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!


 
 
Sidebar: TOOLS

Table-1 lists an array of "audio tools," the Oscillator and Sound Pressure Level (SPL) Meter are respectively available through MCM Electronics 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. 

Table One
Description
Make
Model

( MCM order # )

Price
Notes
         
         
Oscillator
Tenma
( 72-505 )
$59.95
Stepped from 20-Hz to 150-kHz

sine and square out

Oscillator
Tenma
( 72-455A )
$169.00
Continuous sweep

10-Hz to 1-MHz, 7 VRMS, 

sine OR square wave outputs

Digital Voltmeter
BK
5360

(73-870)

$199.00
True RMS w/ 30kHz

frequency response

Digital Voltmeter
Fluke
8060A

(105-005)

$495.00
True RMS plus dBm scale

w/ 12Hz to 200kHz frequency response

Dual-Channel

Analog Voltmeter

Leader
(70-505)
$846.00
AC millivolt and dB meter

No ohms, current or DC volt scales.

Oscillator

Tone Plug

GTC
Gtc-industries.com
$49.95
Multiple test tones primarily for cable testing
Pink Noise generator

Noise Plug

GTC
Gtc-industries.com
$49.95
Pink noise for testing monitor systems
Sound Pressure 

Level Meter

Radio Shack
33-2050
$34.99
Analog Meter Display
Sound Pressure 

Level Meter

Radio Shack
33-2055
$59.99
LCD display
Ear Plugs
     
Your preference
Ear Muffs
     
Your preference

GTC reviews to be posted shortly
 
 
Table-1: Basic tools for general audio evaluation. In bold are the "starter kit" items. Of the voltmeters listed, all measure RMS volts while the more expensive digital and analog meters also measure dB.

 
 
Audio Monitoring Basics: Part Two

Audio Basics and Troubleshooting Procedures

The carpenter’s motto is "measure twice, cut once." With two or more audio channels, we can take maximum advantage of a logical process known as "Comparative Analysis" and extrapolate this axiom to measuring each channel twice to minimize human "data entry" errors. While the goal is to confirm basic monitor performance, the process starts at the console to confirm channel balance across the audio spectrum. The path will be followed all the way to the monitor components to confirm that the "electricity" is equally distributed to all concerned. 

Evaluating a monitoring system is not easy. Comparative analysis will only reveal a difference if one monitor is damaged and a spare set of "mains" is not likely. Otherwise, comparing an equally new or tired pair of monitors won’t be much help. Do consider spare components, especially if the manufacturer can provide matched pairs.

AUDIO BASICS

Remember that audio and power outlet juices are measured in AC volts. Batteries produce DC volts. Wire "continuity" is measured in Ohms. Cables and connectors that are doing their job will measure zero ohms, although most meters do not have the resolution to say exactly how close to zero. It is easier, though more expensive, to measure level differences in dB rather than in volts. Keep in mind that the dB is not a voltage but the logarithmic ratio of two voltages. All gear operates at a "nominal" level, Professional (+4-dBu) and Consumer (-10dBV) equipment being referenced to "0-dBu" (.775-VRMS ) and "0-dBV" (1-VRMS), respectively.

Most oscillators do not continuously sweep from 20-Hz to 20-kHz. Since a "Range" switch typically selects a band of frequencies, be sure to confirm with your ear that the switch is at the correct position. If you see level on the console meters but do not hear it, you may have selected a frequency outside the range of human hearing (above 20 kHz) that is potentially dangerous to tweeters. (A complete list of tools was detailed in Table-1 of the May Issue. You can also view the complete article online at www.tangible-technology.com.)

If you need to chase down intermittent switches, faders or connections, try using a 40-hz sine wave, exercising each of the aforementioned components. Not all oscillators produce pure sine waves at low frequencies (the Tenma 72-505 does a decent job for $59). A pure bass tone makes it easy to spot intermittent fuzz. Please note: at any step in the procedure, it is ok to adjust the signal level to produce a voltage or dB reading that is easy to read and compare. Having an assistant to confirm the readings (and write them down) will make your job easier. Provide ear protection for anyone invited to "the audio tone party."

CHANNEL BALANCE

To confirm left and right channel balance, begin at the console. Set the oscillator to 500-Hz (sine) connected to a single channel of the mixer, panned to the center. Place both the channel fader and the stereo master fader at the nominal position adjusting the oscillator output until the console’s stereo buss meters are at nominal, which could be labeled "0-VU" or somewhere between –12dB and –20dB. 

Measure the console’s left stereo output followed by the right stereo output. (If you haven’t already, it might be helpful to make a few adapter cables to facilitate the connection to the voltmeter.) The monitors need only be loud enough to confirm the oscillator is generating the correct frequency range.


 
Reference Voltage Levels
Relation to Nominal
DB difference
     
1.75 VRMS
+7-dBu
+3.0dB
1.3 VRMS
+4.5-dBu
+0.5dB
1.226VRMS
+4-dBu
0.0 dB
1.15 RMS
+3.5-dBu
-0.5dB
0.870 RMS
+1-dBu
-3.0dB
     
 
Table-2: Referenced to the nominal console operating level (+4-dBu), this table is for voltmeters without a dB scale. The examples are +/- 0.5dB and +/- 3dB relative to nominal.

 
 
A QUESTION OF BALANCE

If the two channels are not within 0.5dB, pan hard-left and then hard-right, measuring each time. (See Table-2 for voltage and dB relationships.) If the hard-panned difference is within 0.5dB, the stereo fader is probably ok. (If the stereo fader is actually two mono faders, make the channels agree.) Center the pan pot and try again. A difference, greater than 0.5 dB but less than 3dB, points toward a defective pan pot. Compare with a number of channels. If the balance improves you can thank comparative analysis. If not…

FREQUENCY TEST

Check left and right balance at 20 hz (bass) and 10 kHz (treble). Remember, the monitors are low or off (headphones can also be used to confirm oscillator settings but do not force-feed these tones to your ears). If channel balance at extreme frequencies is worse, capacitors are suspect. If not, return the oscillator to 500-Hz.

ACTIVE LINE AMPS

In all tests, a channel mismatch of 6dB is a sign of a wiring error or a failed (active balanced) output amplifier. If the channel balance is within 0.5dB, leave the faders and pan pots where they are and proceed to the Control Room Monitor output. Raise the control room level to the 9-o’clock position. If this is hair-raising, there is a gain structure error that is most likely do to the console being referenced to +4-dBu while the power amp (or powered monitors) are referenced to –10-dBV. See both the console and the power amp (powered monitor) literature to determine the output level and input sensitivity options. Once solved, kill the power to the monitors.

Measure both control room outputs, with the control at 9-o’clock, writing down each value. Now raise the control room monitor pot straight up, measure and note each channel. Repeat with the control room pot at the 3-o’clock position. The balance should match at all levels but is likely to be better at higher setting and worse at lower settings. If errors are greater than 1dB, repeat the frequency test.

POWER AMP / DRIVER TEST

By now it should seem obvious that each component in the signal path will be tested. Assuming the Control Room pot passed the low-level test, set it to minimum, fire up the power amps, then raise the control room level until the tone is audible but not life threatening. (Resume use of earplugs.) 

Measure the control room output again to confirm balance, then measure the output of each power amp using a 500-Hz sine wave. For powered monitors, remove the woofer from the cabinet, leaving it connected and measuring at its terminals. 

Adjusting the gain pots on the power amps should solve level discrepancies. If these controls do not seem in the same ballpark repeat the frequency test to rule out the power amp. Signal to the tweeters can be confirmed using a 5-kHz sine wave using ear protection, of course. (When measuring within a monitor, it’s ok to sweep the crossover region to confirm its existence but measure left / right balance using a frequency centered within the useable range of each driver.)

Drivers can be removed from the cabinet and measured with the voltmeter set to OHMS. This is a static DC resistance test, so don’t expect to be closer than 2-ohms of the stated impedance, which is the speaker’s dynamic resistance averaged over the audio frequency range. A dead driver will either be open (infinite ohms) or shorted (zero ohms). Note the values and return the color-coded wires to their respective positions.


 
 
BRING ON THE NOISE

GTC Industries makes two products, The Tone Plug and The Noise Plug, each run on Phantom Power and are built into a convenient male XLR connector for maximum portability. Both can be used for testing cables, preamps, equalizers and amplifiers. The Noise Plug generates pink noise, useful for testing equalizers, crossovers and monitors either by ear or in conjunction with a Sound Pressure Level (SPL) Meter or Real Time Analyzer.

To balance a multiple monitor system, route pink noise to your console’s buss at 20dB below maximum. Place the SPL meter equidistant from each monitor feeding one channel at time. Raise the control room level until the meter reads 85 dB-SPL using the slow, "C" weighted curve. Mark the position of the monitor knob. Check the next monitor, adjusting the power amp level (if necessary) to match the first until all channels have been tested.


 
 
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