First-Order Crossovers: Panacea or Problem?

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First-Order Crossovers: Panacea or Problem?


Thiel-CS17-thumb.pngI've always encouraged critical e-mails from readers because it's good to have the feedback. But a couple of the e-mails in response to my review of the THIEL TT1 got downright nasty. "You are not an audiophile!" one raged in response to my comment that the TT1 is "...a more versatile speaker than any Jim Thiel [the late company cofounder] designed and probably a better value than anything Jim designed..."

I was reminded of this when I recently Googled some information for my upcoming review of THIEL's new SmartSub 1.12 subwoofer. I encountered impassioned complaints about the company's decision to move away from the phase-coherent designs and first-order crossovers that were a hallmark of Jim Thiel's work. In a YouTube video, an audio enthusiast praised Jim Thiel's designs as "light years ahead of what everyone else had to offer" and took the company to task for offering new product that "appears to be the same old crap that we've seen in the loudspeaker world for the past 30, 40, or 50 years."

From a business standpoint, whether or not it was wise for THIEL Audio to abandon Jim Thiel's core design concepts is debatable, but I expect most speaker designers would have made the same decision as the new people at THIEL. That's not because they're dumb. It's not because what they make is "crap" (I don't give out five-star performance ratings to crap speakers). It's not because they're "not audiophiles." It's because they understand that first-order crossovers introduce a surprising number of trade-offs, a point that none of the commenters I've seen lambasting THIEL Audio seems to have bothered to investigate.

I'm fortunate to have spent many hours discussing these issues with Jim Thiel himself. I visited him twice at his product development lab and factory, and he dropped by my workplace a couple of times to help me set up speakers or to chat about what he had in the works. It may be surprising to some that his approach was deeply rooted in science and no-nonsense engineering. To my knowledge, he never indulged in the mystical verbiage and hard-to-support performance claims that are common in high-end audio, and in our discussions he was often dismissive of faddish audio practices that weren't firmly supported by scientific evidence.

First, some basics for those who need them. The crossover is an electrical network that divides the sound into bass for the woofer and treble for the tweeter (and often midrange for a midrange driver). A two-way crossover has two filters: one that filters the treble out of the woofer and one that filters the bass out of the tweeter. (A three-way speaker adds filters that remove the deep bass and upper treble signals from the midrange driver.) These filters are characterized by the frequency at which they begin to attenuate a signal, and by the slope of that attenuation. A first-order filter attenuates at -6 dB per octave, a second-order filter at -12 dB per octave, and so on. These filters affect the phase of an audio signal, slightly delaying some frequencies relative to others.

The benefit of a first-order crossover is that it maintains the phase of the original signal, as long as the speaker that it's used in has the acoustic centers of the drivers aligned in the vertical plane, often through the use of a sloping front baffle. As long as your head is positioned so that your ears are equidistant from all the drivers, the phase of the direct sound that first reaches your ears shouldn't vary by more than a few degrees. That's why THIEL and other manufacturers refer to this as a phase-coherent design.

How does this affect the sound you hear? I've reviewed perhaps 10 THIEL speakers since the early 1990s, as well as several other phase-coherent designs. To my ears, the benefit of phase-coherent design is that you get a more enveloping soundstage, more like what you'd expect to hear from a panel speaker such as a MartinLogan or Magnepan--but with more precise imaging than a dipolar panel speaker can produce. It's a great sound, one I strongly recommend every audio enthusiast check out for themselves in a well-designed dealer showroom or at an audio show.

As nice as the phase-coherent sound can be, it's not "light years ahead" of everything else. If the benefits of phase-coherent design were so great, more speaker companies would use it, because in its basic form it's also the least-expensive crossover you can build. For a two-way speaker, a first-order crossover requires just one capacitor, one inductor, and one resistor. For every added order in a passive filter, you need an additional capacitor or inductor. That's why the most common use of first-order crossovers isn't in audiophile speakers, but in soundbars, wireless speakers, and home-theater-in-a-box systems. (For the record, Jim Thiel's crossovers were amazingly complex, with numerous networks added to correct impedance and frequency response anomalies, but the core functionality was still provided by those three basic components.)

So what's the downside of first-order crossovers? There are three: distortion, driver longevity, and dispersion.

As Jim Thiel told me, because a first-order filter doesn't attenuate as abruptly as high-order filters, a driver used with a first-order crossover has to exhibit good performance two octaves beyond the specified crossover point--i.e., in a two-way speaker with a 2.3-kHz crossover point, a tweeter must be able to handle signals as low as 575 Hz, and a woofer must be able to handle signals as high as 9.2 kHz. This is especially hard on a tweeter, producing distortion at high volume and sometimes leading to driver failure (early Thiel speakers were notorious for blowing tweeters). It may also excite the woofer's "breakup modes," or distortion-producing high-frequency resonances.

Jim Thiel fully acknowledged these problems, and he largely solved them by developing tweeters with an unusually long excursion of about a quarter inch, and through the use of a unique corrugated-diaphragm concentric driver that handled the mids and highs in his more recent models. These problems can also be lessened by going to three- or four-way designs.

The other problem with first-order crossovers is dispersion. Because the filters are relatively shallow, both drivers are simultaneously audible when sounds are within about an octave of the crossover frequency. This is no problem if your ears are at the same distance from the drivers. However, if your ears are closer to one driver--if you stand up, which puts your ears closer to the tweeter, or slouch down, putting your ears closer to the woofer--the sounds from the two drivers will no longer be in phase at all frequencies. They'll be in phase at some frequencies and out of phase at others, which means some frequencies will be boosted and others attenuated, and you will no longer be getting even frequency response. (Speakers with higher-order crossovers can also have this problem, but to a much lesser degree.)

Again, Jim Thiel took great pains to fix this problem; it's why so many of his designs used a coaxial arrangement, with the tweeter inside a midrange driver or woofer. But the exotic driver designs he relied on took years to develop, cost a lot to manufacture, and sometimes prevented him from getting new models out as quickly as his business partners and dealers might have liked.

Thiel-TT1-thumb.jpgI was fortunate to review both the very last speaker Jim Thiel designed (the CS1.7, shown above) and the very first speaker the company designed after Jim passed away (the TT1, shown right). The difference was dramatic. The CS1.7, like past THIEL tower speakers I'd reviewed, was somewhat fussy to position and couldn't touch the bass response and dynamics that many similarly priced speakers offer; it was spectacularly great for purist audiophile recordings of acoustic instruments, but a questionable choice for heavier rock and pop music or for home theater. Meanwhile, the TT1 delivered loads of dynamics, wasn't fussy about placement, and ultimately delivered a more neutral sound that worked with any kind of music.

Of course, fans of the old THIEL speakers may insist there's nothing special about the new speakers, that they're not too different from what B&W, PSB, and Revel produce. That's true. And of course, many audiophiles prefer a product with charming idiosyncrasies to one that delivers objectively superior performance. That's fine. And of course, some may now gravitate away from THIEL and toward Vandersteen, another company known for phase-coherent designs. That's fine, too.

What's not fine is claiming that the phase-coherent speaker (or, for that matter, almost any other audio technology) is "light years ahead of what everyone else has to offer." Just like every other audio product ever made, phase-coherent speakers represent a series of trade-offs. By understanding those trade-offs, we can make intelligent buying decisions. We cannot make intelligent buying decisions if we base those decisions on a quasi-religious reverence for specific technologies, brands, or personalities.

Additional Resources
The Pros and Cons of Multiple Subwoofers at HomeTheaterReview.com.
How to Choose a Subwoofer for Surround Sound or Stereo at HomeTheaterReview.com.
THIEL TT1 Tower Speaker Reviewed at HomeTheaterReview.com.


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