Radical BMR Drivers Make Cambridge Audio’s Minx System Different & Better

Posted by: Chris Martens at 5:05 pm, February 16th, 2011

One of my next projects for The Perfect Vision will be to review one of Cambridge Audio’s Minx-series surround sound speaker systems—in this case the next-to-the-top-of-the-line configuration known as the S325 package (a compact 5.1-channel rig priced at $1399). The system consists of five Minx Min 20 satellite speakers (the larger of the two Minx satellites) and a 300-watt X300 powered subwoofer (the middle model of the three available Minx subs).

At first glance, the system might seem (nay, it just plain does seem) cute, well made, and appealing, but also familiar. After all, anyone who has been exposed to Bose’s wildly popular Acoustimass systems is by now familiar with the “miniature sat/sub surround system produces big sound” concept, right? And really, given that the formula for this sort of system seems pretty well established by now, how different could the Cambridge package possibly be?

Well, the answer to that question is that Minx systems are very different, and for reasons that have everything to do with the BMR (Balanced Mode Radiator) drivers the British firm has chosen to use in its satellite speakers. What brings this point to mind is a cool white paper I received from the Cambridge Audio PR team a few days ago and that digs into the subject of BMR drivers in some detail. Trust me on this point: the more closely you look at BMR drivers, the more you’ll understand how very different from traditional drivers they really are.

Traditional loudspeakers mostly use what are commonly called “dynamic” or “piston-type” drivers—the types of driver most of us know best and have seen in use in nearly all of the loudspeakers we’ve ever encountered (planar magnetic and electrostratic loudspeakers not included). The general concept behind such dynamic drivers is that they use a more or less rigid diaphragm (think “cone” or “dome,” and you’ve got the general idea), which is pushed forward and pulled inward by an electromagnetic motor, thus producing sound waves.

The motor, generally speaking, consists of a tube-shaped voice coil former around which are wrapped closely spaced voice coil wires. The whole shebang (that is, voice coil former and voice coil wires) is suspended within the gap of a magnet assembly, so that when fluctuating audio signals are applied to the voice coil wires the resulting electromagnetic field interacts with the fixed field of the magnet, causing the motor—and thus the diaphragm to which the motor is attached—to move inward and outward, producing sound waves. About all that’s needed to complete the picture is some sort of suspension system to hold the diaphragm and voice coil assemblies in place while allowing them to move fore and aft, and a rigid frame or “basket” that hold all the elements of the driver in precise alignment. So far, so good.

But in order to work well, piston-type drivers have to meet a number of not always easy to manage requirements. First, their diaphragms must at once be very stiff and very light. Stiffness is needed so that the diaphragm’s surface won’t flex when it moves, thus distorting the sound, but lightness is also essential so that driver will be able to respond quickly to subtle “direction changes” in the audio signal (otherwise, the driver’s movements would be sluggish and sonic subtlety would be forever lost). To understand the nature of the problem, let’s consider the fact that a hypothetical driver diaphragm made of steel might be extremely stiff, but too heavy to be responsive. Correspondingly, a driver made an ultra-thin polymer film might be wonderfully light and responsive, but too flexible to precisely follow the vigorous forward and backward drive motions that music requires. Inevitably compromises must be made.

But there are several more issues and requirements that piston-type drivers must also address, and those have to do with inevitable tradeoffs between displacement requirements (that is, the volume of air that the driver must move in order to produce a desired sound pressure level at a given frequency) versus dispersion requirements. In order to produce bass frequencies at satisfying volume levels, it’s necessary to move quite large volumes of air, which means designers typically choose drivers that offer a lot of surface area and that are capable of large fore-and-aft excursions. The trouble is that these large, long-throw bass drivers (commonly called “woofers”) are typically too large (and often too heavy) to handle midrange frequencies well. Thus, designers typically wind up using specialized mid-sized drivers to handle midrange frequencies and even small drivers (typically called “tweeters”) to handle treble frequencies. This is a time-tested approach that works reasonably well, but with some inevitable tradeoffs, some of which I’ll mention below.