Jun 1, 2001 12:00 PM, BY MARK FRINK

Known for its custom stage monitors, Bryan Olson's New York sound hire company, Firehouse Productions, quietly introduced their Firehouse 6500 two-way in-ear monitors (IEMs) last year. Like most two-way IEMs, the 6500s use two different-sized, Knowles-balanced armature drivers with a passive crossover network to deliver highs and lows. As I already had favorite models for different applications, it was with some hesitation that I used a pair on tour.

Custom ear molds are the most frustrating professional sound products to buy, as they are impossible to audition before buying. Yet, musicians who rely on them hear their entire performance through the transducers. An investment in the right pair of IEMs is one of the most important equipment decisions that affects the performers' satisfaction with live sound. Few engineers enjoy the unique privilege of unpressured comparison and critical listening to a variety of IEMs in a controlled environment.

Like Firehouse's highly regarded monitors, objectives for the 6500s included a neutral sound that wouldn't require EQ and a robust tour-worthy design. The twisted-pair cables are typical, except they're permanently attached, avoiding one of the greatest points of failure: the tiny 2-prong connectors that typically plug into the molds. The parts inside are encased in silicon that shock-mounts them, helps with isolation from outside sound, keeps the molds from drying out over time and prevents the accumulation of excess moisture.

The other design goal was to maximize comfort, as most users are put off by the strange sensation of plugging up their ears in order to perform music. Not just the tips, but the entire shell is made of a soft acrylic that is bonded to a hard polyvinyl backshell, providing a degree of comfort when the molds are worn for hours at a time, day after day.

Though the comfort was readily apparent, I did not appreciate their sound until I used them for a long duration, because I was accustomed to the contoured response of other models, which, though pleasant at first, can grate over time.

Individual ear geometry is only one consideration that affects how a particular model of IEM sounds from one person to the next. A variety of subjective factors affect preferences: the kind of music, the content of each mix, the hearing of that individual and, of course, personal taste in monitoring. While no single product will satisfy all users, the 6500s' even response provides a neutral starting point for the engineer.

The 6500s have a natural sound, with smooth highs and an even midrange. IEMs with balanced armature drivers yield more response above 4 kHz than those with dynamic drivers. Many users find these high frequencies promote ear fatigue after several hours, yet there is important musical information in the last two octaves for voices and instruments. The 6500s strike a balance in the highs between the sibilance of typically balanced armature drivers and the attenuated highs found in dynamic models. While most IEM engineers apply mix EQ to tailor response to individual users, the 6500s succeed by presenting the engineer with a neutral palette upon which to paint a mix.

Like all IEMs using Knowles drivers, the 6500s have a peak in the upper midrange at 3.5 kHz, but it is wider and stronger than most. Their lack of color in the lows offers a natural sound, and the smooth high end avoids sibilance that can tire an ear over time.

The 6500s offer more than 20 dB of isolation, which allows monitoring at lower levels in high-SPL environments. Their low impedance and high sensitivity provides 3 to 6 dB more output than similar products, allowing them to get louder without distortion from the chip amps in most packs.

Experienced performers looking for a new set of molds will find the 6500s offer a nonfatiguing, smooth high end, while providing a comfort that allows users to almost forget that they are wearing them. Engineers looking to equip an entire band can choose the 6500s with confidence that they will provide a neutral, uncolored sound. Recording artists planning to tour could even use these to prevent headphone bleed while cutting vocals; write them off against the recording budget and, in the process, get used to IEMs before hitting the road. With a price of $750, they are competitively priced with other professional products in this category, and you can't get a quality custom mold for less money.

Firehouse Productions, 1470 Route 199, Milan, NY 12571; 845/758-9898; www.firehouseproductions.com.

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Mark Frink is Mix's sound reinforcement editor.