Reference Monitors.pdf

IndustryClick Article

Page 1 of 9

GOOD REFERENCES

Brian Knave

Electronic Musician, Jun 1, 2001

Judging by the steady flow of letters and phone calls we get asking our advice about what gear to buy, a

good number of readers are well acquainted with cognitive overload. That's the term psychologists use

to describe the paralysis that can set in when we are confronted by too many options (or too much

information). Freedom of choice is great, but clearly, too many options can bewilder. Case in point: the

EM 2001 Personal Studio Buyer's Guide lists 40 companies presently offering reference monitors, with

more than 200 models to choose from.

Bewildered? If so, you've come to the right place. This article will cover the various designs,

components, and properties (including terminology) of reference monitors, as well as how they work —

in short, all you need to know to make informed decisions when selecting close-field reference

monitors for your personal studio. (Though many of the concepts discussed here apply equally well to

monitors for surround arrays, those interested specifically in monitoring for 5.1 should also see “You're

Surrounded” in the October 2000 EM. )

PRE ROLL

Speakers used in recording studios are called monitors and generally fall into two categories: main

monitors and compact or close-field reference monitors. Mains, as they are called, are mostly found in

the control rooms of large commercial studios, often flush-mounted in a “false” wall (called a soffit );

close-field reference monitors are freestanding and usually sit atop the console bridge or on stands

directly behind the console.

Most personal studios don't have the space or funds for main monitors, so this article will focus on the

compact reference monitor — a relatively recent studio tool. The first “compact” monitor to see

widespread use in recording studios was the JBL 4311, a 3-way design introduced in the late 1960s.

The 4311 was quite large, however (it had a 12-inch woofer, a 5-inch midrange speaker, and a 1.4-inch

tweeter), and today would qualify more as a mid-field monitor.

As engineers increasingly realized the importance of hearing how their mixes sounded on car and

television speakers, smaller reference monitors gained in popularity. One of the earliest favorites

(around the mid-1970s) was the Auratone “cube,” which had a single 5-inch speaker.

Car and home-stereo speakers kept improving, of course, so engineers were always on the lookout for

better close-fields. One compact model that caught on big was the Yamaha NS-10M (see Fig. 1 ). A

bookshelf-type speaker introduced in 1978 for home use, the NS-10M soon became a familiar sight in

commercial studios, and it remains popular — or at least ubiquitous — to this day.

Another significant development was the introduction in 1977 of the MDM-4 near-field monitor, made

by audio pioneer Ed Long's company, Calibration Standard Instruments. The MDM-4s were great

http://industryclick.com//magazinearticle.asp?magazinearticleid=102735&mode=print

25/06/2001

IndustryClick Article

Page 2 of 9

monitors, but it was the then-revolutionary concept of near-field monitoring that secured a chapter in

audio history for Long. (Long also originated the concept of time alignment for speakers and

trademarked the term “Time Align”; more on this later.) Though no one could have predicted how

prophetic the term near-field monitor would prove, Long clearly understood its significance and so had

it trademarked. (That is why EM uses the term close-field monitor instead).

ENVIRONMENTAL ISSUES

Curiously, because close-field reference monitors have become increasingly accurate during the course

of time, the original rationale for using them — to generate a good indication of how mixes will

translate to low-cost car and home-stereo speakers — has waned. But there are also other good reasons

close-field monitors have become all but indispensable in music production. For one, professional mix

engineers are typically hired on a project-by-project basis, which means they may end up in a different

studio from one day to the next. Close-field monitors, because they are portable enough to be carted

from studio to studio, make for an ideal solution and guarantee, at the minimum, some level of sonic

consistency, regardless of the room.

But don't the monitors sound different in different rooms? To a degree, they do. But another advantage

of close-field monitors is that they can partially mitigate the effect of the room on what you hear. As

their name makes clear, they are meant to be used in the “near field,” typically about three feet from the

engineer's ears. At that distance, assuming the monitors are well positioned and used correctly, the

sound can pass to the ears largely unaffected by surface reflections (from the walls, ceiling, console,

and so forth) and the various sonic ills they can wreak.

For the same reason, close-field monitoring is also a good solution for the personal studio, where sonic

anomalies are the norm. As engineer, consultant, and all-around acoustics wizard Bob Hodas has so

well demonstrated, however, it's foolhardy to think close-field monitors entirely spare you from the

effects of room acoustics. “near-field monitors can be accurate,” explains Hodas, “only if care is taken

in the placement of the speakers and room issues are not ignored.” (Find more information at

www.bobhodas.com/pub1.html.)

DIFFERENT WORLDS

A common misconception among those new to music production is that home-stereo speakers are

adequate for monitoring. That is, in fact, not the case. The problem is one of purpose: whereas

manufacturers design reference monitors to reproduce signals accurately, home-stereo speakers are

specifically designed to make recordings sound “better.” Typically, that perceived improvement is

accomplished by boosting low and high frequencies. Although it may sound like an enhancement to the

average listener, such “hype” is really a move away from accuracy.

Home-stereo speakers may also be engineered to de-emphasize midrange frequencies so as to mask

problems in this critical range. That makes it difficult to hear what's going on in the midrange, which

can tempt mixers to overcompensate with EQ. It can also lead to fatigue because the ear must strain to

hear the mids.

Yet another reason home-stereo speakers are inappropriate for monitoring is that they are meant to be

listened to in the far field, where much of the sound is reflected. But as we've seen, close-field monitors

are designed to be used in the near field, in order to help minimize the effects of room acoustics. Of

course, it's important not to sit too close to near fields. Rather, they should be positioned far enough

back to allow the sound from the speakers to blend into an apparent point source and stereo soundstage.

http://industryclick.com//magazinearticle.asp?magazinearticleid=102735&mode=print

25/06/2001

IndustryClick Article

Page 3 of 9

As you move in closer than three feet or so, the sound from each speaker becomes distinguishable

separately, which is not what you want.

ELUSIVE BULL'S-EYE

Everyone can agree that reference monitors are meant to reproduce signals accurately. But what is

accuracy? For our purposes, there are three objective tests that can be performed to help quantify

accuracy in reference monitors. The tests measure frequency response, transient or impulse response ,

and lastly, distortion .

Frequency response is a measure of the changes in output level that occur as a monitor is fed a full

spectrum of constant-level input frequencies. The output levels can be plotted as a line on a graph —

called a frequency response plot — in relation to a nominal level represented as a median line typically

marked 0 dB (see Fig. 2 ). The monitor is said to have a “flat” or linear frequency response when that

line corresponds closely to the median line — that is, does not fluctuate much above or below from one

frequency to the next.

When they are written out, frequency-response specifications first designate a frequency range, which

is typically somewhere between 40 and 60 Hz on the low end and 18 to 22 kHz on the high end. To

complete the specification, the frequency range is followed by a range specifier, which is a plus/minus

figure indicating, in decibels, the range of output fluctuation. For example, the spec “50 Hz — 20 kHz

(±1 dB)” means that frequencies produced by the monitor between 50 Hz and 20 kHz will vary no more

than 1 dB up or down (louder or quieter) from the input signal. (That spec would suggest a very flat

monitor, by the way!) Note that the range specifier may also be expressed as two numbers, for example

“+1/-2 dB, ” which is useful when the response varies more one direction than the other.

Primary frequency-response measurements are made on-axis, that is, with the test mic directly facing

the monitor, often at a distance of one meter. Also helpful are off-axis frequency response plots

(measured with the mic at a 30-degree angle to the monitor, for example), which give an indication of

how accurate the response will be — or how much it might change — as you reach for controls or gear

located outside of the “sweet spot.” (The sweet spot is the ideal position to sit at in relation to the

monitors; it is calculated by distance, angle, and listening.)

Transient or impulse response is a measure of the speaker's ability to reproduce the fast rise of a

transient and the time it takes for the speaker to settle or stop moving after reproduction of the transient.

Obviously, the first characteristic is critical to accurate reproduction of instrument dynamics and

transients (such as the attack of a drum hit or a string pluck). The second is important because a speaker

that is still in motion from a previous waveform will mask the following waveform and thus muddle the

sound (see Fig. 3 ).

Distortion refers to undesirable components of a signal, which is to say, anything added to the signal

that was not there in the first place. For monitors it can be divided into two categories: harmonic

distortion and intermodulation distortion (IM). Harmonic distortion is any distortion related in some

way to the original input signal. It includes second- and third-harmonic distortion, total harmonic

distortion (THD), and noise (which are the types most commonly measured; see Fig. 4 ), as well as

higher harmonic distortions (fifth, seventh, ninth, and so on). Intermodulation distortion is a form of

“self-noise” that is generated by the speaker system in response to being excited by a dynamic,

multifrequency signal; typically, it is more audible and more annoying than harmonic distortion.

Frequency response, impulse response, and distortion levels should all be taken into account to get an

http://industryclick.com//magazinearticle.asp?magazinearticleid=102735&mode=print

25/06/2001

IndustryClick Article

Page 4 of 9

idea of a monitor's accuracy. However, frequency response is often the only measure mentioned in

product literature and reviews, and even it gets short shrift on occasion. (In many instances, I have seen

frequency specs given with no range specifier — and of course, without it the specification is

meaningless). Few manufacturers provide an impulse response graph (even assuming they have

measured impulse response), and often the only distortion specification given is “THD + noise.” In fact,

the lack of established and agreed-upon standards for monitor (and for microphone) specifications —

for both measuring them and reporting them — is a long-standing industry issue. Though it is true that

specs don't tell the entire story, they are useful for corroborating what our ears tell us, and as such they

can help educate us so that we can more exactingly listen.

MIRROR IMAGE

Now that we've established the raison d' être of the close-field monitor, let's take a look at its anatomy.

We'll start with the internal components and work our way outward to the enclosure. Understanding

how monitors are put together will help you know what to look for when deciding which best suit your

needs.

Interestingly, the devices on either end of the recording signal chain — microphones and monitors —

are very similar. Both are types of transducers , or devices that transform energy from one form into

another. The difference is in the direction of energy flow: microphones convert sound waves into

electrical signals and speakers convert electrical signals into sound waves. However, the components

and operating principles of monitors and mics are essentially the same.

The speakers most commonly used in close-field monitors work in the same way as moving-coil

dynamic microphones do, only in reverse. (Actually, there is a correlative speaker for other types of

microphones as well, including ribbons and condensers. However, we will limit the discussion to the

moving-coil type in this article.) In a moving-coil dynamic microphone, a thin, circular diaphragm is

attached to a fine coil of wire positioned inside a gap in a permanent magnet. Sound waves move the

diaphragm back and forth, causing the attached coil to move in its north/south magnetic field, thus

generating a tiny electric current within the coil of wire.

In a loudspeaker, the coil of wire is known as the voice coil. As the electric current (audio signal)

fluctuates in the wire, it generates an oscillating magnetic field that pushes and pulls against the

magnet, causing the voice coil and attached diaphragm (in this case, the speaker cone; see Fig. 5 ) to

vibrate. In turn, the vibrating speaker cone agitates nearby air molecules, creating the sound waves that

reach our ears. (The ear, by the way, is also a transducer. It has a diaphragm — the timpanic membrane

or eardrum — that converts acoustic sound waves into tiny electrochemical impulses which the brain

then interprets as sound.)

DRIVING LESSONS

A loudspeaker's magnet, voice coil, and diaphragm form, collectively, an assembly called a driver .

(The moving-coil driver is the most common type, but there are other kinds as well.) Close-field

monitors usually contain either two or three drivers, and thus are designated 2-way or 3-way ,

respectively. Standard 2-way monitors contain a woofer and tweeter; standard 3-ways contain a woofer,

a tweeter, and a midrange driver. The woofer, of course, reproduces lower frequencies and the tweeter,

the higher frequencies.

Cones and domes are the two most common types of diaphragms used in monitor drivers. Woofers and

most midrange drivers employ cone diaphragms, typically made of treated paper, polypropylene, or

http://industryclick.com//magazinearticle.asp?magazinearticleid=102735&mode=print

25/06/2001

IndustryClick Article

Page 5 of 9

more exotic materials such as Kevlar. (Note that the dome-shaped piece in the center of a woofer cone

is a dust cap, not a dome.) Most moving-coil tweeters use a small dome, typically measuring one inch

in diameter. One advantage of a small dome is that it exhibits fast transient response and a wide

dispersion pattern, both of which are critical to the reproduction of upper frequencies. Domes are

routinely made of treated paper too, but may also be made from a metal such as aluminum or titanium,

or sometimes from stiffened silk, which some people believe sounds less harsh than metal.

When monitors employ separate drivers, as 2-way and 3-way monitors do, the design is termed

discrete . In discrete designs, the drivers are usually mounted on the front face of the enclosure as close

together as possible, which helps the sound blend into a coherent point source at the sweet spot.

Depending on the monitors, the sound can change dramatically as you move away from the sweet spot.

IT'S ABOUT TIME

Some companies, for example Tannoy, employ an alternative driver design in some of their monitors in

which the tweeter is mounted in the center of the woofer cone (see Fig. 6 ). Though more expensive,

this coaxial design is naturally more time coherent than discrete designs because the drivers are

positioned on the same axis (as well as closer together). Indeed, the coaxial driver arrangement is one

of the design elements (among others) that manufacturers have used to meet Ed Long's Time Align

specification, mentioned before.

Before we can understand how time alignment can improve a monitor's accuracy, we must first

understand the timing problems inherent in conventional monitor designs. Discrete loudspeakers cause

minute delays that spread sounds out in time, resulting in lost detail and a blurred or smeared sound.

Specifically, sound from the woofer is delayed more than sound from the tweeter. This problem has

two main sources, one structural, the other electronic. In a discrete monitor with a flat-face enclosure,

the woofer voice coil is naturally set back further than the tweeter voice coil because of the extra depth

of the cone in relation to the dome. The tweeter is therefore closer to your ears, causing the high

frequencies to arrive slightly ahead of the lows.

The problem is compounded by the crossover , an electronic circuit that splits the incoming signal into

separate frequency bands and directs each band to the appropriate driver (more on crossovers

momentarily). As it happens, crossovers also tend to delay low frequencies more than highs.

With his Time Align scheme, Long was the first to specify corrections for these problems, including

physically lining up the drivers and adjusting driver and crossover delay parameters. When correctly

implemented, Time Alignment ensures that the time relationships of the fundamentals and overtones of

sounds are the same when they reach the listener as they were in the electrical signal at the input

terminals of the monitor.

Over the years, some manufacturers have devised their own time-alignment schemes. You may recall,

for example, the now-discontinued JBL 4200 series monitors, which employed protruding woofers

designed to deliver low frequencies to the listener's ears simultaneous with highs from the tweeters.

WHEN I CROSS OVER

As mentioned, the crossover's job is to divide the incoming signal into separate bands and then send

each band to the appropriate driver. In inexpensive monitors, this is typically accomplished using

simple lowpass and highpass filters that split the signal coming from the power amp. This is called a

passive crossover. In more sophisticated systems, an active crossover splits the line-level signal before

http://industryclick.com//magazinearticle.asp?magazinearticleid=102735&mode=print

25/06/2001

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: