By Hugh Robjohns
There are many decisions to be made when choosing a
monitoring system. Infinite baffle, reflex, or transmission line?
Active, powered, or passive? Bi-wired or bi-amped? We help you find the
answers you need.
Choosing a monitoring system can be a difficult and confusing task,
not least because of the enormous number of models and designs on offer.
For a start, there are three basic classes of monitoring loudspeaker:
infinite baffle (sealed box), reflex (ported), and the less common
transmission line. Some monitors use a single wide-band driver, but most
are two-way or three-way, while others use four or more drivers. There
are also systems which require a separate subwoofer. And finally, three
different amplifier arrangements are widespread: passive, powered, and
active, along with bi-wiring options.
So let's have a look at the pros
and cons of each of these designs.
Ports
are used in the majority of project studio monitors, primarily because
they help boost the output level at low frequencies, such as the M-Audio
BX5.Photo: Mike Cameron & Mark Ewing
The simplest kind of cabinet construction used in studio monitors is
the infinite baffle, or sealed cabinet. Theoretically, an infinitely
large baffle will divide the sound coming off the front of the
loudspeaker driver from the opposite-polarity sound coming off the rear,
but both sides of the loudspeaker cone are working into the same
infinitely large volume of air and thus are loaded identically. Of
course, such a concept is not workable in practice, and the closest we
can come to the ideal is to build a large sealed box and place the
loudspeaker in the front baffle, hoping that the sound coming off the
rear of the speaker cone will be absorbed within the box.
Samson Resolv 80A.Photo: Mike Cameron & Mark Ewing
Sadly, it's not quite that easy. Clearly, the rear of the cone is
working against a much smaller volume of air than the front of the cone,
and that volume of air is fixed. Consequently the loudspeaker cone
feels a different degree of resistance when moving inwards than it does
when moving outwards, which affects the distortion characteristics of
the system as a whole. Internal resonances and standing waves can also
be created within the cabinet, despite the use of lots of absorbent
material, and this can produce various audible colorations in the sound.
Finally, the bass response of this kind of cabinet is relatively
limited compared to that of other arrangements, for a given cabinet
size, the low-frequency roll-off starting at a relatively high
frequency. On the plus side, though, the phase response is very smooth,
with relatively little phase shift, and the slope is also quite shallow,
averaging 6dB/octave. Indeed, because of the shallow slope, even small
infinite-baffle speakers can produce audible bass at surprisingly low
frequencies.
In
lower-cost monitors (such as the Event TR5 and Behringer B2030A Truth
shown below), the main side-effect of this design is a smearing of
low-end transients which makes it difficult to judge the balance of bass
instruments.Photo: Mike Cameron & Mark Ewing
For
many, the infinite-baffle design is the most highly regarded and least
compromised solution to loudspeaker monitoring. It is also interesting
to note that the most widely used mixing references — the Auratone and
the Yamaha NS10 — are both infinite-baffle designs. One of the most
revered high-quality infinite-baffle designs was the infamous LS3/5A — a
BBC in-house design dating back to the early '70s.
A couple of more modern and high-tech examples of the infinite-baffle
loudspeaker are the K+H O 300D monitor, most AVI monitors, and the
smaller ATC monitors. These speakers demonstrate the characteristically
smooth, natural-sounding bottom end and associated mid-range clarity of
the closed-box design very well. To many, what the infinite-baffle
approach lacks in raw low-end volume, it more than makes up for in
quality and transparency.
The most common cabinet design is the reflex or ported cabinet, which
makes deliberate use of the resonance of the cabinet to take advantage
of the sound coming off the rear of the loudspeaker cone. Instead of
being completely sealed, the cabinet has a hole in it through which the
internal sound can escape and contribute to the overall sound in the
listening environment.
Behringer B2030A Truth.Photo: Mike Cameron & Mark Ewing
The vent may be located on the front baffle, it may be on the rear,
and it may take the form of one or more round holes or slots. Most
usually, the vent is connected to a tube extending back into the
cabinet, the diameter and length of which are carefully calculated to
achieve the required frequency response. Across a specific frequency
range determined by the various parameters of the port opening, the
sound from the rear of the loudspeaker cone is allowed to resonate
through this port, emerging in the same polarity as the frontal sound to
bolster the low-frequency response of the system as a whole.
Fujitsu Ten Eclipse TD512.Photo: Mike Cameron & Mark Ewing
The
advantage of this approach is that it allows a much greater acoustic
output at lower frequencies than the infinite-baffle design — you get a
far more impressive bass response and overall volume level for the size
of the box. However, there are a few disadvantages, one being that any
resonant system smears transient signals over time. This can most
clearly be seen on the waterfall response charts beloved of hi-fi
magazine reviews, where one or more long resonant tails can usually be
seen at low frequencies.
In monitoring terms, this inherent time-smearing and resonant
behaviour can obscure small dynamic changes in the signal being
auditioned, and may also reduce the transparency of the mid-range. In
practical terms, a poorly designed reflex system can make it extremely
hard to judge the relative levels of bass instruments properly, because
their energy is stretched over time.
The
problems of ported cabinet design can be overcome, and more expensive
models such as the Earthworks Sigma 6.2, Fujitsu Ten Eclipse TD512,
Tannoy Ellipse 10 IDP (pictured here), and ADAM S3A are able to achieve
professional performance.Photo: Mike Cameron & Mark Ewing 
Earthworks Sigma 6.2.Photo: Mike Cameron & Mark Ewing
Another
issue is the frequency- and phase-response characteristics of the port
resonance. While the low-frequency roll-off point can be extended to a
significantly lower frequency using a reflex design than with an
equivalently sized infinite-baffle cabinet, the slope is far steeper,
and the phase shifts far greater. Thus the level of bass output is
greater down to the roll-off point, but then falls away much quicker,
and a reflex cabinet is likely to reproduce very low frequencies at a
far lower level than an infinite-baffle speaker. The inherently large
phase shifts of this design also reduce (or at least affect) the
naturalness of the bass end — although not everyone appears to be
sensitive to this aspect of sound reproduction.
ADAM S3A.Photo: Mike Cameron & Mark Ewing
The extent and impact of these inherent disadvantages depends
enormously on the competence of the reflex cabinet's design and what the
designer was trying to achieve. There are many excellent reflex designs
around, including the larger ATC monitors, all the Genelec models,
various Dynaudios, Mackies, and Tannoys, and many others.
The Mackie monitors are an interesting sub-class of reflex design,
though, because the port is covered by a passive radiator — in essence
an unpowered speaker cone that reacts to the sound pressure inside the
cabinet. This is a more complex arrangement again, sharing some
characteristics with both infinite baffle and reflex designs — although
it falls most comfortably into the latter camp.
- Blue Sky Media Desk: January 2005
- Blue Sky Pro Desk: July 2003
- K+H O 300D & Pro C28: October 2004
- M&K CR2401 & CR480: August 2003
- MJ Acoustics Pro Cinema 1 & Pro 50: December 2002
- NHT Pro M00 & S00: March 2005
- Triple P Pyramid: March 2004
The third type of cabinet is the transmission line, and at the
present time there is only really one commercial monitoring manufacturer
using this approach — PMC in the UK. Like the passive radiator
approach, the transmission line in some ways presents a combination of
both infinite baffle and reflex characteristics, arguably offering the
best aspects of both worlds. Essentially, a transmission-line speaker
places the driver cone near to or at the end of a long large-diameter
tube which is very heavily damped with absorbent material. To make the
cabinet practical, the tube is generally folded several times
internally, allowing a line length of several metres to be enclosed
within even fairly compact cabinets.
Across most of the low and middle frequency range the transmission
line is so well damped that all of the sound energy from the rear of the
driver cone is completely absorbed and none of it reaches the outside
of the cabinet. In that regard, it operates like a true infinite-baffle
design — none of the rear sound reaches the listener. At very low
frequencies, though, the line absorption becomes less effective and some
very low-frequency sound reaches the end of the transmission line, much
like the sound leaving a ported speaker. This allows a near flat
response which extends down to at least an octave below any similarly
sized reflex cabinet. One other advantage is that the overall frequency
response varies very little with monitoring volume — the balance stays
more or less constant regardless of listening level — which I personally
find very useful.
Single drivers can't really handle the entire audio spectrum at
monitoring levels, so the vast majority of monitoring speakers employ
two drivers — a low-frequency/mid-range woofer and a high-frequency
tweeter. The former generally handles frequencies below about 2kHz and
the latter everything above, the actual changeover point being called
the crossover frequency.
Getting two drivers to match each other in terms of level, phase, and
dispersion at the crossover point is far from trivial, and the on-axis
frequency response of a loudspeaker is only one aspect of its
performance that must be right. The relative phase through the crossover
region is just as important, and the smoothness of the off-axis
responses arguably more so — after all, most of the sound energy we hear
in a room is reflected off-axis sound rather than direct sound. This is
often what differentiates a really good monitor from a less good one.
All
of the monitors in Mackie's HR series, including the HR626 shown here,
use a variation on the reflex design, where a kind of passive speaker
cone is fixed over the end of the port. This design retains some of the
advantages of the infinite baffle, even though a port it used.Photo: Mike Cameron & Mark Ewing
Loudspeaker monitors have polar responses just like microphones or
acoustic instruments. Some designers argue that a loudspeaker should
have an omnidirectional polar response, and there are commercial designs
built to do that — but in most typical studio situations a directional
speaker works far better with typical acoustic treatment designs. At
very low frequencies, speakers tend to radiate omnidirectionally because
the wavelengths of low-frequency sound are generally far larger than
the speaker cabinet. As the frequency rises, the cabinet starts to
influence the dispersion of sound, and so the polar response starts to
narrow into a more directional lobe. At higher frequencies still, the
size of the driver itself starts to influence the dispersion, and the
sound lobe reduces to something more like a beam.
Through the crossover region, the sound will be generated by both
mid-range/woofer and tweeter, but given the relative size of the two
drivers in relation to the wavelengths being produced, the woofer's
polar response is likely to be very 'beamy', while the tweeter will have
a much broader dispersion. Such a disparity in dispersion angles will
cause a huge step in the off-axis frequency response, and consequently a
very coloured off-axis sound. This is one reason why a speaker can
sound very different when placed in a highly damped room than it does in
a more lively, reflective room. It's only in the last twenty years or
so that the importance of the off-axis sound and the careful matching of
dispersion has been realised. So the width of the front baffle, the
relative size of the drivers, and their crossover frequencies and filter
responses are all chosen very carefully to optimise the response of the
complete system.
Many systems these days employ waveguides around the tweeter to help
control dispersion and sometimes to create different polar responses in
the horizontal and vertical planes. This is usually to reduce early
reflections from console and ceiling, and it's also one reason why
turning a nearfield monitor on its side is not a good idea!
Designing a two-way speaker is hard enough, but most designers agree
that a three-way system offers the best overall performance. Although
there are two crossover regions to perfect, the disparity in size from
woofer to mid-range driver to tweeter is much smaller, so the dispersion
matching between adjacent drivers is easier. Each driver also has to
operate over a much narrower frequency range, which enables each to
deliver far better performance. Indeed, the improvement in the mid-range
resolution and clarity of a good three-way system compared with a
two-way system is very significant. Systems with additional drivers —
four-way systems and systems with multiple tweeters, bass units, and so
on — become a lot more complicated, and often the advantages are
outweighed or at least balanced by the disadvantages.
- PMC TB2SA & DB1SA: May 2005
The traditional way to build a loudspeaker is with a high-level
crossover that accepts the full-bandwidth, high-power output of a power
amplifier and splits that signal into two or more separate frequency
bands to feed the appropriate drivers. However, the quality of such a
'passive crossover' can affect the sound dramatically, and there are
limitations as to what can be achieved in terms of response shapes and
phase alignment using passive filtering. Additionally, passive systems
are inherently lossy in terms of power dissipation.
A passive loudspeaker is powered from a separate amplifier, typically
installed some distance from the speaker and connected via a two-wire
cable. Given the need to transfer power from the amp to the speaker and
the relatively low impedance of the speaker itself, this cable has to
have very low resistance and be able to carry large current pulses. Bell
wire is not recommended, but any relatively substantial two-core cable
will do. Two-core lawn-mower mains cable is ideal in most circumstances,
and very cost-effective — far more so than the esoteric cables promoted
in hi-fi shops.
Assuming good clean and tight connections and a competent amplifier, a
passive speaker connected with respectable cable will perform very
well. However, there are potential quality gains to made, if the
speaker's innate resolution warrants it, by doing what is known as
'bi-wiring', where the tweeter and woofer are connected to the amplifier
by separate cables. The passive crossover must be designed for bi-wired
operation, and must provide separate pairs of terminals for each driver
(normally linked with bars or brackets which must be removed for
bi-wiring). This allows the amplifier to control the damping of each
driver more effectively, as this bi-wiring separates the large sustained
current flows to the bass driver from the smaller high-frequency
signals. However, the crossover filter for each driver is still placed
at the end of a long piece of connecting cable.
A related configuration is called 'bi-amping'. Here separate power
amplifiers are used to drive the bass driver and tweeter. Each amp is
fed with a 'Y'-cord so that it is amplifying the same signal through
both channels, and is then connected to the relevant terminals of the
speaker. The idea is to remove the interaction between bass and treble
signals completely, but the practical disadvantages of this approach
usually outweigh any performance gains.
Bolting the amplifier directly to the back of the speaker cabinet
reduces the length of the speaker cable considerably, and thus also
improves performance. The result of this approach is known as a 'powered
speaker', but it is important to remember that the crossover circuitry
is still passive.
The final step is to remove the crossover function from the speaker
and perform it at line level using active electronics. Speakers designed
this way are known as active speakers, and potentially have a lot of
advantages. Firstly, far more complex filter shapes and characteristics
can be implemented with active electronics than with passive circuitry,
and the need for large capacitors and inductors is removed. This
facilitates better matching of drivers through the crossover region.
Most active designs also incorporate room equalisation and tailored
response facilities that can be very useful too.
By performing the crossover separation ahead of the amplification, it
is necessary to power each driver with its own amplifier. This affords
the second advantage, which is that the amplifiers' responses and power
ratings can be tailored precisely to the speakers they are driving and
driver protection systems can be easily built in. Most active systems
intended for home studios are fully integrated solutions, so the cabling
between the amplifier and driver is very short, although larger
high-end active systems usually employ separate amplifiers with
rackmounting active crossover units.
There are also some disadvantages with active designs, the most
obvious one being that the number of amplifiers required has doubled or
trebled compared to a passive design, and good amplifiers are inherently
expensive. There are cost savings to be made in translating a passive
crossover into an active one, but nothing like the amount needed to fund
the kind of amplifier usually employed with good passive speakers.
In order to make active speakers for the budget end of the studio
market, most manufacturers have to employ cost-effective 'chip
amplifiers' — fully integrated designs — rather than traditional
discrete circuits. Alternatively, many have chosen to use very efficient
Class-D digital switching amplifiers, but in both cases the signal
quality is often compromised in comparison with a good rackmounting
amplifier. Whether these potential amplification losses are outweighed
by the filtering and connection gains and careful system optimisation
within any specific system is largely open to debate. However, overall I
would say that, on pure resolution and transparency grounds, the losses
generally outweigh the gains on most active speakers costing less than
about £250 each in the UK, and true monitor resolution doesn't start to
emerge until at least double that figure.
- ADAM ANF10: November 2004
- Alesis M1 Active MkII: August 2002
- Alesis Prolinear 720DSP: January 2004
- Behringer B2030A Truth: September 2004
- Dynaudio Acoustics Air Series: September 2002
- Earthworks Sigma 6.2: April 2003
- EMES Black TV & Amber: September 2003
- EMES Pink TV Active: October 2002
- Event TR5 & TR8: December 2003
- Fostex PM1: November 2003
- Fujitsu Ten Eclipse TD512 & A502: June 2002
- Genelec 8040A & 7060A: December 2004
- JBL LSR6328 & LSR6312: May 2005
- K+H O 100 & O 800: November 2002
- KRK Rokit 5 & Rokit 8: August 2004
- KRK V Series 2: March 2005
- M Audio BX5: December 2003
- Mackie Tapco S5: February 2004
- Mission Pro SM6A: July 2004
- Mission Pro SM6P: September 2003
- Samson Resolv 80A: October 2003
- Tannoy Ellipse 10 IDP & TS212 IDP: June 2004
- Tannoy Ellipse: March 2003
- Wharfedale Pro Diamond 8.1 Pro Active: October 2004
- Yamaha MSP10 Studio: May 2003
