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).
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.
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.
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.