Excerpt from Bob's interview with Gordon Brockhouse of Audio Ideas Guide:
Link:
http://www.audio-ideas.com/interview/carver.htmlExcerpt:
BC: Having left Carver I wanted to build a product that people would really notice. A very small subwoofer would certainly attract attention…provided it had performance. The smallest box I could fit a driver with a 10-inch basket into was 11 by 11 by 11inches.
GB: The power specification for the True Subwoofer - 2,700 watts - has been controversial.
BC: No one believes it, because the amp’s the size of a candy bar, albeit a big $2.25 Hershey bar. When Don Keele, a world-respected subwoofer scientist and reviewer, measured it, he measured 2,500 watts continuous for at least two minutes. And it’s capable of twice as much peak output - maybe 5,000 watts.
GB: Is it just raw power that enables you to get so much bass from so small a box?
BC: The True Subwoofer is a small box with a small driver that produces the same output and bass extension as a large box with a large driver. It has not been possible until now to build a small subwoofer with this kind of output. Conventional drivers are not very efficient at converting input power into motion. They operate close to “stall mode” where a lot of power is turned to heat. To get lots of output from a small box, we have to increase input power by a factor of 10 or more. Two hundred watts becomes 2,000 watts, and that burns out the voice coil. What was necessary was to make a driver that would remain highly efficient even in a small box. This requires a big magnet and lots of motion, which pulls the driver out of stall. When it’s out of stall, it’s efficient and there’s far less heat.
This flies in the face of conventional driver design, because that type of driver generates vast amounts of back emf [back electromotive force, which results when the motion of the voice coil in the magnet gap generates a voltage]. The back emf prevents current from flowing through the voice coil, so there isn’t any output. To overcome the back emf, you need a special amplifier that can swing 100 volts, rather than 20 or 30 volts. When you compute the input power - 100 volts squared divided by four ohms, the resistance of the speaker - that’s over 2,000 watts. That’s what the amplifier would deliver if the load were purely resistive. But it’s not. It’s a loudspeaker load with high back emf. The real power delivered to the voice coil is only a few hundred watts.
GB: Is that the explanation of the controversial power specification of the True Subwoofer? That its amplifier delivers a few hundred watts into the resistive component of the load, but that it has to be capable of delivering far more power in order to deal with the reactive component of the load.
BC: You can think of it in those terms. That’s called “imaginary power.”
GB: I lose the ability to think when people start talking complex arithmetic and imaginary numbers.
BC: It’s hard. I’d forget about it at this point.
GB: What was the basic insight that led to the True Subwoofer?
BC: Necessity is the mother of invention. This woofer began two-and-a-half years ago with an eviction notice from my wife Diana. “Get this stuff out of the room,” she said. “If you want to have a big woofer in the room, it has to be invisible.” I began thinking of ways to do this. I’d think about when I was driving around, when I’d go to an opera, when I was going to sleep at night. Gradually, it occurred to me that one of the fundamental defining equations for efficiency, (Bl)2/re [where B is magnetic strength, l is voice-coil excursion, and re is voice-coil resistance], is true over all frequencies, not just for the midband as the equation is classically interpreted. It turns out I was right. That meant I could increase output and bass extension by using a big magnet and long-excursion driver.
GB: It sounds like a very unusual driver design.
BC: Oh yes. It requires careful attention to the suspension, surround and the magnetic geometry to keep it linear. They can’t collapse under the box pressures, which are around 150 pounds. So the rubber-rolled surround has to be sufficiently rigid so that it doesn’t cave in and act like a passive radiator in reverse.
GB: How do you obtain acceptable transient response with materials sufficiently massive to withstand these pressures and produce these excursions?
BC: Transient response is the mirror of frequency response. If you have good amplitude response, you have good transient response. If you have good transient response, you have good frequency response. The trick is to get the amplitude response correct, which is challenging enough. It requires good motor structure, good surround design, a good amplifier, the proper equalisation and so on.
GB: What about sensitivity?
BC: Sensitivity is so low it’s preposterous. It’s not so much the mass, although that does drive down sensitivity. The main thing is the back emf associated with the large magnet. The sensitivity is somewhere in the 60s or 70s [dB @ 1 watt @ 1 metre]. But it doesn’t matter because I’ve got this super amplifier to drive it.
GB: Isn’t distortion high with such a small box and driver?
BC: At full output, distortion is less than 10 per cent from 18 Hz upward. Most subwoofer manufacturers don’t specify distortion, because it’s very high: 10, 20 per cent or more. Even the Velodyne, which has a high-gain servo system, has one per cent distortion at low levels; but at full output, distortion is close to 10 per cent.
GB: How tolerant is human hearing of distortion at very low frequencies?
BC: Incredibly tolerant. It’s all but impossible to hear distortion at very low frequencies.
GB: Some speaker manufacturers maintain that distortion is important at those frequencies.
BC: Believing in low distortion is like believing in motherhood and apple pie. Low distortion is a good thing, and every manufacturer strives for ever-lower distortion. But it’s well-known how much distortion we can hear, and at what levels and under what circumstances we can hear it. If we apply those rules artfully, we can design products with vastly improved performance.
Take DVD as an example. DVD contains immense amounts of distortion as a result of the compression algorithms used for audio and video. A lot of information is thrown away. The components in the signal that are tossed out represent tremendously high distortion. But we don’t notice it because the designers know what’s important and what’s not.
Philosophically and conceptually, any audio product can be approached that way. It’s impossible to hear 20 per cent distortion if the thing’s operating at 9 Hz, because the first overtone is below the threshold of audibility unless it’s so loud that it’s hurting your ears � 130 dB or more.
GB: What about when you move higher in frequency?
BC: The thresholds change, but the rule still applies.
GB: In addition to the active driver, the True Subwoofer has a mass-driven unit that moves out when the active woofer moves out, and moves in when the active unit moves in. You’d expect the outward movement of the active unit to pull the mass-driven cone in, and vice-versa. How is the mass-driven woofer made to operate in phase with the active woofer?
BC: The force that moves the driven woofer is the electricity going through the voice coil. The force that moves the other woofer is the reaction force. To understand how this works, imagine standing on a chair and holding a Slinky in your hands. Pretend there’s a little weight on the other end of the Slinky. Your hand represents the voice-coil-driven woofer. The springs in the Slinky represent the spring of the air in the box, and the mass on the bottom represents the mass glued onto the other woofer. If you start moving that Slinky up and down with your hand, when your hand goes down, the bottom end of the Slinky comes up, and when your hand comes up, the bottom end of the Slinky goes down. It’s just the opposite of what you’d expect.
GB: That’s when the rate of movement is above the resonant frequency of the system.
BC: Correct.
GB: So you get this spring effect in the subwoofer whenever the input frequency is above the resonant frequency of the subwoofer, which, given the driver design, must be very low.
BC: It’s 18 Hz.
GB: Is the new 13-inch version of the True Subwoofer just a bigger cousin, with a little more output and lower extension?
BC: The Signature Subwoofer is the same system, but slightly larger. It has 6 dB more output, 116 dB as opposed to 110 dB, and it’s tuned to 16 Hz rather than 18 Hz. It uses the same amplifier, but larger drivers � equivalent to a 10 1/2-inch driver rather than eight inches.
