10MM 154 - Monitor Design
Monitors are a crucial component in every studio, from project setups to pro facilities, but what distinguishes one pair from another? Mark Cousins speaks out.
Choosing a pair of monitors has to be one of the hardest decisions you’ll make when it comes to equipping your studio, with a wide variety of models on the market all claiming to offer near-perfect reproduction. It’s no surprise, therefore, to find as many different approaches to monitor design, from active and passive choices, through infinite-baffle and bass reflex enclosures to moving-coil tweeters versus ribbon designs.
Listen ear
How, then, do these different approaches impact on the listening experience? Are active units really better than passive ones? And, ultimately, what design provides the most accurate reproduction of music for recording, mixing and mastering?
Getting active
It’s often the use of active electronics that distinguish hi-fi speakers from pro studio monitors. In active designs, the speaker, amplifier and crossover electronics are mounted in the same enclosure; passive designs see the speaker and amplifier kept separate. In theory, active designs will be sonically superior, but why should the placement of the amplifier make so much difference to the reproduction of sound?
One of the most difficult challenges any loudspeaker faces lies in effectively distributing sound between the two drivers – the woofer and tweeter, in other words. The crucial component in this process is the crossover, which filters the input into two constituent bands (bass and treble) before feeding it to the respective driver.
In a hi-fi speaker, the amplifier feeds the speaker a full-bandwidth mix at its amplified power level, with the speaker’s passive crossover filtering sound ahead of the two drivers. In an active studio monitor, the powered crossover takes a line-level mix and filters it into two frequency bands, which are individually amplified and then fed to the drivers.
Crossing over
Clearly, making an active monitor isn’t simply a case of strapping an amplifier onto a speaker enclosure, but there are sound sonic reasons for going to the extra effort. A passive crossover is somewhat less efficient than an active crossover, resulting in a small dip in the frequency response around the crossover point (usually, 2kHz). If you’re listening for pleasure, this dip isn’t too problematic (especially as most hi-fi speakers are already ‘tuned’ to accentuate bass and treble). For mixing, however, the dip can cloud your judgement, especially in respect to balancing midrange instrumentation such as vocals or snares.
Having said that, it’s worth remembering that just because a monitor is active doesn’t mean that it always delivers sound that’s superior to that of a passive speaker. Match an audiophile passive hi-fi speaker with the right amp and you’ll be surprised at the level of critical listening you can achieve. Indeed, the quality of the amplifier itself is arguably just as important as the crossover, and with many entry-level active monitors employing cheaper amplifier designs, it isn’t always true that active is best.
Feeling baffled?
Although it’s easy to be absorbed by the electronics behind monitor design, it’s important to remember that the enclosure itself has a fundamental impact on the sound, especially as a speaker will be pumping as much sound into the cabinet as it will the room.
One solution is to effectively enclose the whole design – sealing the cabinet off and filling it with acoustically absorbent material to soak up internal reflections. This so-called infinite-baffle design was used for Yamaha’s NS10s, and theoretically negates any phase cancellations brought about by the ‘rear’ sound interfering with the direct ‘frontal’ sound emanating from the speaker cone.
However, one major downside to the infinite-baffle design is the lack of bass response, which was a much-touted criticism of the NS10s (although this didn’t stop them from becoming one of the most popular studio monitors to mix with).
Reflex action
A much more common approach, therefore, is the bass reflex system, whereby a port or tunnel is mounted in the wall of the cabinet, directing sound either to the front or rear of the enclosure. This ‘tuned’ port reinforces the bass response of the speaker, as well as enabling it to move air more effectively than sealed infinite-baffle designs.
Probably as a result of this improved bass response, ported designs tend to be the most popular solutions for studio monitors, although it’s worth noting that the bass reflex design has its negative points in relation to extra unwanted bass resonances as well as some compromise in transient performance.
An interesting compromise between both approaches is PMC’s Advance Transmission Line (ATL) concept. In this design, the driver is mounted at the end of a long tunnel wrapped around the insides of the cabinet, which is filled with acoustically absorbent material. In theory, the ATL design will deliver improved bass extension (much like ported designs), although thanks to the absorption mounted along the tube, only a negligible amount of sound leaves the speaker’s enclosure. The net result is less distortion and timbral uniformity at a variety of listening levels.
Aesthetically, it’s also worth noting how monitors have become increasingly curved in recent years. Rather than being designed simply to look good, the curves of the cabinet are deliberately engineered to minimise so-called diffraction distortion. This distortion is created by sharp corners at the cabinet’s edges, which effectively end up acting like secondary sound sources, confusing the otherwise precise imaging coming directly from the speaker cone itself. Monitors such as Genelec’s 8000 series or KRK’s VXT series are designed to reduce diffraction distortion as much as possible.
The third way
The final pieces of the puzzle are the drivers themselves – both in respect to the number of drivers used as well as the material they are constructed from. Most nearfield monitors are two-way designs, using a bi-ampflied set of drivers (the woofer and tweeter) to deliver the mix. Even with the benefits of active crossovers, though, two-way designs still divide the mix in an uncomfortable way, potentially slicing important midrange sounds in two.
A three-way midfield monitor, though, divides the mix in a more sympathetic way, using a single driver for the entirety of the midrange and two further drivers for the extremes of the spectrum.
As you’d expect, a three-way active monitor is more expensive to produce than a two-way design, with the drivers needing to be tri-amplified (rather than bi-amplified) as well as necessitating a larger cabinet. As a result, three-way designs are often significantly bulkier and more expensive than two-way designs, resulting in them being positioned much further away from the listener. For many project studios, the small sonic improvement offered by three-way designs are often far outweighed by space and cost considerations.
Newer monitors have now adopted woven Kevlar as the material of choice in the construction of the woofer. By comparison to a traditional paper-based cone, a woven Kevlar woofer is both stiffer and lighter, enabling it to retain its shape even at high energy levels as well as improving transient response.
Use your ears
While it’s easy to become absorbed in the technical merits of different designs, it’s important to let your ears be the judge. What might seem a perfect design on paper doesn’t necessarily translate into a sound that you’ll enjoy working with on a daily basis. The success of ‘flawed’ speakers such as the NS10 are testament to this fact, illustrating that it’s often best to understand and appreciate the sound of your monitors rather than simply opting for the most expensive set you can find.
Tech Terms
Woofer: a loudspeaker driver specifically designed to reproduce low-frequency sounds between 50Hz and 2kHz.
Tweeter: a tweeter reproduces sounds between 2kHz and 20kHz (the theoretical limit of human hearing).
This feature first appeared in Music Tech Magazine issue 75
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