Calibrate your home-theatre audio

Sound is a big part of a satisfying home-theater setup, and people tend to give a movie higher marks when they watch it with good surround sound audio than with simple stereo.

Sound is a big part of a satisfying home-theater setup, and people tend to give a movie higher marks when they watch it with good surround sound audio than with simple stereo. So improving the audio listening environment in your family room or living room may yield a much more immersive HDTV viewing experience.

But how do you set up your audio system to achieve the best sound? The secret lies in understanding your room. A few years back, I attended THX technician certification training. Most of the classroom time was devoted not to electronics, but to understanding room acoustics. Once you know a little about that, you can start down the road of improving the quality of your surround-sound audio system.

Work the Room

Not everyone can afford a custom-designed home-theater space. My own family-room space, where my audio gear and HDTV live, is a fairly difficult environment. In this article I won't discuss such complex issues as room and wall treatments (modifications to walls and the internal space behind your walls and ceilings). Altering those variables can have a big impact, but doing so can be expensive.

Let's take a look at a floor plan of my family room (left).

At about 15 by 15.5 feet, the room is essentially square, which creates problems bass frequencies, which in turn complicates subwoofer placement.

A standing wave occurs when a particular room dimension is equal to the wavelength of the sound. In most rooms, such a wave typically occurs at very low frequencies.

You can calculate the standing wave frequencies pretty easily.

F = (n x 1130)/2D

where F is the frequency, n is the harmonic (1 for first level), 1130 feet per second is the speed of sound at sea level, and D (in feet) is the room dimension (length, width, or height) under consideration. You'll need to adjust the constant for speed of sound (given here as 1130 fps), depending on your altitude above sea level.

A 15-by-15-foot room can have a pretty noticeable dead zone at 36Hz to 38Hz near its center. Rectangular (nonsquare) rooms can have two different, but less intense, bass dead zones in different locations. That frequency happens to be precisely where a lot of low-frequency effects occur in surround-sound movies. You'll notice a distinct lack of deep bass where the resonance frequencies overlap. The problem may be harder to detect with music, unless the music has very deep bass content.

The intensity of standing waves depends to some extent on the composition of the walls. Walls composed of extremely stiff materials like brick or concrete may produce dead zones that are quite noticeable. Walls with more flex--cheap drywall, for example--may decrease standing-wave intensity.

Returning to the example of my family room, I should note that it isn't really a square room. In the lower right corner of the floor plan, you'll see an opening with no door. That's a stairwell going up. Since it's permanently open, it bahaves like an infinite baffle, soaking up some audio volume, but also diminishing the effect of standing waves. Ultimately I placed my Hsu Research subwoofer in the left corner, a few inches away from the converging walls.

Deciding where to position the remaining speakers is fairly straightforward, but it depends on where the viewers will be sitting. In my family room, because of the TV location, the center-channel speaker has to be above the HDTV, but I angled it slightly downward. I placed the left and right front speakers roughly equidistant from the number one listening spot (the large circle labeled "1").

The positioning isn't exact, however, because the location and size of other furniture can interfere with geometrically perfect speaker placement in an imperfect space. And if the front speakers aren't equidistant from the listener, small delays due to sound arriving at different times can smear the overall soundstage.

Surround speakers are mounted high on the walls, right at the intersection with the ceiling. They're located to the side, and immediately behind the listening positions.

I have a 7.1-speaker setup in place. Since I had no wall space to use in mounting the rear channels, I mounted them in the ceiling instead, behind the listening area and a few feet apart.

So much the room and the speaker placement. Now let's move on to calibration.

First-Level Calibration: Audyssey MultEQ

More and more AV receivers these days ship with some form of automated calibration software incorporated into the system. One common type of calibration software is designed by Audyssey. The Onkyo TX-NR1007 that I use has Audyssey MultEQ built in.

Automated calibration tools used to be restricted to high-end receivers, but today many receivers contain these types of tools. Even some "home theater in a box" models ship come with automatic calibration equipment. When you unpack the box, you'll find a calibration microphone attached to a very long cable.

When you plug the microphone into the jack specifically designed for it on the receiver, the calibration tools start up automatically.

At this point you just follow the instructions on your HDTV, one step at a time. The first step is to place the microphone at the number one listening location, at roughly the height of the listener's head.

The receiver generates a set of sweep tones, one speaker at a time. The sweeps happen very fast, so you hear them as a series of loud whooping sounds. The microphone captures the tones, and feeds them back to the receiver. After all of the speakers have completed their tests, the digital signal processor inside the system spends some time crunching the data. Eventually, at the other end, the system spits out the crossover frequency for the subwoofer and calculates the distance from the speaker to the listening position.

Many receivers allow you to repeat the testing for more than one listening position. Doing this is worthwhile, because it enables you to fine-tune speaker distance and delay settings.

At the end of this process, you unplug the microphone; and if you like, you can stop here. Audyssey and similar automated calibration commonly produce a calbration that most people find good enough. The soundstage imaging often improves, and subwoofer levels sound mostly right.

The calibration routines will have set the levels for all the speakers, based on some internal reference. For THX-certified receivers, the reference value is 85dB at the listening position when the receiver volume knob is set to 0. (Now you know why many receivers have volume controls that are negative numbers, and go up to 0.)

You'll find that Audyssey also sets the speaker distance levels. I have found that Audyssey actually works fairly well for the fronts and surround speakers, and that it does a reasonably good job with the subwoofer. Still, it doesn't work flawlessly.

If you want to get a little closer to perfection with your audio system's bass, there's more you can do.

Manual Tweaking

For tweaking your audio manually, you'll need a couple of additional tools. One is a sound-pressure-level meter.

SPL meters of various kinds are available from Amazon for a moderate price. For maximum usefulness the device should support A and C weighting and let you set it to slow response. You don't need one that measures very slight sound levels, since the tests generally occur at high volume.

You'll also need a signal source to run through your speakers. For this purpose I recommend the Goldline 5.1 Audio Toolkit, an affordable DVD chock-full of test signals.

I use the toolkit to confirm that the Audyssey calibration tool accurately set the reference levels emerging from the speakers. Though Audyssey generally works well for fronts and surrounds, its subwoofer levels sometimes aren't quite right. So I pop the Goldline DVD into my DVD player and use it to determine a reference level. I normally use the center-channel broadband pink-noise signal and push the receiver volume to 85dB (C-weighted.)

Next, I run the low-frequency-effects signal from the Goldline DVD, adjusting the subwoofer's level to 75Hz. Since I don't want to disturb the Audyssey settings on the receiver, I use the subwoofer's own volume knob to make these adjustments.

By this point, your home-theater audio setup should be 80% to 90% of the way to a fine-tuned listening experience. For most people, that's fine. My family room is imperfect enough, that I'm fairly happy with this level of calibration. Stereo and surround-sound imaging is sharp, and bass is clean at the listening positions.

If you have great ears and you have even more seats to worry about, you'll want to do more. Pricey wall treatments are an option, or you can use a calibration microphone and software for your laptop to tweak the audio settings. With patience, you can achieve success by manually tweaking all of the speaker settings, using calibration software such as TrueRTA.

Don't forget, though, that your goal is to have a great time watching movies or TV. While it may be a pleasure to tweak your room and audio settings to the nth degree, you can't sit back and enjoy your system's superb film and video content until you put your tools away.

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Loyd Case

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