Studio Sound Techniques (Part 1)
An exciting new series for the home electro-musician running a home studio
This series of articles is aimed at the operators of any recording equipment which might be called a studio. The intention is to relate the theory of sound recording to the practice of getting tapes which are up to scratch, both from the technical and artistic viewpoints. Although the sound engineer is to some extent at the mercy of his equipment, by knowing its strengths and weaknesses, it is possible to optimise the performance of the entire studio system. To do this we have to familiarise ourselves with the basics of sound reproduction. Remembering that, for all its faults, the average ear, once trained, is one of the most sensitive measuring instruments-cum-analysers readily available.
I hope to pass on to the reader many useful tips by going through an imaginary session i.e. from checking out the equipment prior to recording, to playing back test pressings or freshly cut acetates. Also there will be a 'what to do when the worse happens' section covering the many calamities that befall the hapless recording engineer on 'one of those days'.
The following diagram (Figure 1) may be useful in assessing the importance of getting to know your equipment. I have included an approximate indication of where you might expect to find some common instruments. The actual sound pressure level (SPL) of drums, say, is naturally dependent on both how hard you hit them and how far you are from them. So the SPL and frequency content vary enormously from moment to moment. It is these two dimensions which we perceive at any given point. The time axis would run into the page on this diagram. Figure one also represents the dynamic range and frequency range covered by our aural system. Consequently it defines the area over which our equipment has to work. The closer our equipment comes to covering this area, the more faithful the reproduction will be. However, this should not become an aim in itself, for the aim is to cover as much as we need to.
Most modern music has a limited dynamic range, so there is not much point in paying out for expensive gear which may not be fully utilised. Frequency response is another matter. It is generally speaking possible to cover the entire spectrum provided a few basic rules are observed. More on these later. So before the session starts let's make sure the equipment is giving its best.
Microphones (mics) are delicate electro-mechanical devices and should be treated with care if they are to perform consistently. They are easily upset by fine dust from cigarette smoke, spit from vocalists and dents in the protective wire mesh from careless handling or inaccurate drumming. Most mics can be taken apart with a set of jeweller's screw drivers. If you are the sort of person who has taken the back off a mechanical wrist watch, cleaning most mics should present you with few problems. You will need:
1) A clean, flat, well lit bench or table top.
2) A set of jeweller's screw drivers.
3) Two small, fine camel hair paint brushes.
4) Distilled water.
5) Soft white tissues.
6) A number of small clean containers (egg cups will do fine).
7) A pencil and pad.
The basic procedure is the same for all devices: disassemble to reveal diaphragm; clean diaphragm and inside of wire mesh; allow mic to dry; reassemble. The actual cleaning of the diaphragm is quite delicate work. So before you start, go through the procedure mentally and work somewhere free from interruptions or anyone nudging your elbows at the wrong time!
Half fill two of the cups with distilled water. Keep the others for small screws etc, allocating one for each mic you intend to clean. Lay the screw drivers, paint brushes, etc, out in convenient positions.
Most mics are screwed together in some way or other. If there are no obvious screw heads showing, check around any cylindrical rings where they are often hidden. It may be that you will have to hold the mic top and bottom and twist it anti-clockwise to reveal the insides. Beware of diaphragm assemblies (inserts) which are not fixed themselves, but rely on pressure from the parts which you have removed to keep them in place. Normally the gaskets they are seated in will hold them for the required duration. If not, unsolder the leads, noting their position and colour with a diagram in your pad, so that you can work on the insert itself. You may also want to make a note of what goes where as it comes apart so you can reassemble it confidently.
Having revealed the diaphragm, make a detailed inspection of the surface. If the mic has never been used for vocals or live gigs, it may only require a gentle blow across the diaphragm to remove whatever dust may have built up. If that is insufficient, take one of the paint brushes and work from the centre outwards with gentle strokes. Have only the tips of the bristles in contact with the surface.
If the diaphragm looks tarnished (from condensation), or appears to have small particles stuck to the surface, light brushing will not be enough. Take the other paint brush and use it to transfer one or two drops of distilled water to the centre of the diaphragm. Work gently towards the edge in a spiral motion, mopping up the drops of water with the tissue paper at the edge. Normally the capillary action of the tissue paper sucks up any particles suspended in the water. However, this can happen in the bristles of the brush as well, so use the other container to swish out any particles and keep the brush clean. Repeat this procedure until the diaphragm is free from surface asperities.
Leave the mic/insert in a warm (safe!) place for about half an hour to dry. While it's drying, check the inside of the wire mesh for dust and dents, etc and do whatever seems appropriate. Some people may argue that all of this effort can be avoided if a windshield or some other type of screen is used. And that these have no effect on the performance of the mic. Fair enough. Some people have not had a producer asking them for a very intimate female vocal sound full of breaths!
If you have a number of mics of different makes, it will certainly be worth checking them to ensure that they are phased uniformally, i.e. all the same. Phase is often misunderstood so a few words here on, what it is, and what it means to the sound of something, should not go amiss. You are probably aware that if you accidentally reverse one of the speaker connections on your stereo, the bass response virtually disappears. In non-technical parlance this is because when one is sucking, the other is blowing and the pressure waves tend to cancel out.
Figure 2 shows that 'A' is 180° out of phase with 'B' and, 'C' is 180° out of phase with 'D'. What does this mean? If we take the points 'X' and T we can see that they correspond, i.e. both are going positive, also we note that 'Y' is at the 180° point. One could be forgiven here for thinking that we simply displace the waveform by half of one cycle to render it 180° out of phase. However, when we look at 'C' and 'D' we find we can't make the same evaluation because 'C' and 'D' are asymmetric waveforms, whereas 'A' and 'B' are symmetric, i.e. the positive half cycle is the same shape as the negative half cycle. This throws into sharp focus the fact that relative phase response is an instantaneous or simultaneous phenomenon, not strictly related to time at all. Time modulation devices or reflected waves often produce phasing type sounds because the input waveform is symmetric and fairly well sustained. The crunch comes when we start mixing various waveforms together. If we mix equal amounts of 'A' and 'B', the resultant sound will be next to nothing. Similarly, if we mix equal amounts of 'C' and 'D', the net result is nearly zero. This is because when one is going positive, the other is going negative by the same amount at the same time. It is, therefore, better to think of out of phase signals as being mirror images of each other, as this embodies the 'simultaneous aspect' which is so crucial in music and sound engineering.
In practice, sounds rarely cancel out completely, which often makes it difficult to tell if there is a phase problem or not. If we go back to our suspect microphones, we can do a simple experiment to make sure their phase response is similar. You will need a constant low frequency sound source (preferably sine wave) e.g. synth or oscillator through a combo amplifier at roughly 100Hz and a way of mixing the output of two microphones together. Arrange the mics so that they are close to each other, pointing at the speaker, at least three feet from the floor and six feet from the speaker. Plug the mics into the mixer and set each channel gain one by one for a level of -7dB (VU) or PPM 4 on a group output meter. Compare the first two mics by mixing them into the group output. If the mics are in phase the output will rise approximately 6dB to about -1dB (VU) or, halfway between marks 5 and 6 on a PPM. If they are out-of-phase it will fall. Sort the mics out by marking them type one or two, depending on whether they add or subtract. If they all add relax, you have no more to do. If not, select the type in the minority for further attention. These will require the leads reversing at the output connector (pin 2 and 3 for XLR types). If that is impractical the voice coil leads may be reversed on dynamic mics.
Most professional mics are 200 ohms impedance and of the balanced/floating configuration which means that the signal is not directly connected to earth or the screen on the input cable. The most common connector, and by far the best, being the standard three pin XLR series. If your mics have 'single ended' outputs i.e. only two connectors, one of which is earth or screen, then you may have trouble reversing the phase. One course of action is to use a phase reversing transformer which may also adjust the impedance to the preferred 200 ohms. When you have corrected the errant mics, give them a final check as outlined before. It may be instructive at this point to play some music through the combo amplifier and move the mics into different positions. You will find that the balance of the sound changes according to whether specific frequencies are adding or subtracting. This is also a good way to get to know your room.
Photos kindly provided by The Manor Recording Studio, Shipton on Cherwell, Kidlington, Oxon.
Feature by P.A. Becque
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