Not necessary for good subwoofer performance, but there are many good subs using this technology.
Sensing the right thing
A servo system is a generic name that has many meanings depending on who is using it. To me, a servo woofer system involves a motional feedback element capable of measuring the woofer’s acceleration and position. To quite a number of others, a servo woofer can be as simple as another coil of wire on the woofer. Both definitions are correct but the results of each are radically different.
Of the few servo woofer systems available today most rely upon a second woofer voice coil. The first voice coil acts as the magnetic motor that drives the woofer’s cone back and forth in response to the amplifier’s output. The second voice coil generates a small voltage in proportion to the woofer’s movement. If the sensing voltage is compared to the driving voltage an approximate difference signal can be derived that can be used to correct for the box enclosure’s restrictions. What you get is a flatter output from the woofer, thus we have a servo system.
In the servo systems I prefer, an accelerometer or positional sensor is mounted to the woofer that can not only measure the woofer’s movement but additionally offers precise information about its rate of acceleration and exact position. When this signal is compared to the original amplifier’s input far more information is available. Because we can now trace the exact position of the woofer cone in real time our difference signal can be used for more benefits than a simple second coil. An accelerometer-based system flattens frequency response but then goes beyond what a second coil offers: lower distortion, reduced overhang, improved transient response.
And here’s where you can tell if a system is really giving you what you assume to be advanced technology. Can it be achieved by other, simpler methods? The simple servo can be replicated by EQing a particular woofer to a specific box—but it’s far sexier to call it a servo system.
The next time you hear a fancy technology label applied to a product, take a peek under the covers to discover what the claimed benefits of that system are.
Kevin in Wappinger Falls New York (wherever that is) asked me an interesting question recently.
“Arnie Nudell popularized the line source loudspeaker design and many of his most revered Infinity speakers such as the Quantum Line Source, the IRS 1B, the IRS V, and the Genesis 1 (and apparently at least the two larger members of your forthcoming AN series loudspeakers) are all line source designs. Why don’t we see more line source loudspeaker designs? Is it strictly due to the cost because of the many more drivers, the larger cabinet, the increased manufacturing labor cost – or are there other attributes that have made them less popular?”
This is a really good question and one we don’t talk about much. Perhaps it’s a good idea to first get on the same page. The classic two-way or three-way loudspeaker has two or three drivers in a box. The multiplicity of drivers—tweeter, midrange, woofer—is needed to break apart the frequencies so each driver only has to handle a specific range: tweeters handle the higher frequencies about 2kHz and the woofer handles everything from that point down.
A line source handles the frequency divisions in exactly the same way—tweeters, midrange, and woofers—but instead of relying upon a single driver for each range multiple drivers are instead employed, typically with a line of many tweeters and sometimes many midranges. The advantages of multiple drivers in a line are manifold: each driver has fewer demands and the waveform comes out in a long, vertical, cylinder rather than a single driver’s ever-expanding circular wave.
The advantages of a line source vs. a point source can be summed up fairly easily. A point source sound radiates in all directions from the driver and quickly loses energy as it floods the room in a 180˚ plane. Worse, this expanded radiation pattern hits the ceiling, walls, and floor and reflects back into the room out of synch (time) with the initial launch. Only those listeners in a narrow sweet spot get to enjoy the best sound. A line source radiates a more focused pattern in the shape of a tall vertical cylinder that, above about 500Hz, has nearly no floor, ceiling, or sidewall reflections to dissipate energy and add to sonic confusion.
All that said, tomorrow we’ll look at some older Infinity designs.