Studio Reference Monitors

Jun 1, 1994 2:41 PM, By George Petersen

UNDERSTANDING THE BASICS

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A proper listening system is perhaps the most fundamental (yet most often ignored) of all the tools available to the audio engineer. If your speakers are not giving you an accurate reproduction of what’s on tape (or coming out of the console), then it can be difficult or impossible to make informed decisions concerning microphone placement/selection, equalization or balancing elements in the mix.

Another problem stems from the extraordinarily wide range of playback systems that any product will eventually be played on, ranging from 3-inch television speakers to elaborate audiophile systems costing $50,000. One obvious solution for the sound engineer is that of small reference monitors to be used with (or in lieu of) a studio’s main monitors. Thus, reference monitors serve two purposes: providing a “real-world” picture of how a project sounds when played on compact speakers and a consistent “reference” point for an independent engineer working in an unfamiliar room.

MONITOR TYPES
Most speaker systems for music reproduction fall into two basic categories: those having sealed enclosures and those with vented enclosures. Sealed designs—also referred to as acoustic suspension types—mount the speaker components on the outside of an air-tight cabinet, which prevents the escape of the air displaced by the woofer’s rearward motion. The increased air pressure within the cabinet acts as a springboard to push the woofer’s cone forward. Vented systems—sometimes called bass-reflex systems—have one or more openings (vents or ports) that create a small amount of phase cancellation at the woofer’s resonant frequency, while increasing bass response at a lower frequency.

Vented monitors typically require a larger enclosure than their sealed counterparts; however, this disadvantage is offset by an increase in efficiency (more sound-per-watt input) and extended low-frequency response.

One novel design approach applied by several manufacturers is the use of coaxially mounted speakers, usually some sort of tweeter or other high-frequency unit mounted in the center of a bass reproducer. This method gives the impression that the sound is coming from a singe point source, which improves coherence, especially at close listening distances.

Not surprisingly, the actual components used in a loudspeaker system have a greater effect on a monitor’s sound than the enclosure itself. In the present state of studio transducer technology, low-frequency reproduction is handled by large-diameter woofers with cones ranging from six to 15 inches. As a rule, speakers used for mid- and high-frequency reproduction in studio monitors are either of the direct-radiator (such as cones and domes) type or some sort of compression driver/horn combination.

Direct-radiator designs offer a smooth response and a wide dispersion of the reproduced sound, yet they are less efficient than horn/compression drivers. Because efficiency is lower, direct-radiator driver systems cannot produce the very loud levels attainable by horn systems. In near-field listening, extremely loud reproduction is seldom required, as the monitors are placed in close proximity to the listener.

INTERPRETING SPECS
Interpreting specifications from speaker manufacturers can be regarded as either high art or a real chore. Yet you can tell a lot about a company and its speakers by taking a good look at the spec sheets.

The most important (yet over-hyped) bit of information on that glossy brochure is “frequency response.” Simply put, this is a measurement of the range of frequencies that a given loudspeaker can reproduce. Unfortunately, that figure alone is virtually useless without some sort of qualifier, often expressed as something like +-2 or +-3 dB. The ideal loudspeaker system would reproduce all frequencies at a constant level; this is referred to as a “flat” response.

Theoretically, a 3-inch transistor radio speaker could have a frequency of 20 to 20 kHz; what’s left out is the fact that its actual frequency response may be -90 dB at the extreme ends of the scale. Because every 10 dB of change represents a doubling (or halving) in perceived volume, the amount of 20Hz energy being reproduced by the 3-inch speaker is infinitesimally small.

A monitor’s sensitivity rating gives an indication of the system’s overall efficiency, and is expressed as the sound pressure level (in decibels) the monitor will produce, given a 1-watt input measured at 1 meter. Because console-mounted speakers are usually heard from a distance of about 1 meter, the sensitivity rating is quite important. Sensitivity ratings for loudspeakers range from about 80 dB to more than 100 dB (1W/1m). Most direct-radiator monitors fall into the 87 to 93dB range, while horn-type systems occupy the upper end of the scale, typically 95 dB and higher.

What does this all mean in real terms? Let’s say your monitor has a sensitivity spec of 90 dB (1W/1m). In that case, the monitor will provide 100 dB from a 10-watt input; 110 dB from a 100W input; and reaching 120 dB would require a 1,000W input—at which point, your poor little monitors would have emitted a pungent whiff of black smoke and then lain very quiet. By now, you should have figured out the pattern: Each 10dB increase in volume requires 10 times more amplifier power. Because the scale is based on a logarithmic progression, we also know that each 3dB volume increase requires a doubling of amplifier power. By combining this knowledge of sensitivity with a monitor’s maximum power rating, you can tell whether any given monitor will be loud enough to suit your tastes.

There seems to be a considerable amount of confusion regarding the question of what is the “right” number of components in a loudspeaker system. Obviously, there is no single speaker that can faithfully reproduce the entire audio spectrum. But is a two-way system (utilizing a woofer and a combination mid/high-frequency driver) any better than a three-way system (with woofer, MF and HF components)?

The concept of dividing the load of audio reproduction among many specialized drivers seems to be a good idea, at least in theory. Unfortunately, the reality is that each crossover network used to route the sound to its respective driver introduces a certain amount of phase error. Also, the physical realities of close-in listening imparts some real limitations in the number of drivers you can cluster together and still have a coherent sound image. Therefore, most—if not all—compact studio monitors are either two- or three-way systems.

WHAT TO LOOK FOR
Starting from the back, one thing to check out is the input terminal, which can range from a simple screw fastener to gold-plated, five-way binding posts. The latter are extremely versatile and can accommodate banana plugs, spade lugs, test prods, bare wire bent to go around the post and holes in the post itself for inserting wires of various gauges.

Self-powered speakers with internal amplification seem to be gaining in popularity. These offer the advantage of providing a complete, matched monitoring system, and may be a good choice for the traveling engineer who is looking for a reliable, consistent reference when working in different studios. JBL/UREI makes the Model 6210, a compact power amp that can easily be mounted on the back of most monitors.

On the monitor’s face, removable front grilles are a plus. While the grille “cloth” may be said to be acoustically transparent (and may come close to this ideal), other problems stem from diffraction effects and edge reflections caused when sound from the high- and mid-frequency drivers reflect off the wood or plastic frame that holds the grille cloth.

With computer and video monitors becoming a regular fixture in studios today, many reference speakers incorporate internal magnetic shielding to cut down or eliminate the picture distortion that occurs whenever a large magnetic structure is placed near a video display. Even though a speaker may be said to have magnetic shielding, the degree of protection offered varies widely. With some models, this means within three to six inches without picture degradation, while others could be placed alongside a video or computer monitor without ill effects.

A final point to consider is mounting accessories. Some manufacturers offer optional mounting brackets for their speakers, while third-party suppliers (such as OmniMount Systems of North Hollywood) provide a wide range of versatile, high-quality mounting systems. If you plan to place your reference monitors on top of your mixing board, first be sure that the board’s meter bridge can handle the weight, and second, see that the speakers can be safely balanced. Simple, secure mounts can be fashioned from a few plywood pieces and a little ingenuity. Another alternative is to make floor stands to hold the speakers at a convenient listening height.






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