If you've purchased a new AV receiver or surround sound processor in the past seven or eight years, chances are good that it included some form of auto setup and/or automatic EQ (also known as room correction.) On the surface, all of the different auto-calibration systems - from Audyssey to ARC to MCACC to YPAO - 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 two different purposes. Firstly, the software will automatically adjust the relative levels of all your speakers, so that the center, mains, surrounds, and subwoofer are all playing at the same loudness. It sets the crossover point between your satellite speakers and subwoofer, and it makes an educated guess as to how far each of your speakers is from your main listening position and adjusts the delay settings in your receiver or processor accordingly. Secondly, it looks at those measurements to determine what sort of deleterious effects your room's acoustics are having on the performance of your speakers, and then applies some mixture of equalization and/or filtering to compensate.
It's worth noting, though, that no amount of digital room correction can compensate for all of the acoustic 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 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 space. The reflected sound waves from your walls interfere with the sound waves coming directly from your subwoofer 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 50Hz tone may be twice as loud in your listening position as it's supposed to be, whereas an 80Hz tone may be practically nonexistent. 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, which makes digital room correction an ideal solution. Well, it's an ideal solution to at least half of that problem. While equalization can handily deal with nasty boosts in bass caused by standing waves, most room correction systems cannot effectively deal with dips in bass caused by interference. In other words, if the size and shape of your room are causing, say, 80Hz tones to fall into a black hole 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. But, when it comes to combating the boomy, bloated bass caused by the reinforcement of standing waves, most room correction systems can work wonders.
Above 200 or 300 Hz or so, though, your room's effects on the sound that reaches your ear have less to do with the size and shape of the space and more to do with the qualities of the surfaces within it. 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.
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 many of us who aren't overly enthusiastic about many digital room correction systems describe the results as deadening or dull.
That said, I'll be the first to admit that better digital room correction systems - those with a higher filter resolution 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 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, though, combat ringing artifacts (a bit of pre-echo or post-echo accompanying high-frequency sounds). 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 the different room correction systems you're likely to come across when shopping for a new receiver or AV processor.