Burning the Midnight Monitor Oil
by Eddie Cilletti

 

Speaker components have improved considerably during the past 20 years. Distortion is lower, power handling is higher, and bandwidth is wider. Despite the improvements, however, monitoring systems are still the weak link when compared to any electronic hardware, digital or analog. The obstacle is not the lack of progress but the fact that the largest collection of variables ever thrown at transducer technology is called “a room with stuff in it.” This obstruction can make it difficult to assess the health of your monitoring system.

In Mix’s February issue, Bob McCarthy wrestled with that thorny beast known as “room tuning.” This article focuses on monitor health and channel balance issues, including troubleshooting tips—from the console to the drivers—for both live and studio applications.

HOW DEEP?

SOME
BASIC
TOOLS
For General Audio Evaluation
(click for chart)


Compared to buying the latest audio processing box, tube mic or hot software plug-in, audio tools aren’t always high on anyone’s list of “must-have” future purchases.
However, here are some recommendations of basic items for anyone who’s serious about getting into audio maintenance/ troubleshooting. Test gear can get expensive, so Itried to go easy, offering some lower-cost products as well. Better quality items, such as the Fluke 8060A, are not cheap, but they make a tech’s life easier. Note that all the listed voltmeters measure RMS volts, and the more expensive digitaland analog meters also measure dB.
Most of these products are available via mail-order tool suppliers such as MCM Electronics (www.mcmelectronics.com). As an alternative, check out sites such as www.big-list.com, which features used gear from several dozen dealers-listed alphabetically and regularly updated. -EC

There’s plenty of talk about the sound of outboard gear. From converters to mic preamps, equalizers to compressors, these are devices that can be abused without fear of destruction and can be appreciated for what can be rather subtle nuances. There is nothing subtle about monitoring systems. A cabinet at the near-field position will sound much different on a bookshelf, on the floor or next to an identical system. All this before we drive ’em hard, blast ’em with feedback or accidentally remove the reference clock from a piece of digital gear. Ouch!

Have you ever wondered how much abuse a monitor system can take before its performance is degraded? Precise evaluation requires test equipment as well as a test environment (an anechoic chamber), not to mention a level of technical expertise that goes well beyond the scope of this article and the green in your wallet.

However, a basic investigation requires only a minimal electronic tool kit and some comparative analysis. In the sidebar “Basic Tools,” Table 1 lists the necessary electronic accessories, including alternatives for those of you with more cash. More extensive troubleshooting techniques will be detailed next month in this column. For example, with an oscillator and a voltmeter, it will be possible to confirm signal integrity from source to destination. Additional tools include a pink-noise generator, a sound-pressure level meter and your earplugs of choice.

ZOOM THIS
Whether built-in or freestanding, large speaker systems have the ability to move more than air. Using a continuously variable oscillator, start at 1 kHz and sweep downward into the bass region. You might be horrified to hear how easily walls, floors, racks and light fixtures can be coaxed into a cacophony of sympathetic vibrations. It is not necessary or recommended to go beyond a “comfortable” listening level for this test.

Other “room interaction” issues will reveal themselves when panning any sound source from left to right (or right to left, in some parts of the world). Try this test with kick, snare, vocal, crunchy electric guitar or pink noise. If differences were noticed, especially at low frequencies, would you suspect the monitors or the room? You can swap power amp channels easily enough—crossovers and drivers require a little more effort. Be sure to exercise every connection between the console and each speaker component. If nothing changes, the problem is with the room and/or cabinet placement.

LITTLE DAVID
Near-field monitors minimize room interaction, improving mid- to high-frequency accuracy (imaging). Yet many fall short of providing adequate low-frequency information. Subwoofers may seem to be a gimmick and are admittedly tricky to set up, yet the two octaves below 80 Hz are important enough to be worth an experiment. Satellites with less than an 8-inch woofer may need help in the octave above 80 Hz.

Absolute (coarse) polarity is critical—an out-of-phase subwoofer will create a “hole” at the crossover region rather than a smooth transition. Most times you simply want the subwoofer to pick up where the satellite rolls off. For greater flexibility, get a subwoofer that includes a crossover for itself as well as the “satellite” speakers. There will always be phase shift in the crossover region—it’s the inherent nature of the filters—so look for the ability to fine-tune the phase, in addition to polarity.

Hearing and feeling the bottom helps the listener/engineer create better mixes. Cleaning up subsonic slop makes room for the “real” bass instruments. Reducing the guesswork and using less EQ will improve mix compatibility—no more bottom-heavy mixes. All of the aforementioned items serve a dual purpose by reducing the stress on Chihuahua-sized woofers.

LESS FATIGUE

Figure 1: Cross-section of a typical woofer (not to scale)

Anytime a monitor chart shows near-flat frequency response, you are looking at the soft-focused Reader’s Digest version. It’s way more complex than that! Monitors are measured in an anechoic chamber, not in a studio environment, where each boundary—wall, floor, console—can increase low-frequency response. A freestanding monitor with no nearby boundaries will have less bass than a soffitt-mounted monitor, assuming the environment is well-designed.

Each driver in a monitoring system is much more like a stringed instrument than you might have ever imagined. I could show you charts for days, but Fig. 1 gets to the crux of the biscuit with a down and dirty crosscut view. Like a guitar that was “in tune when you bought it,” a driver starts out with a resonant frequency and a series of complex harmonic overtones that constitute its sonic fingerprint. A loudspeaker played hard will eventually go flat, shifting the resonance downward and permanently altering the fingerprint.


Continued on Page 2


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Reprinted with permission from Mix Magazine, May 2000
© 2000, Intertec Publishing, A Primedia Company All Rights Reserved