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

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.

Materials/size-induced sonic discontinuities:

As most critical listeners can attest, there are almost always some audible discontinuities to be heard whenever sound output transitions from drivers of one size to another, or from drivers made of one material to another. While these discontinuities can be pretty subtle, and heaven knows that speaker designers burn barrels of midnight oil working to minimize them, they are there nonetheless. In practice, this means that our ears can and often do pick up on what I’ll call “signature differences” between different sizes and types of drivers. (If you don’t believe these exist at all, try listening to identically-sized tweeters made of different materials—say of doped fabric, polymer films, or various metal alloys—to see if you can spot “signature difference” (I’m betting that, with a little practice, you can and will spot them).

Dispersion-induced discontinuities:

As we ask drive units of a given size to reproduce higher and higher frequencies, their dispersion (that is, the ability to produce evenly balanced output levels not only directly to the front, but also well off to the sides) falls off dramatically. At lower frequencies, where the wavelengths of the sound being reproduced are much longer than drivers are wide, dispersion is good. But at higher frequencies, as wavelengths get shorter and shorter—so that they may be about the same length (or even smaller than) the diameter of the driver—dispersion falls way off, so that drivers are said to be “beaming” (as in the way that a flashlight typically throws most of its light output straight ahead, and not off to the sides). In practice, this means most multi-driver loudspeakers can produce reasonably balanced sound “on-axis” (that is, when measured from directly in front of the speaker), but have distinctly lump-looking response curves when measured to the sides or from above or below the central axis of the speaker, which again causes discontinuities that the ear can and does detect.

Crossover-induced discontinuities:

Most multi-driver loudspeakers use electronic crossover networks, which essentially route different portions of the incoming audio signals to the appropriate drive units—bass frequencies to woofers, middle frequencies to midrange drivers, and so forth. But crossover networks also take on other tasks, such a balancing output levels between drivers, and also governing the “steepness” and phase (or timing characteristics) of the overlap between one driver (or set of drivers) and the next. Given the inherent complexity of the crossover network’s job, it’s inevitable for crossovers to contribute discontinuities and distortions of their own. If you doubt this, you might want to check out some of the relatively rare crossover-less speaker designs on the market to see what happens when you remove crossover networks from the sonic equation.

How do BMR drivers help/what makes them different?

Balanced mode radiators (or BMR drivers) have been around for some time, but have only recently undergone the extensive development work necessary to make them suitable for true hi-fi applications. But here’s the general concept, distilled down to its simplest and most rudimentary form.

Suppose you had a driver that seemingly started out as a conventional piston-type driver, and that was equipped with a light, flat, medium-sized, disc-shaped diaphragm (picture a driver about the size of a traditional midrange driver, but one whose diaphragm was neither a cone nor a dome, but rather a flat disc). But here’s where things get really interesting and—truth be told—a little bit strange. Imagine that this disc-shaped diaphragm behaved pretty much like a rigid piston at lower and middle frequencies, but that as frequencies climb higher we deliberately allowed the diaphragm (and in fact, deliberately designed the diaphragm) to flex with so-called “bending modes,” so that its once rigid and flat disc-shaped surface would instead begin to ripple, with waves of motion that spread out in concentric circles from the center of the diaphragm (where the voice coil is attached) to its outer rim (where the “surround” is attached). But let’s be frank: ordinarily, we would consider such flexing of the diaphragm to be undesirable “break up” and would try to avoid it like the plague (or to damp it out). The key in a BMR driver, however, is that flexing of the diaphragm isn’t random or uncontrolled; instead, it is carefully balanced and used to our advantage, so that—by design—the driver transitions from pistonic (fore-and-aft) motion to ripple motion as frequencies climb higher and higher. This transition buys us several things.

•No need for midrange-driver-to-tweeter crossovers: a BMR driver as used in the Cambridge Audio Minx satellite speakers can and does cover both midrange and treble frequencies, so that no crossover network is needed. At their present state of development, BMR drivers can handle everything from lower midrange frequencies on up, so that a traditional woofer—a small powered subwoofer in the case of the Minx system—is still required. But who knows what future development possibilities BMR technology might hold?
•No dispersion problems: a BMR driver does not “beam” as frequencies climb higher and higher; on the contrary, dispersion remains broad and evenly balanced from the bottom of its range to the top. This is true because the transition to ripple-mode operation allows even fairly large diameter drivers to disperse well—even at extremely high frequencies.
•No phase or timing problems: a BMR stays in phase with itself (well, plus or minus a bit as in any drive unit) across its entire operating range. What is more, there are “lobing” effects to contend with as in normal multi-driver speakers with separate, dedicated midrange and tweeter drivers.

Put all of these factors together, as Cambridge Audio has done in the Minx system, and you have the recipe for a compact system that can produce an unusually big and sophisticated sound.

Just consider this: until the Minx system came along, the only other firm to apply BMR drivers in a high-end audio context was Naim Audio, which uses BMR drivers in its brilliant Ovator-series floorstanding loudspeakers. Having heard Naim’s Ovators, I can vouch for the fact that they sound terrific, but they are also relatively large and expensive (the top Ovator models cost many thousands of dollars per pair). What’s exciting to me about the Minx rig is that, by design, it makes the advantages of BMR technology accessible in compact format and at Everyman prices.

Stay tuned for a full review of the Cambridge Minx S325 system in an upcoming online issue of The Perfect Vision.