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Friday, December 9, 2016

Choosing Monitors

By Hugh Robjohns
Choosing Monitors 
Photo: Mike Cameron & Mark Ewing

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.
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

Infinite-baffle Designs

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. 
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. However, 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, and ADAM S3A are able to achieve professional performance. 

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.

Reflex Cabinets

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. 
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. 
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. 
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. 
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. 
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.

Transmission-line Systems

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.

How Many Ways?

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. 
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.

Powered Models

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.

Active Electronics

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.

SOS Reviews: Reflex Monitors

  • ADAM ANF10: November 2004
  • ADAM P33A: April 2005
  • ADAM S2.5A: June 2003
  • ADAM S3A: March 2004
  • Alesis M1 Active MkII: August 2002
  • Alesis Prolinear 720DSP: January 2004
  • B&W DM602 S3: July 2002
  • Behringer B2030A Truth: September 2004
  • Dynaudio Acoustics Air Series: September 2002
  • Dynaudio BM5A: June 2005
  • Earthworks Sigma 6.2: April 2003
  • EMES Black TV & Amber: September 2003
  • EMES Pink TV Active: October 2002
  • Event ASP8: April 2004
  • Event TR5 & TR8: December 2003
  • FAR OBS: February 2005
  • 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
  • KRK V4: April 2002
  • 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
  • Yamaha MSP3: March 2002

Published July 2005

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