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TMHLabs Audio Test Discs  Print E-mail
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Written by Kim Wilson   
Sunday, 01 August 1999

Introduction
From TMHLabs and the mind of Tomlinson Holman (creator of THX) comes a set of powerful test discs that provide a series of test tones for the calibration and performance measurement of two to 5.1 channel music and theater systems. The test tones were generated in the digital domain by computer programs that ensure accuracy and stability without actually needing tremendously expensive and hard-to-use instruments. All editing and mastering was conducted digitally, with no analog conversions along the way. The finished discs are guaranteed to correctly represent the original signals.

Most acousticians and installers will tell you they need at least an AC voltmeter, an oscilloscope and a spectrum analyzer to do what this disc series promises. These discs were designed to make system tests simpler, requiring only a sound pressure level (SPL) meter that can be purchased at Radio Shack, a stopwatch (on occasion) and a functioning pair of ears. The Stereo and Surround System Setup Test Disc is priced at $49. There is a four-disc series that also includes this disc, for professional facilities with a much wider array of tests, which sells for $300.

Evaluation
The Stereo and Surround System Setup tests are used for two-channel as well as surround sound systems. While not all of the 58 tests are suited for the home theater environment, there are a number of useful tests that check channel polarity, room equalization and speaker placement. You can test your room acoustics for background noise, the origin of rattles, or find the high and low frequency range extension of your system.

I know this sounds dorky, but bear with me on this one: my favorite test was the four bands of pink noise that alternate between the left and right channel. As the pink noise travels from left to right, if you hear a shift in the frequency response, it indicates improper speaker placement. Using the A/B loop on my CD player, I could make this 47-second track repeat itself, allowing me to sit at my listening position, analyze the tones, make some adjustments, and then listen some more. In about two minutes, I was shocked to hear the best imaging ever on my Genesis APM-1s. Actual speaker movements were quite small, but the effect of this one relatively simple test was nothing short of phenomenal. The minor amount of time and effort was impressive, and there was no need for any big-dollar, high-tech test equipment or portable super-computers. This test alone is worth $500! Shake, Rattle and Roll.

No doubt, during explosive scenes of your favorite action thriller, you've noticed a rattle here and there, but how do you find the source of these rattles? Since the sympathetic vibrations in a room (the source of the rattles) respond only to a very narrow frequency band, it can be difficult to find the exact location of resonance, since the tone causing the rattle is often very brief. The rattle test is a 20Hz to 20KHz slow sweeping tone that elicits rattles from elements in your room, including your loudspeakers, your equipment and/or your furnishings. Once rattles are detected, they can be treated with simple physical adjustments and or damping materials like foam or Blue Tac.

I knew of one window in my listening room that would rattle at a particular subsonic frequency, but I was aghast at how many transient rattles appeared throughout the room. I was able to fix quite a few of them very easily and with little expense or effort. The cumulative effect of room rattles and background noise can be dramatically bad. With a point in the right direction from these test discs and a little bit of effort on your part, you can easily move a few steps closer towards audio perfection.

As I have advocated in the past, a useful test tone CD in favor of the built-in test tone generator found on receivers and digital processors is preferred for calibrating multi-channel sound systems. All of the tests you need, including subwoofer levels, are included here. Moreover, there's a test to assist in determining the best placement for your subwoofer. Contrary to popular AV mythology, physical subwoofer placement and crossover set-up are critical to an excellent-sounding playback system.

Finding the useful high and low frequency limit of my sound system or, more precisely, my ears, was enlightening. Starting at 8kHz, the tone slides upwards at a rate of 500Hz per second. As the frequency sweeps, I identified the fundamental and used a stopwatch to time how long I heard it. Using the calculations provided, I could determine the high frequency limit. Knowing that you're getting older and experiencing high frequency loss is one thing, but measuring your top limit at about 15k is kind of distressing. Testing for the low frequency limit works essentially the same way but, of course, the sweep starts at 120Hz and slides downwards at a rate of 2Hz per second. 55Hz isn't bad for a living room that wasn't designed for the extreme low frequencies of ultra-high-end audio and I could detect subsonics down to 25Hz. These are just a few of the many relevant tests found on this disc. I suppose you could spend just as many hours tweaking your sound system with this test disc as you would without it. The advantage, of course, is that with the disc you know the adjustments are accurate, plus they can provide you with some valuable information about your system's capabilities.

Conclusion
Listening to a 1kHz test tone or pink noise isn't very exciting, but it's actually quite amazing what this assortment of test tones can tell you about your sound system and its relative environment. For a reviewer like myself or a custom installer, this disc is a must-have. The booklet that comes with the disc is a tremendous resource, providing details for each test and instructions on how to evaluate the test properly, along with fascinating in-depth background information. Even the limitations of each test are well documented.

Does a product like this negate the work of professionals who use advanced instruments and powerful computers to equalize a room such as Bob Hodas, the Revolution's own Miracle Man? Hardly, though not everyone can or even wants to go to the expense of such exacting detail. If you have 15 free minutes and $49, you are on your way to dramatically better sound from your system. Will you want to spend more time with this disc to elicit even better performance from your system? Damn straight. Could this be the best-sounding $49 you ever spent on your AV system? Most likely.





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