Audio Monitoring Basics: Part One

Burning the Midnight Monitor Oil

© 2000 by Eddie Ciletti

Speaker components have improved considerably over the past twenty 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 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 the February 2000 issue of MIX, Bob McCarthy wrestled 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?

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 — 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. 

To proceed with a basic investigation requires minimal electronic tool kit plus a bit of comparative analysis. Next to the "Tools" Sidebar, Table-1 lists the necessary electronic accessories including alternatives for those of you with more cash. More extensive troubleshooting techniques will be detailed in Part Two: Audio Basics and Troubleshooting Procedures. 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 to ability to move more than air. Using a continuously variable oscillator, start at 1kHz 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, it’s back to "thornsville," a.k.a., 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 that are admittedly tricky to set-up, yet the two octaves below 80 Hz are important enough to encourage everyone to 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 sub-sonic 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

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 soffit-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 Figure-1 gets to the crux of the biscuit with a down and dirty cross-cut 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. 

 
MARGIN OF SAFETY

Powered (active) monitors typically have some sort of driver protection either in the form of a peak limiter or a safety relay. Passive monitors are further down on the food chain relying on the installer to include not only a safety valve (a fuse), but also a healthy gain structure between source and power amp. Your system should be calibrated so that a comfortable listening level occurs when the control room level pot "lives" between straight up and the three-o’clock position.

Once upon a time I was fast winding an analog tape past what should have been the end of any recorded material. A sudden screech and then an eerie silence jolted me back to consciousness too late to pull my finger away from the heads. A one-amp fast-blow fuse did not react in time to protect all of my beloved Radio Shack Minimus-7s — four in a passive-surround configuration. At least they were fused. Figure-2 shows the distorted bobbin after the voice coil overheated. The two surviving tweeters (made in Japan) didn’t sound anything like the two new replacements from Malaysia. Yeah, I know these are cheap consumer-grade speakers. My "real" monitors are by Dynaudio Acoustics, model BM15A.

 
MORE FATIGUE

Monitor fatigue gradually occurs over time so you may not notice the change until a driver fails and is replaced. The difference may also be related to manufacturing tolerances. If you read no further, at the very least keep two spares and always replace in pairs. If one tweeter dies, replace both, dating the new and labeling the used (working) driver. For (near-field) monitors that are cheap enough, sometimes an extra pair is less expensive than buying spare drivers. (You can always use them in a passive surround configuration, to be discussed in a future article.)

YOU HEAR FOOTSTEPS?

Monitor selection is not an easy task. The terms "accurate" and "pleasing" can be disparate qualities. The former may appear unremarkable at first audition, while the latter might provide instant gratification suitable for those moments of "easy listening," when the ear is off-duty. Great sounding speaker systems are easy to come by, but the path toward greater accuracy requires many deliberate steps. 

The natural, free-air resonance of a woofer changes when installed in a cabinet, sealed or ported. Disconnect one wire from a passive monitor and tap on the woofer with your finger, listening closely for its natural resonance. Next, alternate between connected and disconnected (with the power amp on). You should notice more resonance with the amp disconnected. This demonstrates Damping Factor (DF), the relationship between the power amp’s output impedance and the speaker’s nominal impedance. DF is undermined by cable resistance, hence the hunger for cable a la monster.

Powered monitors made Damping Factor less of a variable by reducing cable length — from power amp to driver — to about 12-inches. In addition, by using an Active Crossover Network, pre-power amp (rather than a passive network, post-amp) both the woofer and the tweeter are better damped. An active system may seem a little less exciting than it’s passive cousin, but it is now more accurate (tighter bass, smoother treble).

REAL MAGIC (not snake) OIL

All wire has resistance (stated in ohms-per-foot and referenced to room temperature) that increases when hot and decreases when cold. This applies to voice-coil windings as well, so when you continuously blast those boom boxes, heat makes the coil less effective at delivering transients. If you listen loud over an extended period of time, the monitors will sound dull and floppy from fatigue. (Your ears get tired too.)

There is one, quite literally remarkable "solution," called Ferrofluid (www.ferrofluidics.com), a magnetic material suspended in a viscous liquid. It is used to fill the magnetic air gap into which the voice coil is suspended. The fluid provides three benefits: greater transfer of magnetic energy, "hydraulic" damping of the resonant diaphragm and the transfer of heat from the coil to the magnet assembly. It’s not hard to see (and hear) that Ferrofluid makes a driver more efficient, less colored and better equipped to deliver transients even when driven hard. 

The manufacturer’s sample pack included fluids for tweeters through subwoofers including one optimized for midrange and high-frequency compression drivers. I was willing to sacrifice a Minimus-7 or two to see how much magic I could conjure out of them.
 

 
Figure-3 shows the powerfully focused magnet assembly attracting the fluid from its container. It took some experimenting to achieve an optimum, repeatable mic placement (solely for the purposes of demonstrating the effect for this article). Using an AKG C300 with an omni capsule and a Great River transformerless mic preamp.

 
Figure-4a shows a 500Hz square wave through an unmodified tweeter. 

 
Figure-4b shows the same tweeter after ferrofluid treatment. Notice that the amount of overshoot decreases with treatment.

End of Part One: Ciao!

Table-1 lists an array of "audio tools," the Oscillator and Sound Pressure Level (SPL) Meter are respectively available through MCM Electronics (www.mcmelectronics.com) and Radio Shack. Maintenance and tools may not be high on your list of essentials, so I tried to go easy, highlighting in bold the essential items. Better quality items like the Fluke 8060A are not cheap but they make geek life easier. A quick search on the net revealed the ultimate used equipment "network," www.big-list.com, where several dozen dealers are alphabetically listed and regularly updated.