Audio Odyssey: Ken Kreisel of M&K Page 3

We know that the ear hears differently as a function of volume, and we know that a lot of well-respected audiophile speakers are not accurate. So when people say that a THX speaker system sounds harsh, I'd say that there are a variety of reasons for this perception. The first is that the Home THX specification demands that a speaker play at 105dB SPL in a 3000 cubic-foot room, which is a lot louder than most audiophile speakers can play. Some designers meet this spec by crossing the midrange over at a high frequency in order to prevent the tweeter from having to deal with more power than it can handle, and they also cross over at a high frequency to meet the THX vertical directivity requirement. Those factors can make speakers sound harsh.

Furthermore, THX doesn't let you design in the usual friendly Fletcher-Munson-type "smile curve" response—and anechoically, they have to measure flat. So some of that "harshness" might be a poorly designed crossover; some might be that these speakers put out a lot more energy in the 2 to 3kHz region than people are used to hearing from "audiophile" speakers; and some of it can certainly be attributed to designs that—while they make spec—are not really competently thought-out.

WP: At a recent trade show, you mentioned that a lot of your admiration for the THX program has to do with the fact that you've been so active with subwoofer development. What did you mean?

KK: It's very important to understand what THX has done with filters. They assume that a home theater will have a subwoofer, and they embraced 80Hz as the crossover point. Having made subwoofers for over 20 years, I'd say that's a reasonable frequency. THX also decided to use a 24dB/octave fourth-order Linkwitz-Riley crossover, because it's about the only crossover design that has the high- and low-pass filters in phase at the crossover point. There are certain benefits to that. A fourth-order Linkwitz-Riley filter is essentially two 12dB/octave Butterworth filters in series with one another, which produces the specified 24dB/octave roll-off.

Simply put, THX front speakers are designed in such a way that half of the 24dB/octave forth-order filtering is accomplished acoustically in the speaker itself, while the other half is accomplished electrically in the THX processor—there's essentially no difference between an acoustic filter and an electronic filter. So a THX processor feeds the satellites with a 12dB/octave electronic filter, and the subwoofer with a 24dB/octave fourth-order Linkwitz-Riley filter. The sum of the whole system adds up to a fourth-order Linkwitz-Riley crossover that puts the speakers in-phase at the 80Hz crossover frequency. It's just so elegant.

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