Digital room correction can seem almost like a miracle. The improvements are so spectacular that it may seem like all problems have been solved. But the final result still depends on what you start with—room correction is powerful, but not all-powerful. It is logical to expect the best end result when the uncorrected system has been set up to be as good as possible. But the system I want to tell you about here is different, because it doesn’t work at all without digital signal processing. It’s the next step in DSP control of the room/speaker interaction, and one more step into the future of audio.
Like many really good ideas, this one is simple. Decades ago, describing what is now known as the “Allison effect,” Roy Allison pointed out that it is difficult to generate bass from speakers that are out in the room. The room boundaries load the speaker in a way that varies erratically with frequency, causing big peaks and dips in response and timing errors, too. Corners are where woofers belong. (Corners are where the Allison Model 3 goes, too; see my review in Issue 150). But if you try to put the midrange and treble in a corner, early reflections will cause coloration and imaging problems.
Of course, there were and are “satellite/ subwoofer” systems that allow you to place the woofer in one spot and the midrange/tweeter in another, but they never sound coherent unless the crossover to the subwoofer is way down low. And even when it is as low as needed some of the bass still has to come from the satellites (out in the room), so much of the effect of the corner placement of woofers is lost.
What occurred to TacT was that: (1) the lack of coherence of the satellite/subwoofer system had to do with the physical separation of the woofer and satellites causing separation in timing; and (2) in the world of digital signal processing it is easy to introduce time shifts that line things up temporally. In other words, you could fix the coherence problem by time-delaying the satellites until the (sub)woofer signal from the corner had moved out to where the satellites were physically located.
Using this idea, the $3990 TacT 2.2X (X for crossover) lets you do a crossover-plus-time-delay at, say, 200Hz from a corner-placed woofer to a main speaker out in the room (where midrange and treble response are smoother and imaging better). The crossover can be configured in various ways; I used the “default” tenth-order high pass and low pass (in DSP, steeper slopes can be used without problems that would be common in analog crossover design). The RCS 2.2X also does the overall “room correction” to bring the in-room response to whatever “target curve” you choose, but it is the corner woofer arrangement that I am going to talk about here. (The TacT Room Correction System has been reviewed before in TAS by AHC in Issue 150 and by me in Issue 126.)
The theory of this appealed to me right off, and I knew from my experience with the corner-placed Allison Model 3 how good corner bass could be. But I was still taken by surprise when I first heard this system in my own room. Anyone who has worked with DSP already knows what bass sounds like when it is truly flat and phase-linear. The TacT Room Correction System already makes the bass of my Harbeth M40s exceptionally smooth and precisesounding. 1 But the RCS 2.2X cornerwoofer system pushes things further in the right direction—quite a bit further.
The first thing I played was the second movement of Dag Wiren’s Serenade for String Orchestra [Paula]. This charming music has prominently-plucked lower strings (cello and doublebass) that are strong and very precise, so it makes a great bass demo. My jaw dropped when I heard this on the TacT RCS 2.2X system (using the woofers of the Allison Model 3s for the corner woofers and the Harbeths M40s for “satellites,” even though they are almost full-range to begin with). Not only was the bass pizzicato line more defined than ever, but the whole sense of the recording venue was enhanced. And the sense of sound coming from speakers was greatly diminished.
We are always talking about speakers “disappearing,” and indeed many do in terms of side-to-side imaging (see my Audio Physic Padua RR review, Issue 156). But front-to-back imaging is a different story—so much of a different story that many audiophiles have come to believe that the front of the soundstage is intrinsically associated with the front plane of the speakers. But it should not be. With the TacT corner-woofer system, the speakers “floated” much better from front to back, and the soundstage positioned itself front-to-back according to the recording, as it should have, not automatically receding from the front plane of the speakers. Piano music is another realm in which the corner-woofer system excelled. Piano bass varies with time in a complex way, and it is really hard to get it right. But with the TacT cornerwoofer system the big Yamaha concert grand on the “Liebeslied” transcription from Freddy Kempf Plays Rachmaninov [Bis] really sounded like a concert grand piano, with all the low-frequency precision and weight that entails.
(Incidentally, people who think that the room-dependent frequency range [250Hz down] is a sort of incidental part of music and not as significant as the top end might want to look at a piano some time. Note that 250Hz is right around “middle C,” which is 261.6Hz; more or less half of music’s fundamentals fall below this.)
Orchestral music got its rewards from the corner-woofer system, too, as the opened-up sense of venue gave this physically larger music a more natural sense of scale. The Dallas/Mata Rachmaninoff Symphonic Dances [Audio+] sounded exceptionally spacious and detailed in the bass. This is a system where you could transcribe the bass parts of the orchestral score with ease. And if people are not tired of my mentioning Waterlily’s recent Mahler Fifth, here there were improvements in the already-excellent sense of the hall, imaging, and power, right down to the bottom.
Readers who have been following the DSP room correction saga from the beginning may be a bit mystified about some technical aspects of all this. If inroom bass is a minimum-phase phenomenon at a sample listening position, and if the TacT Room Correction System already made it as flat as the measurements on my Web site show, isn’t it basically perfect already? Good question. And yet the audible improvement from the TacT RCS 2.2X corner-woofer system is real.
My preliminary explanation is that the corner-woofer system does not so much improve performance at a single listening point as expands the area over which this improvement is realized. As I pointed out with measurements in my initial TacT review, the perfection of bass flatness is not always stable as a listener changes positions, even if the changes are small. Because corner-bass drive energizes all room modes in-phase, it’s a good guess that the RCS 2.2X correction is more stable than that of the Room Correction System, which is trying to equalize the partially out-of-phase signals of a woofer that is “out” in the room. (More on this later, I hope.)
There is also the distortion issue. Corner placement maximizes woofer loading—the woofer requires minimal motion for a given power transfer into the room, and woofers that do not need to move much are woofers that exhibit less distortion. Moreover, the maximized acoustic loading increases the dynamic limit of the woofers and diminishes the draw on amplifier power—often by as much as 10dB or more. You really are getting more bang—or, in this case, boom—for your buck.
This matter of experiencing the recording venue instead of your own room seems important to me, psychologically speaking, as one needs to be “transported” to experience music ideally. I (and many other people) have had quite good luck setting up systems that do not change much tonally when they are DSP’d to an appropriate “target curve.” Good speakers, careful placement, and room treatment can do a lot. I know audiophiles are phobic about analog EQ, but it can help if used wisely. Such a system will be very truthful— and very attractive—in tonal terms. But it is harder to get that elusive but important feeling of erasing the acoustics of the room around you. At some level, the ear/brain is still locking onto your listening room. This seems especially true in the bass.
To my ears, the Harbeths plus corner woofers provide the best bass/lower-mid performance I’ve ever had in my room. Only large dipoles like the original Carver Amazing have been comparable, and they had to be out in the room to the point of domestic incompatibility. This level of perfection can be had from a system that, in principle, is quite inconspicuous. The Harbeths are big, but their deep-bass capabilities are irrelevant here. I got the same outstanding results (albeit with less dynamic capacity) using the Allisons as corner woofers and the diminutive Metronome 7 as satellites—a system almost invisible by high-end standards.
In case you are wondering, the corner- woofer system should be set up so the main speaker lies on the single straight line from the corner to the listening position. The time synchronization is perfect on this line, but in principal somewhat imperfect elsewhere. But if you study the geometry a bit you’ll see that the error remains small over a quite broad area around the listening position.
When satellite/subwoofer systems appeared some time ago there were declarations that the days of monolithic full-range speakers were numbered. These declarations were obviously premature. But times change. The TacT corner-woofer time-delay system solves the coherence problem that was the Achilles heel of satellite/subwoofer setups before. And more importantly to the committed audiophile, it actually offers better performance—not just uncompromised, but better. Non-dipole bass drive belongs in corners and works best there. This time, the full-range monsters actually are starting to look like dinosaurs to me.But don’t take my word for it; hunt down a properly adjusted corner-woofer system. I can all but guarantee that you’ve never heard anything like it—at least since your last live concert.
Robert Harley comments: I heard an ideal demonstration of the DSP-corrected corner-woofer technique at the home of DALI president Peter Lyngdorf last summer, in conjunction with my visit to the DALI factory (see Issue 155). His system included two custom woofercabinets designed to locate each of two 10" drivers about an inch from walls. The woofers have very high sensitivity (100dB/1W/1m) and very low moving mass, enabling them to be driven by as little as 10W. These were crossed over to a pair of DALI Helicon 400 loudspeakers positioned out into the room. The crossover frequency was an astonishingly high 325Hz, with fourth-order slopes, realized via the TacT RCS 2.2X. A TacT Millenium Mk.II digital amplifier, which takes in PCM audio and converts it to a pulse-width modulation signal that turns the output transistors on and off, powered the Helicons.
This system’s bottom-end power, resolution, and freedom from coloration were jaw-dropping. The complete lack of bloat, thickness, or heaviness in the bass and midbass seemed to liberate the entire bottom end, allowing me to hear deep bass without the masking effect of excessive midbass. Bottom-end dyname ics were unlike anything I’ve heard from a hi-fi system. The system resolved the attack and decay of bass guitar, acoustic bass, and kick drum with an astonishing lack of transient smearing. Dynamics jumped out of the speakers, then disappeared with equal rapidity. The entire presentation had massive weight and impact, yet was amazingly quick and agile. Finally, this bottom-end clarity made it easy to hear the pitch of individual notes. In short, this system was revelatory in its reproduction of the bottom four octaves.
A similar system, composed of the Lyngdorf Audio W210 woofers (a production version of the experimental units I heard) and Lyngdorf TDA-2200 digital amplifier with full DSP room correction, will be demonstrated at this coming CES. Lyngdorf reports that even higher crossover frequencies (up to 600Hz) with steeper slopes further increase bass clarity and dynamics.