It has been common practice in studios for some time to have at least two sets of monitoring loudspeaker systems. The reasons behind this are evolutionary as much as anything else, but it is important to realise that at different stages in the recording process eg. backing tracks, overdubbing and mixdown a different approach to listening is required. In the initial stages one needs a 'reference system' against which to compare and check particular parameters. In the later stages a domestic type of system is useful in assessing the overall sound clarity. If you don't lose the sound of any instruments when switching over from one system to the other you are off to a good start.

It is difficult to make any generalisations about monitoring because it is a matter of taste. However, since you are the person who will have to sit and listen for hours you will need a system on which you find it easy to hear many different sounds. If you are having to concentrate too much to perceive the sound you will end up with a headache. Besides which, hearing should be automatic as any effort expended in that process detracts from the work at hand. So far so good.

There are inevitable complications when comparing monitor systems and some of them are listed below:

1. A given set of speakers will sound different if you move them round the same room let alone put them in a different room.

2. Control rooms are full of things which tend to upset the overall sound picture eg. mixing console, tape machines etc.

3. A change in temperature or humidity will affect the sound propagation.

4. A change in listening position will affect what you hear to some extent.

5. A change in listening level (SPL) will affect how you perceive the sound.

6. Your ability to hear varies over the course of 8 hours or so.

7. Different frequencies at different levels are reflected or absorbed by different surfaces.

The best thing to aim for is a well dispersed sound above and behind the mixing console. To achieve this we require a reverberant sound field in this area which is then absorbed. That is; the sound arrives as a dynamic picture which then passes the listener never to return. In practice this means having harder surfaces at the front of the room and softer ones behind the consoles.

There are three definite frequency regions for us to concentrate our attention on: Bass, Mid and Top.

Bass frequencies are to all intents and purposes omnidirectional. This means that if you put someone blindfolded in a room with low frequency notes only being reproduced, they could only guess at where the sound source is. This is because at these frequencies our ears are not far enough apart to give us the phase difference we require to calculate directionality.

Bass frequencies also possess a lot of momentum, in so far as they are difficult to absorb. Because most rooms have dense walls (brick, breeze block, plaster, etc.) bass frequencies tend to be reflected making a lot of rooms naturally 'bass heavy'. The problem with this is that you think there is more bass on tape than there really is when you are mixing, resulting in weak sounding tapes when played back on a 'domestic system'.

This problem is compounded by two of the above complications (6,7). The first is known as the Fletcher/Munson effect, which means that the average person hears more Bass and Top at higher sound pressure levels (SPLs) than at lower ones. The second (7) is that at these higher SPLs there is proportionately more bass being reflected, (ie. not absorbed like the mid and high frequencies).

The Bass Trap

The second of our problems may be solved by the use of a bass trap. This is basically a method of ensuring that any incident bass frequencies will be totally absorbed. Figure 18 shows the general arrangement.

A 2 inch framework is fixed to the walls and covered with half inch insulation board. The insulation board has 2 to 3 inches of fibreglass stapled to it.

Baffles of half inch insulation board covered with 2 inches of fibreglass on both sides are suspended by wires and hooks from a false ceiling of half inch block or chipboard. The front is then covered with fine net curtain to keep the glass fibres in the enclosure and with a decorative curtain, or anything you fancy, to improve the looks.

The idea behind these devices lies in the fact that the wavefronts hit the baffles which absorb the energy, hence it is not returned into the listening area to 'muddy' the sound. An open trap will absorb all frequencies, though at high SPLs (above 100 dB) there will come a point where the trap saturates and starts to reflect some sound (starting with the lowest frequencies first) back into the room. This is not usually a problem except with very high powered monitoring systems.

It may not be obvious to many readers why all major studios use such high-powered monitoring (often potentially 600 watts in each corner of the room). The main reason is that to reproduce undistorted sound in the control room, as it is at the microphone, requires the monitoring to cover the entire audio dynamic range (softest to loudest sounds) of 120dB SPL. An average rock drummer pushes out these sort of levels right across the audio frequency range. So if you want to hear that sort of sound you have to build in heavy monitoring. A pair of 15 inch speakers in many cases will be insufficient as the peaks and overloads will wreck the drivers in a relatively short space of time. Mind you, this is largely due to inefficient speaker cabinet design in the first place.

Interestingly, horn-loaded drivers may be anything up to 20 times more efficient than straight forward 'infinite baffle' (sealed box) designs. Horns require correspondingly less drive power for the same acoustical power output (SPL). This is because the air pressure waves are 'matched' between the vibrating driver surface and the surrounding air in the room, rather in the same way that a transformer matches one impedance to another. Other efficient types include 'transmission line' and well designed 'ported' enclosures.

Mid Frequency Considerations

This area (from 200Hz to 4kHz) carries the most recognisable sound patterns and structures so it has to be well balanced. If not, errors of judgement will be passed on to the successive stages of recording, resulting in final tapes which sound virtually unrecognisable except over the system on which they were mixed.

Any sound that you hear is made up of two major components: direct and reflected sound. If you were put in a situation where you could only hear direct sound (eg. an anechoic chamber) you would find it difficult to discern what you were actually hearing. The ideal situation is where you hear the direct sound first closely followed by a measured amount of reflected or secondary sound (reverberation). This is due to the way in which our aural facility requires a frame of reference or, reverberant sound field in which it can compare events as they are juxtaposed to one another. So when we are listening to music it is the flow of change that we are aware of rather than a series of 'snapshot' style pictures one after the other.

Much has been made in recent years of 'time-aligned' speaker systems. In the final analysis though you have to choose a system that you basically like the sound of and then arrange the equipment in the room so that you obtain a well balanced sound. You may decide to forego the heavy monitoring and opt for a medium power (50-100 watt) main system and a smaller (10-20 watt) domestic system. As an indication of what can be done on smaller speakers, the Fox hit record 'Single Bed' (of some years back) was made on Auratones while we were sorting out the main monitoring! We did however have to make a big allowance for their predominantly 'middley' sound and go easy on the bass and top frequencies.

High Frequency Considerations

High frequencies are naturally attenuated by air and in any case are very directional. So you have to point the speakers straight at the listening area above the console in an attempt to get an even spread of sound. You may well find that some hard, uneven, reflective surfaces adjacent to the main speakers help to disperse the upper middle and high frequencies into the listening area. If you are working alone you will be able to find the best listening spot for yourself. If there are two or more people, however, you may find there is disagreement about how bright or clear the sound is due to everyone hearing something different.