Carver Amazing Loudspeaker (Platinum Edition)
It was the promise of greatness that led me to request a review sample from Carver. What eventually arrived was the Amazing's new Platinum Edition. There's also a Silver Edition, a little-brother version with only three woofers per side as opposed to the Platinum's retinue of four 12" woofers. These new Amazings have not been reviewed elsewhere, and differ from the old Amazing in several ways: the ribbon is now pleated for better low-frequency extension and power handling; the new ribbon has more excursion and, to minimize acoustical interference, is one long, continuous design instead of the two shorter segments of the older design.
The elegant fascia of the speaker commands respect and bears more than a passing resemblance to the Apogee designs. However, to characterize the Amazing as a ripoff of the latter would be grossly unfair to Carver. For example, it so happens that the asymmetry of the front baffle is important for smoothing out the bass response of any dipole radiator. A rectangular baffle would have worked, but not as well as an irregular one. So to accuse Carver of mindless imitation in this case makes about as much sense as accusing Ford of copying GM because all of Ford's cars also have four wheels.
The woofer section
More accurately, the subwoofer section, because undeniably the Amazing is capable of subwoofer performance. The 1980s saw the resurgence in several subwoofer designs of the finite baffle, once the low man on the totem pole of baffles. Some of you may recall the Enigma subwoofer, now out of production, and of course more recently the Celestion 6000 subwoofer. In both, electronic equalization is used to compensate for the baffle's LF rolloff down to the woofer's free-air resonant frequency. At frequencies where the average baffle dimension is smaller than half a wavelength, the front-to-back cancellation characteristic of a dipole radiator takes place.
One of the nice features of finite baffles is that the LF rolloff is a gentle, predictable, easily equalizable 6dB/octave down to the driver free-air resonance—below which the response goes to hell at the rate of 18dB/octave. Even so, the bugaboo for such designs remains achieving sufficient dynamic headroom in the deep bass. Push the woofer or woofers extra hard and you run out of voice-coil excursion sooner than you would with that same driver in a box.
The Celestion 6000 is a good case in point. The small-signal response looks terrific with a half-power bass frequency of about 25Hz. But below 40Hz, you'd be lucky to hit 100dB at a realistic listening position even in a small room. Folks, in the deep bass 100dB spls are quite polite.
Bob Carver's innovation lies in his startling reversal of the roles of driver and baffle. Carver's correspondence on this subject is quite eloquent, and the following description is based largely on his writing. Traditionally, the drive-unit alignment is of fairly low Q. (Q stands for "Quality Factor" and is a measure of how sharply defined a resonant peak or dip is.) For example, a woofer Q of 0.42 is considered about optimum for a bass-reflex design. The cabinet, being of high Q, pushes the overall response Q to about 1 in a properly designed system. Bob's insight was to regard the finite baffle as the low-Q or overdamped element in the system and let the high Q of the woofer bring about an overall system Q of 1. Simply put, the idea is to introduce an under-damped or peaky woofer into the baffle and let the peak in the woofer response compensate for the front-to-back bass cancellation of the baffle.
Sounds simple at first blush, but boy, are there technical difficulties lying in the bush, as Bob will only too gladly tell you. He's spent the last two and a half years addicted to the notion of perfecting this planar speaker—until recently to the detriment of Carver the Company and his own personal life. Speaker designers are a breed apart, junkies whose highs are derived from doing battle with the laws of physics. And to judge from my recent encounter (of the third kind) with Bob, the fire still burns bright. His enthusiasm for the Amazing and his untiring drive to improve them clearly go beyond the call of duty.
To underdamp a woofer you have to reduce the size of the magnet. As the magnet size is reduced, the electrical Q of the driver increases, and with it its total Q. The problem is that the efficiency of the woofer is reduced at the same time. The absurd conclusion of this scenario is zero magnet, zero efficiency, and of course zero cone motion. To improve the efficiency of a woofer with a miniature magnet it is therefore necessary to drastically reduce its moving mass. The idea is to try to keep constant the ratio of force to mass, hence a constant woofer acceleration factor. Because most of the moving mass is represented by the cone, that's where you have to cut. What you end up with at the end is a woofer with a gossamer-thin cone and an unbelievably tiny magnet—quite unlike the traditional woofer with its heavy cone and beefy magnet.
So radically different was Carver's woofer design that Tonegen, its Japanese manufacturer, refused at first to produce it. It took a personal visit by Bob to convince them that that was truly what he wanted. The danger, according to Bob, is that there is a tendency on the part of the audiophile who sees this tiny magnet to say, "Ah-ha, Carver has out-cheapied himself and is just screwing us and ripping us off." But it just would not work any other way. Reducing the moving mass, however, is in itself inadequate to confer a decent efficiency; ultimately, one has to resort to a different strategy to achieve a reasonable system sensitivity.
There it is: sensitivity. A term so often confused with efficiency that, in many audiophiles' minds, the two concepts are synonymous. Efficiency is a fundamental driver property, determined essentially by driver parameters, that relates (for example) how efficiently a woofer converts electrical input power to acoustical output power. Sensitivity, on the other hand, is a system parameter. For example, a system with ten inefficient woofers may put out more total power than a system with a single more efficient woofer. Thus, there is combined "strength" in numbers. And from a practical standpoint, most of us could care less about driver efficiency—as long as the bottom line is excellent sensitivity.
Those of you familiar with Small-Thiele theory may be reaching for a pen and pad just about now, getting ready to fire off a letter to the Editor. I know what you're thinking: "How can DO say that? Haven't I seen a design equation somewhere that shows efficiency increasing with box volume"? Well, let me save you the postage. It's only a myth; appearances can sometimes be deceiving. And even Richard Small himself will tell you that driver efficiency is determined almost entirely by driver parameters, not box volume. It can't be any other way; if efficiency did increase with box volume, you'd have infinite efficiency in an infinite baffle. Nonsense. Big, efficient woofers do require big boxes for proper bass alignment, and for that reason big box designs are more efficient. But it's not because of the box; rather, the woofer.
Back to the idea of using multiple woofers to improve system sensitivity. This is not a practical approach for a box speaker. Once you optimize the design for a given woofer to yield maximally flat bass response or whatever, mounting another woofer on the front baffle screws up the bass alignment (the exception is "isobarik" or compound loading). The two woofers in the same box will produce a peaky, non-flat response with reduced bandwidth. The reason is that the acoustic compliance of the woofer pair is doubled, which in turn requires a box volume twice as large. So to keep adding woofers to a box design requires an ever-increasing penalty in terms of box volume. This is not a problem, however, for finite baffle designs.