[Editor's note: Dennis Burger originally penned the primer Automated Room Correction Explained back in late 2013. Given the changes that have occurred--both to the audio landscape (like the arrival of Dolby Atmos and DTS:X) and in the room correction business itself--we decided it was time for an update. This article includes many of Dennis's original explanations and incorporates newly added information where appropriate.]
One of the realities of shopping for an AV receiver or surround sound processor these days is that a quick glance at specs, streaming features (or the lack thereof), and performance simply isn't enough to make the most informed purchasing decision. Now, more than ever, the sort of room correction software built into or supported by a preamp or receiver likely factors heavily into your choice of home theater (and sometimes even stereo) audio gear.
In the days of 5.1 or 7.1, that wasn't necessarily the case for all people; but, with the advent of object-based surround sound formats like Dolby Atmos and DTS:X--where you're likely either placing speakers in the ceiling that differ radically from the timbre of your main speakers or bouncing sound off the ceiling to achieve that overhead surround sound effect--automated speaker-setup features and room correction software are almost a necessity.
On the surface, all of the different auto-calibration systems--from Audyssey to Anthem Room Correction to MCACC to YPAO and newer solutions like Dirac--seem similar in implementation. Each requires you to place a microphone in your room (either in your main listening position or at several points in the room, depending on the sophistication of the system), which measures test tones played through each of your speakers, then uses those measurements for several different purposes.
First, most automated speaker setup systems will automatically adjust the relative levels of all your speakers so that the center, mains, surrounds, overhead speakers, and subwoofer are all playing at the same loudness. Most will also adjust the crossover point(s) between your satellite speakers and subwoofer, make an educated guess as to how far each of your speakers is from your main listening position, and adjust the delay settings in your receiver or processor accordingly, such that the sound from each speaker is reaching your ears at the same time.
Second, those measurements are used to determine what sort of deleterious effects your room's acoustics are having on the performance of your speakers. The systems then apply equalization to the speakers to "correct" issues with magnitude response.
The most sophisticated room correction systems will also then apply filters (either finite impulse response or infinite impulse response or, in some cases like Dirac Live, a combination thereof) in an attempt to correct for timing errors caused by your speakers and/or room. In other words, these filters are designed to compensate for the fact that sounds of different frequencies that are intended to reach your ears simultaneously don't do so, for one reason or another.
It's worth noting, though, that no amount of digital room correction can compensate for all of the acoustical problems in a room. To understand why, we need to break the audio spectrum into two different chunks and look at them separately (with apologies in advance to any acoustic experts in the audience for the gross oversimplification that follows). From 20 Hz to somewhere around 200 or 300 Hz, standing waves are your biggest enemy. Standing waves are an unavoidable byproduct of placing loudspeakers in an enclosed, rectangular space. At these low frequencies, sound waves reflected by the geometry of your room interfere with the sound waves coming directly from your subwoofers and/or speakers, resulting in an increase in volume at some spots in the room and a decrease in volume at other spots, with different dips and spikes at different spots depending on the frequency.
In other words, even if your subwoofer is creating low-frequency sounds at the same loudness level across the entire bass spectrum, a 50-Hz tone may be twice as loud in your listening position as it's supposed to be, whereas an 80-Hz tone may be half as loud. Switch to another seat in the room, and the opposite may be true (probably not exactly, but you get the point). And all of this is determined primarily by the size and shape of your room, as well as where the speakers are placed. Standing waves are also really tough (if not impossible) to combat with physical acoustic treatments alone, short of changing the architecture of your room completely, which makes digital room correction an ideal solution for dealing with them.
Or perhaps it would be more accurate to say that room correction is an ideal solution to at least half of that problem. While equalization can handily deal with nasty boosts in bass caused by standing waves, not all room correction systems can effectively deal with dips in bass caused by interference. In other words, if the size and shape of your room are causing, say, 80-Hz tones to nearly disappear at your favorite seat in the room, digital room correction isn't really your best bet for solving the problem. You're much better off moving your subwoofer, rearranging your furniture, or adding another subwoofer to your system--not to generate more bass, but to give you more even bass coverage, with one sub filling the nulls caused by the placement of the other. However, when it comes to combating the boomy, bloated bass caused by the reinforcement of standing waves (in other words, the crest of a reflected wave meets the crest of a direct wave right near your main listening position), many digital room correction systems can work wonders.
Above 200 or 300 Hz or so, your room's effects on sound reproduction have less to do with the size and shape of the space and more to do with the qualities of the surfaces within it--namely, how reflective, absorptive, or diffusive they are, and where they're located. Here, we're dipping into controversial territory because not everyone agrees that a digital room correction system based on microphone measurements and calculated target equalization curves can successfully deal with problems in these frequencies. As acoustic engineer and president of Performance Media Industries Anthony Grimani once explained to me, "An omnidirectional microphone does not listen the way a human being does. The main difference is that a human being hears high-frequency sounds as mainly directional, directly from the speaker, and low-frequency sounds as integrated from the speaker plus the room. And then between low-frequency and high-frequency, there's a splice where those different ways of hearing gradually shift from one to the other." Needless to say, a microphone doesn't hear things that way, and it takes a lot of processing power and some sophisticated filtering to make up for that fact.
Acoustic experts ranging from acoustic consultant Floyd Toole to Pro Audio Technology founder Paul Hales agree that room correction shouldn't be applied above this switchover point, primarily because our brains are perfectly capable of compensating for things like the difference between direct and reflected sound above 200 or 300 Hz. The way Hales explains it is that a great concert grand piano sounds like a great concert grand piano no matter where you put it; you don't EQ it differently for different rooms. Likewise, whether you and I are talking in an anechoic chamber or a tiled public bathroom, you recognize my voice as being my voice. That's why those of us who aren't overly enthusiastic about many digital room correction systems describe their results as deadening or dull.
That being said, the better digital room correction systems--those with a higher filter resolution and a smarter application of filters, and those that measure not only the relative loudness of different frequencies but also when those frequencies arrive at the microphone--can do a good job of combating some acoustic problems in the higher frequencies. They can't, though, effectively deal with things like long reverberation decay, at least not in my experience with setting up home theater systems for friends with very reverberant rooms. If you play guitar, you're probably well aware of the effect that a long decay can have on sound. If not, sit in your room and sing a few notes. If your voice sort of hangs in the air for a bit, what you're hearing is long decay. Or you can stand in the middle of your room and clap your hands. If it sounds like you're in a cathedral or parking deck, no amount of digital room correction can effectively compensate for that. You're going to need to invest in some absorptive acoustic treatments for your room, whether they be professional products or just some strategically placed draperies.
Good digital room correction can, on the other hand, combat ringing artifacts (a bit of echo accompanying high-frequency sounds). What's more, as I said earlier, the introduction of object-based surround sound formats like Dolby Atmos and DTS:X have made digital room correction more important (indeed, more of a necessity) than ever before.
Of course, not all digital room correction systems are the same, and not all of them deal with different acoustic artifacts in the same way. So it's worth discussing a few of the more popular (as well as a few of the more advanced) room correction systems individually. This isn't a comprehensive list by any means; the intent is merely to give you a selective overview of some of the different room correction systems you might come across when shopping for a new receiver or AV processor and how the market has changed over the past several years.
It may not appear on nearly as many products as it used to, but Audyssey is still probably the first name you think of when you think "room correction." Audyssey's platform appears in products ranging from Denon's mid- to entry-level AV receivers to Wisdom Audio's $6,000 SC-1 System Controller. Of course, both ends of that spectrum aren't covered by exactly the same Audyssey technology. The company's auto-calibration is available in several different flavors. Thankfully, its most basic solution, 2EQ, seems to have been phased out. You may recall from my review of Cambridge Audio's Azur 751R AV Receiver that I wasn't overly fond of 2EQ, which completely ignored the bass frequencies that most benefit from digital room correction and featured a limited number of filters for high frequencies.
The standard MultEQ solution improves things a bit, with twice the number of high-frequency filters and some much-needed correction for bass frequencies. But with all due respect to Audyssey, in my experience, I still find that MultEQ does more harm than good to the mid and high frequencies in anything approaching an acoustically acceptable room.
Move up to MultEQ XT, though, and now we're talking. With eight times the high-frequency filter resolution of vanilla MultEQ, XT allows for more precise equalization and, as such, sounds less processed, less lifeless, and more spacious. It also allows for eight measuring positions (compared with MultEQ's six). More measuring positions means that the system has a more comprehensive view of your entire listening space, allowing it to make more intelligent decisions about which problems it should and shouldn't try to correct. When reviewing a receiver with MultEQ XT, I tend to turn it off when listening to multichannel music, but I quite enjoy the effect it has on movies.
For most consumers, the step-up MultEQ XT32--usually only included with top-of-the-line receivers--is good stuff, indeed, with 256 times the high-frequency filter resolution of MultEQ and four times the bass filter resolution. Like MultEQ XT, it also benefits from eight measuring positions and, also like MultEQ XT, it gives professional installers access to MultEQ Pro, a kit that includes a professional-grade microphone and preamp, and allows for as many as 32 measurements in the room. MultEQ Pro also allows your installer to dial in customized target EQ curves better suited to your listening environment and preferences, rather than the one-size-fits-all target EQ curves of 2EQ, MultEQ, XT, and XT32.
These days, if you find Audyssey in a new receiver, it's most likely either a Denon or Marantz, who now group the room correction system together with other Audyssey technologies into four tiers labeled Audyssey Bronze, Silver, Gold, and Platinum. Bronze includes MultEQ, Dynamic Volume, and Dynamic EQ. Silver includes MultEQ XT, Dynamic Volume, and Dynamic EQ. Gold also features MultEQ XT, along with Dynamic Volume, Dynamic EQ, and Dynamic Surround Expansion. Audyssey Platinum bumps things up to MultEQ XT32, Dynamic Volume, Dynamic EQ, Dynamic Surround Expansion, Sub EQ HT, and Low Frequency Containment.
New Denon and Marantz receivers have also been upgraded to support the MultEQ Editor App for iOS and Android, which allows you to adjust your target curves and--praise be to the Baby Buddha!--set a maximum EQ frequency above which equalization is not applied. At the time of this writing, the $19.99 app for iOS and Android is still pretty new, so expect some bugs and kinks, but this is a huge step toward alleviating many of the concerns that people have with Audyssey.
My, how times have changed. When the original version of this primer went live nearly four years ago, Dirac was such a minor blip on my room correction radar that I overlooked it completely. Fast-forward to 2017, and it's not only one of the best-regarded room correction systems--it's also appearing on new gear at an ever-increasing rate. You can find the system on everything from Emotiva's XMC-1 and upcoming RMC-1 home theater processors up to DataSat's $23,000 RS20i, with offerings from Arcam, AudioControl, Lexicon, NAD, StormAudio, and Theta Digital in between.
Will we start to see it appear on more mass-market receivers anytime soon? Perhaps. One thing sets Dirac apart from the room correction systems found on most consumer units: the fact that it requires a Windows or OS X computer to run. It's also a bit more complicated than most basic room correction systems, in that it requires you to adjust input and output gain before running your measurements; and, if clipping occurs at any point during the process, you have to back out and make adjustments. The software also allows you to tweak your target curves by channel pair (or individually for the center speaker and subs) and easily set a maximum frequency past which no correction is applied.
As with Audyssey (but unlike other highly regarded room correction systems, such as Anthem Room Correction), Dirac Live works in the time domain, meaning that it doesn't merely tweak the equalization of your processor or receiver's output, but it applies filters that compensate for temporal distortion. Unlike Audyssey, which relies on Finite Impulse Response filters, Dirac Live uses mixed-phase filters to achieve superior time alignment or impulse response correction. Given this, and my further experience with (and understanding of) Dirac Live, I've recently changed the way I use the software. Until recently, I sculpted my own filters to apply correction no higher than 400 or 500 Hz. These days, I find that I get better results by applying filters that cover the entire audible frequency range, but I spend much more time sculpting them. Below 400 or 500 Hz or so, I let Dirac set its own target curve points. Above that region, I craft a curve that very carefully traces the broad strokes of my speakers' in-room response, while smoothing out any egregious wiggles. In other words, I'm maintaining the voice of my speakers while alleviating the harm done by my room, and I'm also applying temporal correction to the entire audible frequency range.
Given that it requires nine microphone measurement points that form a sort of elongated cube (yep, you have to raise and lower the microphone as you move it forward, backward, and side to side), as well as the fact that its measurement and filter-creation screens can be pretty densely packed with data and choices, Dirac Live isn't for the faint of heart--but it's currently tied for first place amongst my favorite room correction systems.
Anthem Room Correction
Also tied for first place? Anthem's proprietary room correction system. ARC is, of course, limited to use with Anthem preamps and receivers, as well as wireless music systems and soundbars from Paradigm and MartinLogan. As noted above, ARC doesn't work in the time domain, but it does, in my opinion, deal with standing waves better than any other room correction system--even Dirac.
In its fullest form, ARC requires the use of a Windows computer, and the microphone that comes with it is a beast compared with the little hat-wearing hockey-puck mics packed in with most room correction systems. The use of a computer provides ARC with much more processing power to take its measurements and create its filters. Perhaps due to that fact, ARC has never failed to perfectly nail crossover points and speaker levels in the dozens of times I've run it on both my D2v 3D processor and all of the company's receivers that I've auditioned.
What I really love about ARC, though, is that it allows you to make all sorts of decisions regarding crossover slopes, room gain, and of course maximum EQ frequencies. In my main home theater, with the Anthem D2v 3D, I've always found that a max EQ setting of roughly 400 Hz was ideal. In my secondary home theater in the back of the house, I set my Anthem MRX 1120's max EQ to 600 Hz so that the system can smoothly deal with a spike in that room at right around 500 Hz.
It should also be noted that, until the release of the second-generation MRX receivers, Anthem didn't give ARC different names for its different implementations. If you ran ARC on the D2v, it ran at its full capabilities; running ARC on the first-generation MRX 500 or 700 gave you the same measurements, but applied fewer filters. ARC also originally required that the connection between your PC and pre/pro or receiver come in the form of an RS-232 connection, which required the use of a USB-to-serial adapter for most users. With the introduction of the newer MRX 510 and 710 receivers, Anthem also debuted ARC M1 (now simply known as ARC 2), which benefits from more filters for the receivers (not as many as the D2v is capable of, but still more), along with speedier calibration and network capabilities. The company also recently introduced ARC for iOS devices, which may not benefit from quite the same quality of microphone and functionality but certainly delivers more in the way of intuitiveness and ease of use.
Trinnov calls its room correction system Acoustic Correction, and by all rights I should hate it. Uniquely, Trinnov works not only in the frequency and time domains, but in the spatial domain, as well. The microphone included with Trinnov is actually a microphone array that includes an arrow that must be pointed directly at your screen. During its measurements, the system maps your speakers' locations in three dimensions and can actually move the perceived location of one or more of your speakers to compensate for less-than-ideal speaker placement. It can take front left and right speakers that are spaced too closely together--or too far apart--and virtually relocate them to a pitch-perfect 22.5 or 30 degrees out from your center channel. It can even bring the image of your center speaker up to the level of your main left and right speakers if it's placed too far below (or, heaven forbid, above) your TV. The effect is downright spooky, and again--on paper--I absolutely hate the idea of it. But in practice, it's fantastic. I reviewed a Sherwood Newcastle receiver a few years back that included the Trinnov Optimizer and absolutely fell in love with the effect. The system in its newest form also features impulse and magnitude response correction on par with (or technically better than) Dirac Live, and it allows for all the sorts of target-curve-shaping tools I detailed in that system's overview, to an even finer degree.
There's one major caveat, though. Outside of select systems like JBL Synthesis' ultra-expensive SDP-75 processor, Trinnov Optimizer is only really available on Trinnov's own expensive 16-or-more-channel preamps.
Over the past few years, Onkyo and Integra have made the switch from being Audyssey licensees to introducing their own room correction system dubbed AccuEQ. Early implementations of the system were lacking, to say the least, since they applied no EQ to the front speakers or the subwoofer. Newer versions have improved greatly, though, and in its current form AccuEQ is actually pretty good. One thing that sets the system apart is that it runs two rounds of measurements. The first round is used to adjust levels, delays, and crossovers, whereas the second is used to calculate EQ curves. In my experience, unlike earlier implementations of AccuEQ, the newer version of the software actually performs most of its correction on the bass frequencies, for the most part leaving the upper mids and high frequencies alone. It does struggle somewhat to set crossover frequencies properly, though, so be prepared to tweak those manually.
MCACC (or Advanced MCACC) is Pioneer's proprietary room correction system, and it has improved dramatically over the past few years. It comes in three varieties these days. The basic, un-adjectived version of MCACC simply measures your speakers, balances the levels, sets your crossover, tweaks rudimentary EQ settings, and applies phase correction to the subwoofer. It doesn't, on the other hand, do anything to combat standing waves, since no equalization is applied to the sub. Advanced MCACC and MCACC Pro, on the other hand, do.
Advanced MCACC also adds phase control for each speaker, more advanced equalization (including sub EQ), and niceties like speaker polarity checks. MCACC Pro is the big kahuna, with such advanced features as Auto Phase Control Plus (filters that apply temporal correction selectively across the entire frequency range), independent dual-sub measurement and correction, advanced bass management for object-based surround systems, and a Precision Distance tool that allows you to adjust speaker distances (and hence delays) at the millimeter level.
I'm purely speculating here, but I have to wonder how much longer MCACC is going to hang around. I mention this not because Pioneer is any sort of trouble, nor due to any deficiencies in MCACC as compared with other proprietary solutions from Onkyo, Yamaha, Sony, and the like. This speculation is purely based on the fact that Pioneer recently developed a working relationship with Dirac, whereby the former actually serves as the distributor of the latter's software in Japan.
YPAO (Yamaha Parametric Room Acoustic Optimizer)
YPAO, as you may have guessed from its name, is Yamaha's proprietary room setup system. YPAO comes in two varieties: one is simply called YPAO, and the other is called YPAO R.S.C. The former measures speaker levels and delays and applies parametric EQ with varying degrees of success, whereas YPAO R.S.C. also applies impulse response filters to combat reflected sound. More recently, Yamaha also added the ability to tweak your own parametric EQ settings on YPAO R.S.C., with the impulse response filters being copied over from its own calculated settings.
You'll likely only find YPAO on the lowest-priced AVRs from Yamaha these days, with YPAO R.S.C more commonly found at mid levels and up. If you're in the market for a Yamaha receiver, there is one more crucial thing to consider: YPAO R.S.C comes in two varieties--one with multipoint measurements and one that measures from only one seating position. The latter doesn't do a very good job with levels and distances, in my opinion, so it's probably worth it to step up to a model with multipoint measurements unless you plan to set up your receiver completely manually.
Digital Cinema Automatic Calibration (DCAC)
Another proprietary room correction system found on mass-market receivers is Sony's DCAC (which comes in varieties such as DCAC EX and ADCAC). This one is, quite frankly, a bit of a mystery to me, since Sony doesn't publish much in the way of clear information about what it does or how it works. In my experience, it's hit or miss, delivering results that are either subtle or bad, and its speaker level and distance settings always need some tweaking. What's more, the new dual microphone that ships with units like the STR-ZA5000ES can deliver very inconsistent measurement results from run to run, even with the microphone placed in the same position. The one major upside to DCAC is that it seems to do very little to the signal above 500 Hz or thereabouts. The downside is that sometimes its automatically calculated settings can actually exacerbate standing wave problems, which completely defeats the purpose. When it comes right down to it, the one thing DCAC could really benefit from is multiple measurements, but all of the Sony receivers I've reviewed thus far only allow for one microphone position.
Room EQ Wizard
A popular solution for DIYers, Room EQ Wizard (or REW) isn't for the faint of heart; but, if you have a receiver that allows you to dial in your own parametric EQ settings and you don't mind digging deep into the weeds, it delivers pretty amazing results. REW does require that you purchase your own USB audio interface (the PreSonus AudioBox is a pretty popular choice, but others like the Tascam US-2 2 will work great, as long as you have phantom power), as well as your own calibrated microphone (like the Behringer ECM8000 microphone, if you're looking for an affordable option), but the nice thing is that the software itself is free. Again, this can be a daunting option at first, since the software doesn't hold your hand in the slightest, but there are tons of great video tutorials out there, as well as extensive help files on the REW website.
Perfect Bass Kit
Paradigm/Anthem also has another proprietary room correction system called the Perfect Bass Kit, which as you may have gathered only works on Paradigm and MartinLogan subs. Think of it as a simplified version of ARC (with simplified connections, too; it includes the same USB microphone as ARC and only requires a USB connection from your computer to the subwoofer). The great thing about PBK is that it applies room correction where your room needs it the most--in the bass frequencies. The downside, of course, is that this only applies to the subwoofer channel, which is probably crossed over at 80 Hz by your processor. So it won't catch problems in the main channels, which also generate bass between the crossover frequency and that critical 200- to 300-Hz point where standing waves can plague upper bass performance.
Sunfire Room EQ
Sunfire doesn't have a fancy name for the room EQ system included with many of its subwoofers, but it doesn't need one. (I've run it with excellent results on both the AtmosXT in my bedroom and the SubRosa Flat Panel Subwoofer in my home theater.) It's quick. It's simple. It gets the job done and deals with most of the acoustic problems in both of my rooms. But, as with PBK, it can only apply correction up to the crossover point between your subs and satellites.
Of course, as I said, this list overlooks many other notable room correction systems, from HARMAN's ARCOS to Lyngdorf's RoomPerfect to Meridian's excellent MRC, along with other bass-only solutions like Velodyne's SMS-1. So, if I've left out your favorite, let us know in the Comments below.
Do you love what room correction does for your sound system, or do you absolutely loathe it? Or does your relationship with room correction, like mine, fall under the fabled Facebook category of "It's Complicated"?
• AV Receivers vs. AV Separates: Which Is Right for You? at HomeTheaterReview.com.
• The Good, Better, and Best AV Receivers on the Market Today at HomeTheaterReview.com.