Monitoring is one of the most important links in the audio chain, and one of the most misunderstood, being affected by a multitude of parameters. Here, Stuart Arrowsmith highlights some of the more common factors that can influence your choice of monitoring system.

Building a home studio on a budget is hard enough at the best of times; the choice of mixer and tape machine, microphones and acoustic treatment is vast. The subject that is probably the most important and gets least attention is how the whole thing sounds - the monitoring. The requirements of a good monitoring system vary considerably from the needs of simple listening; the system has to cope with a number of very specialised parameters:

1) The audio power output shall be enough to reproduce loud sounds without stress or severe distortion.

2) The frequency response must be flat and even throughout the audio range - and remain so over a wide range of levels.

3) The loudspeakers shall be capable of producing excellent stereo imaging.

4) The placing of the loudspeakers shall be designed for good stereo pictures.

5) The acoustics must be 'soft' enough to allow the reproduced sound to dominate the natural sound of the studio/control room.

Power

Loudspeakers vary considerably in efficiency, for example, there is no point in using 500 watts of power amplifier to drive a Tannoy - the loudspeaker is highly efficient and can be fully driven with 50 watts. In order to reproduce transients properly, this power should be doubled (there is a lot of short term energy in transients!). However, many popular monitor loudspeakers are much less efficient, so the 'rule of thumb' to choose the rating of the power amplifier, is to take the manufacturer's recommendation for what the speaker will stand - and double it. If the acoustic output of the speaker is easily adequate for the size of, control room, then this will cause no problems — but beware of underspending on the loudspeakers themselves, it's the worst sort of false economy.

It is also worth remembering what 'power output' really means: 100 watts is a lot of audio power, but for an amplifier to deliver anything like this, considerably more power is needed in reserve to handle the transient peaks - otherwise the effect will be loud, but vaguely distorted (listened to a disco recently?). For the average power to be, say, 50 watts; 400 to 500 watts of reserve power is needed to be sure that what you put in, comes out.

Response

Published frequency responses of loudspeakers mean very little, particularly the curves of multi-element arrays. There are many different ways of measuring them and wildly different results can be got from the same loudspeaker just by moving the test microphone an inch or two. As long as the manufacturer assures you that the speakers will give reasonable outputs down to 40Hz and up to 16kHz, and as long as they sound good after about half an hour of concentrated listening, then they stand a good chance of being OK.

There is absolutely no point in expecting your monitoring to be flat from 20Hz to 20kHz - leave those sorts of fairy stories to Hi-Fi addicts; they can't hear it either! Some speakers have a nasty habit of changing in tone when they are not being driven hard enough and this is worth watching out for.

Stereo Image

In 90% of today's recording, the stereo picture is created by 'amplitude panning' which is just a posh way of saying that if an individual signal is louder from one loudspeaker than the other, then it will appear to come more from the louder direction.

For the image to be sharp and well defined, the mid and high frequencies must come (together) from single points - this is why the stereo image effect is always so good from tiny 'bookcase' speakers. With larger studio speakers this is more difficult to achieve. Multi-element speakers do it by getting the high frequency unit as physically close as possible to the midrange driver. Co-incident speakers like Tannoy have the problem solved for them at the expense of needing very careful design to get the crossover frequencies right so as to avoid phase shift problems that would ruin everything.

Bass Response

Low frequencies play no part in stereo imaging, they are not directional, but cause their own problems.

In monitor loudspeakers there are two ways of producing good, even bass; the first is the Infinite Baffle or IB type of enclosure which relies on all the bass energy being transmitted by the front of the loudspeaker. The efficiency depends upon how large the 'baffle' (mainly the front surface of the cabinet) is and upon the density of the baffle material. The performance of the IB system is good in its linearity, but extreme low bass requires huge baffles, and very high sound pressures are difficult to achieve. In a small control room they are recommended.

The more common way to achieve the 'bottom end' is the Reflex Enclosure. Here, the cabinet is vented at the front and an internal series of baffles controls the low frequency phase so that power from the rear of the speaker (wasted in an IB) appears at the front, in phase with the pressure waves from the front of the cone.

The design of the baffles within the box, the size of the 'port' at the front, and the material the box is made of, are all very important to the final result.

Speaker Placement

Figure 4 shows the recommended distances for ideal listening to a stereo pair of loudspeakers. These dimensions may be increased, but the further the listener gets from the speakers, the greater the chance of 'hole in the middle' effects, so for good stereo listening, stay within 2-5 metres.

Note that in the diagram, the speakers do not face directly at the listener; this is because if the triangle is perfect, the only person able to listen correctly is the person sitting at the apex. By widening the angle slightly, a good image is retained over a larger area.

Acoustics

Getting the studio sounding right is 80% common sense and 20% knowing your materials and their effect on sound.

The common sense part is recognising the more obvious snags before they occur, for example: isn't it obvious that if there are parallel walls facing each other in a studio, and if they have hard surfaces, then high frequencies will skip between them and produce 'flutter echo'? Isn't it also obvious that the more soft and absorbent material that there is around, the less lively the room is - and that a symmetrical room doesn't sound as good as one with unequal surfaces?

These 'obvious' facts form the basis of simple and effective acoustic design and, with a little experimentation, anyone with a pair of ears can design a reasonable sounding room, given time and opportunity.

First the windows, any glass work at all MUST be angled, and preferably slightly downwards - it looks better. This is so that high frequency sound hitting the glass is reflected towards the floor, which will probably be carpet covered and therefore sound absorbent. If you can go to double glazing, leave at least 5 cms between the sheets of glass and don't make them parallel. The gap is to avoid standing pressure waves between the glass - these cause strange mid frequency effects.

Avoid making the room completely 'dead' to high frequencies, a few bright surfaces around make the place easier to work in. Even if the budget is tight, it isn't impossible to make the room unsymmetrical. Figure 5 shows a simple and cheap way - and the little bit of false wall takes very little space; and acts as a very good bass frequency absorber.

The design is a conversion of a simple rectangular room into a workable home studio/control room. The two existing windows are retained but improved by the addition of angled double glazing. The fireplace is a bit expensive to move, so use has been made of it as a solid area between the speakers, which are shelf or cupboard-mounted on each side.

This type of room usually has a problem with low frequency standing waves and these can be minimised by the addition of the false wall on the door side. This absorbs the bass frequencies and assists with soundproofing into the rest of the building.

A small area of hard wall has been left on the false wall to add 'brightness' to that side of the room to compensate for the glass in the windows. The wall opposite the speakers is hung with soft material. This provides a useful, uneven surface to reduce sound reflections.

Soundproofing

It's a sad fact that soundproofing (as opposed to acoustic treatment) is always expensive. There is no cheap way to keep sound in or out; and the louder, the more expensive! The basic rules are simple: keep it airtight, and nothing stops sound like sheer mass.

The airtight rule brings its own complications in that it is necessary to breathe! In any serious attempt at building a studio, some thought has to go into getting air in and out. The cheapest way is to use simple 'Vent Axia' type fans and to make your own ducting out of chipboard with acoustic baffles inside. The amount of air to be moved can be worked out quite easily - it should be about 5 complete changes per hour. If there are plenty of baffles inside the ducts, make a 50% allowance for losses due to their slowing down effect.

Don't forget that to pull air out, it must get in somewhere and the way in is just as important for soundproofing.

Once the ventilation has been taken care of, the room should be sealed when the door is closed. The door is often the most critical place. Rubber sealing strip around a standard frame will usually work well enough, but the mass of the door may have to be increased by lining it with a very dense chipboard or similar material.

The treatment and soundproofing of a room are messy, expensive and very time consuming, but once it is done well, the results are nearly as satisfying as the recordings themselves.