Earphone and In-Ear Monitor Driver Technologies
Most universal-fit earphones and some custom-fit in-ear monitors use traditional voice-coil-powered, piston-type, "dynamic" drivers that may be housed either in sealed or vented earpiece enclosures (although a few designers have also experimented with enclosures that use various other resonance-control systems). You could think of these drivers as being conceptually similar to the pistonic drivers used in any number of loudspeakers, but executed in miniaturized form. One key difference, however, is that these drivers typically are run full-range, so that crossover networks generally are not required.
Example: The German firm Sennheiser's flagship IE 800 earphone ($999) uses a single very high-quality driver that is fitted in a rigid earpiece housing equipped with a set of resonance- control vents.
Some universal-fit earphones and the majority of custom-fit in-ear monitors use alternate types of drivers known as balanced- armature drivers. From the outside, balanced-armature drivers look like very tiny rectangular metal canisters, with sound outlet tubes on one side. Internally, each balanced armature-type driver features a tiny rod-like metal armature balanced in the center, almost like a teeter-totter (hence, the name "balanced armature").
The armature is surrounded by an electrical coil and positioned within the center of the gap of a magnet. As musical signals are applied to the coil, the metal armature becomes magnetized (as you may recall from physics class), so that it now reacts to the magnetic gap in which it is positioned. As the music signal fluctuates back and forth, the armature responds by swinging back and forth, tilting up and down (again, somewhat like a teeter-totter in motion). Sound is produced because one end of the armature is coupled to an extremely small diaphragm that in turn produces sound waves in the sound outlet tube.
What critical listeners appreciate about balanced-armature drivers is that they are light, small, self-contained, inherently well balanced in a mechanical sense, and extremely responsive (because they have exceedingly low moving mass). What is more, balanced-armature drivers are so small that several of them can be fitted within an earpiece housing, where this is desirable.
Examples: The U.S.-based firm Westone's flagship Westone 4R earphone ($709), which is a marvel of miniaturization, contains four balanced-armature drivers (configured as two woofers, a midrange driver, and a tweeter) per earpiece. JH Audio's flagship JH16 PRO in-ear monitor ($1149) uses a whopping total of eight balanced-armature-type drivers grouped as "double dual" woofers, dual midrange drivers, and dual tweeters. What is more, the latest JH16 PRO units feature JH Audio's proprietary "freqphase" technology, which provides a high degree of time/phase alignment between the drivers across the entire audio spectrum.
Although these are relatively rare, some high-end universal- fit earphones and custom-fit in-ear monitors use a hybrid combination of dynamic drivers and balanced armature type drivers. For example, AKG's very exotic flagship K3003 earphone ($1299) uses a dynamic driver-type woofer and two balanced armature-type drivers to cover midrange and treble frequencies.
Custom-Fit In-Ear Monitor Earpiece Technologies
As mentioned above, the performance of in-ear monitors hinges in a very significant way on the quality of their custom-fitted earpieces. You might think that since all such earpieces are based on mold impressions taken from your own ears they would be virtually identical, but in practice that is not necessarily the case. Different manufacturers have different processes and techniques for transferring ear-mold impressions to finished custom earpieces and there are significant differences-both in fit and in resulting sound quality-between brands.
There can also be significant differences between the earpiece materials used in custom-fit in-ear monitors. Many manufacturers (e.g.,JH Audio and Logitech/Ultimate Ears) mold earpieces from relatively rigid acrylic materials. Other manufacturers (e.g., ACS Custom and Sensaphonics) fabricate earpieces from much softer cold-cure silicone-gel-type materials. Still others (e.g., We- stone) split the difference by molding earpiece bodies from acrylic, but with the ear-canal section of the earpiece molded from a temperature-sensitive material that softens just a bit after the in-ear monitors have been worn for a minute or two.
One key point to bear in mind both with universal-fit earphones and custom-fit in-ear monitors is that superior fit always, always translates into superior sound quality. Think of getting the fit right, then, as the earphone or in-ear monitor equivalent of getting loudspeakers correctly positioned within your listening room.
While some may think of devices in these product categories as mere "toys," my view is that an open-minded audition of today's better quality earphones and in-ear monitors will show they have genuine sonic merit. What is more, high-performance earphones and in-ear monitors open a world of serious musical enjoyment for listening at home (ideal, for example, for late night listening) or on the go. Indeed, many serious audiophiles wouldn't dream of traveling without their preferred earphones or in-ear monitors.
In the next article in this series, I will focus on full-size headphones and their several variants.