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Monitoring

Sound Workshop


No matter how sophisticated your recording equipment or your instruments, the final judgement of a track has to be made by ear — and without an accurate monitoring system, you don't really know what you're hearing. Paul White explains how monitors work and gives some tips on their choice and placement.

The technology of music making and recording has come a long way over the past decade or two, but when it comes to monitoring, most loudspeaker systems appear to have changed little since the '60s. This fact is not so much an indictment of the slow progress being made in loudspeaker technology, but rather an affirmation of the fact that a great deal of the pioneering work was right first time around. Nevertheless, there have been advances in both the design of loudspeaker drive units and the amplifiers that drive them, and now is as good a time as any to review the situation and find out exactly what constitutes a good monitoring system.

The simple aim of a studio monitoring system is to provide an accurate reference so that the mixing engineer can have confidence that his or her work is going to sound as intended. This apparently modest requirement isn't as straightforward as it seems, however, because there is no such thing as a standard monitor loudspeaker, and even if there was, it would sound different depending on the room it was used in, its position within that room and even the type of amplifier used to drive it. To make matters worse, the studio engineer doesn't know what speaker system the end user will have, because they too are all very different. Some users will have properly set up hi-fi systems, while others will use cheap music centres, complete with grossly inaccurate speakers that are probably positioned in entirely the wrong place and, in all probability, partially or completely obscured by rubber plants.

WHAT'S A MONITOR?



What exactly do we mean by an accurate monitor loudspeaker — and what's the difference between a monitor loudspeaker and a hi-fi loudspeaker? An accurate speaker might be defined as one that faithfully reproduces the entire audio spectrum (allowing some compromise at the bass end due to size restrictions), with the minimum of distortion or coloration. Even this isn't as simple as it seems, because while some speakers might come close to the above ideal when you're standing right in front of them, their off-axis performance might be far from accurate. But, assuming that you're going to set up the speakers so they are pointing directly at you, why should you worry about off-axis accuracy?

It is a fact of life that sound bounces, and what we hear when we listen to a pair of loudspeakers is a combination of the direct (on-axis) sound, added to a proportion of the off-axis sound after it has bounced off the walls and other objects within the listening room. No matter how accurate the on-axis sound, if the off-axis sound is grossly inaccurate, then it stands to reason that any reflected sound we hear will also be inaccurate. It is largely because of the differences in off-axis performance that two similarly specified loudspeakers can sound so different in the same room. In an ideal world, the only difference you should notice as you move off-axis from a loudspeaker is that the sound level drops, but in reality, the HF end will drop away first, and in the case of a bad speaker, it can change drastically.

Having reached the conclusion that a good music loudspeaker must be accurate both on and off-axis, what is the difference between a studio monitor and a good hi-fi speaker? To start with, many so-called hi-fi speakers are nothing of the kind, and indeed, many are designed with deliberate inaccuracies to make them sound more impressive. For example, a small cabinet usually signifies a limited bass response, but by tuning the cabinet so that it accentuates an area of the spectrum around 80Hz, bass drums and bass guitars can be given the illusion of having more punch and depth. That's fine in a bedsit or in the car, but if you mix on speakers like these, you end up trying to compensate for the character of the speakers; the result is that your mix sounds wrong played on any other speaker system.

Assuming we have a speaker that's sufficiently accurate both on and off-axis, the main attribute of a studio monitor is that it can handle a considerable amount of power and produce very high SPLs (Sound Pressure Levels). Though it isn't necessary to monitor at high volumes, many studio engineers like to be able to solo a bass drum and hear it at the same level as the drummer is playing it — and that peaks at 300 Watts or more! Similarly, if you're recording a full symphony orchestra and want to hear it played back at its natural volume, you're going to need hugely powerful monitors. I've already said that it isn't necessary to monitor nearly so loudly at home, but even home monitors need to be quite robust to withstand the onslaught of uncompressed drum machines or high-pitched, high energy synth sounds.

THE ROOM



Many professional monitoring systems are designed to work in a particular room, or if a studio is being built from scratch, maybe the room and monitors will be designed together as a system. And they are a system, because the room has a very profound effect on the overall sound. In larger studios, monitors are generally built into the walls, and this is known as soffit mounting. (Speakers designed for conventional stand mounting should not be soffit-mounted, as the resulting bass performance will be incorrect.)

Domestic rooms tend to be reasonably absorbent because of the profusion of carpeting, curtains, soft furnishings and so on, but even so, they are unlikely to be as well damped as a commercial recording studio. Having said this, most of us are comfortable listening to music in such a domestic room, and because this is also the type of room the typical end user will be listening in, it makes sense to mix in a room that sounds as much like your typical lounge as possible.



"Unless the room is very large, using full-range loudspeakers will only produce a confused and inaccurate bass response, even though the speakers themselves may be very accurate."


Another very important point to keep in mind is that you can't expect to generate very deep bass in a small room. Indeed, unless the room is very large, using full-range loudspeakers will only produce a confused and inaccurate bass response, even though the speakers themselves may be very accurate. This occurs because the dimensions of the room start to become small when compared to the wavelengths of the lower notes being reproduced, which results in the reflected sound interfering with the direct sound. For example, a complete cycle of a 50Hz bass note is around 20 feet long.

A sensible option is to use a pair of speakers that have a flat response down to, say, 60 or 70Hz and then roll off gradually towards the low end of the spectrum; this way the room won't affect the bass end so much. A good two-way system with a bass driver of between six and 10 inches in diameter is usually more than adequate for home studio use, and the models with soft-dome tweeters are, to my mind, far more smooth and natural sounding than those using hard metal or plastic tweeters. Horn loudspeakers have no advantages at all for low to medium level monitoring; their main advantage is that they can produce high sound levels, but the trade-off is higher distortion, particularly in the upper mid and treble regions.

POSITION



It's not just room size that's important, but also where in the room the speakers are positioned. At mid and high frequencies, the sound radiates from the loudspeaker in the form of a cone, but at low frequencies, the radiation becomes omnidirectional, with almost as much energy being directed backwards as forwards. This low frequency energy will bounce off the walls adjacent to and behind the speakers, resulting in an increase in the perceived bass level within the room — another good reason to choose speakers with a gentle bass roll-off.

Ideally, the distances between the speaker and the nearer surfaces should be as random as possible to stop all the reflections combining at the same frequency. The worst thing you can do is to put a speaker in a corner, mid-way between floor and ceiling, as this will produce a bass response that is interspersed with large peaks and troughs. This will result in some bass notes sounding much too loud, with others being unusually quiet. If the distances from the speaker to the nearest two walls, the floor and the ceiling can all be made different, the reflections will add together in a more random fashion, producing a smoother bass response with less severe peaks and dips. As a rule, try to keep speakers at least a foot from the wall in front of you and at least 18 inches from the side walls. Figure 1 shows a typical speaker position. In rectangular rooms, it is usually preferable to set up the speakers along the longest wall so as to keep them as far from the corners and side walls as possible. This reduces the intensity of any reflections reaching the listener and reduces the extent to which the direct sound is compromised by the reflected sound.

Figure 1: Positioning a monitor relative to a wall or corner.




"It is a good idea to keep speaker cables as short as possible and to ensure that both speaker leads are the same length; make sure that the speakers are wired in phase."


There is a fashion for perching monitors on the meter bridge of a mixing console, but this isn't without its problems. With the speakers so close to the console, a lot of what you hear is sound reflected from the surface of the console, and when that combines with the direct sound from the speakers, it creates peaks and dips in the mid-range. Far better to put the speakers on proper stands a little way behind the desk.

On the subject of unorthodox mounting, most monitors are designed to be positioned with the tweeter above the bass unit, not to be set on their side. The reason for this is to ensure that the sound from both the bass driver and tweeter reach the listener at the same time, even if the listener isn't sitting exactly between the speakers. Where the speakers are very small with the bass units and tweeters close together, this isn't quite so important, but even so, the result will always be better, and the acceptable listening area wider, with the speaker mounted properly. Figure 2 shows the effects of mounting a speaker sideways.

Figure 2: The effect of mounting a monitor on its side.


Because speakers are designed to be at their most accurate when used on-axis, it is important that the tweeters are pointed towards the head of the listener. If this causes a problem with positioning, it is permissible to site the speakers upside down with the tweeter at head height, or it may be more practical to angle the speakers up or down slightly using wedges. It is important that speakers don't rattle or vibrate on their stands, and though I don't feel spiked speaker stands are really necessary, it can help to park the speaker cabinets on top of four blobs of Blu Tak. Figure 3 shows the optimum speaker setup for a home studio. Note that the tweeter axes converge slightly behind the listener's head so as to allow a wider listening angle for those standing directly behind the engineer.

Figure 3: Speaker angle relative to listening position.


SUMMARY



Always choose an accurate speaker rather than a flattering one. What matters is not so much how good your mix sounds at home, but how good it sounds to the end user. Set the speakers up on proper stands, ideally between six and eight feet apart, and between four and six feet from the listening position.

Don't put the speakers in corners, avoid putting them on the meter bridge of the desk and don't mount them on their side unless you really have to.

Be certain that the amplifier you have chosen can drive the speakers at a reasonable listening level without going into clipping on the peaks. It is better to use an amplifier that's too powerful at a sensible level than it is to use an underpowered one flat out. Use heavy speaker cable, but don't be conned into spending a lot of money on something supposedly esoteric.

Always try to take a rest between recording and mixing, and if you can find time to play a few records over the system to regain your sense of perspective, it helps. Check the mix sounds good at both high and low levels, and make your final mix decisions when replaying the mix at the sort of level the end user is likely to hear it. This isn't always practical in the case of club mixes, but you can always take a copy along to your local club to try over the PA!

Finally, don't go thinking that headphones are just for wimps — check your mix at least once using phones as they'll show up distortion and minor clicks that you may never hear at all on loudspeakers.

SPEAKER WIRING

If you believe everything you read in the hi-fi press, there's a good argument to suggest that you shouldn't be reading a down-to-earth journal such as SOS at all, but you must have wondered at some of the myths and legends surrounding loudspeaker cable. While it is undeniably true that using flimsy wire will compromise the sound of your speakers, there is really very little to choose between one heavy-duty cable and another. You should always use the heaviest speaker cable practical, ideally rated at 1OA or more, but it doesn't make much difference whether the cable is multistrand or solid — pure copper or only 99.9% oxygen free. For large installations, 30A cooker cable is fine.

It is a good idea to keep speaker cables as short as possible and to ensure that both speaker leads are the same length. Make sure that the speakers are wired in phase (red terminal on the amplifier to red terminal on the speaker), and check the cable ends are firmly clamped at both the speaker and amplifier end. If you want to hide your speaker cable under the carpet, try some of that nifty 2D stuff advertised in the SOS Reader Offers at the back of the magazine. It is heavy enough for all home monitoring requirements yet flat enough to wallpaper over. It's also self-adhesive!


AMPLIFIER POWER

Conventional wisdom used to suggest that buying a small power amplifier was the best way of protecting your speakers from overload, but in recent years the converse has been proven to be true. Most speakers will stand short periods of overload providing the input signal isn't unduly distorted or clipped. However, an underpowered amplifier can easily be driven into clipping, which results in a harmonically rich, clipped waveform being fed into the tweeters. Aside from sounding disgusting, this can easily overheat the tweeter's voice coil, causing it to burn out.

The best bet is to buy the largest amplifier recommended for your speakers rather than the smallest — ideally one that has clip indicators to show you when you're running out of steam. If you can hear your speakers distorting, turn the level down before it's too late. It's difficult to suggest an ideal amplifier power for home use, as much depends on the efficiency of the speaker and on the level you like to work at, but a reasonable estimate might be between 50 and 150 Watts per channel. Amplifiers lower in power than 50 Watts can very easily be driven into clipping, especially when used with less efficient speakers.


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Souper Trouper

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The Logical Choice?


Sound On Sound - Copyright: SOS Publications Ltd.
The contents of this magazine are re-published here with the kind permission of SOS Publications Ltd.

 

Sound On Sound - Oct 1993

Feature by Paul White

Previous article in this issue:

> Souper Trouper

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> The Logical Choice?


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