. Bob is available for consultation as well as in-home tunings starting at $500. To contact Bob Hodas, email firstname.lastname@example.org or call (510) 649-925