Wildlife Recording in Stereo Part 2
This article appeared in Wildlife Sound March 1973 ff.
Medium range recording
In the case of recording at long range, the only solution to the problem so far, you will remember, was the large stereo parabolic reflector, which, with an angle of pick-up of about 10 can, when the background sounds are very low, usefully pick up subjects at up to 500 metres away.
By medium range I am thinking of any distance between 5 and 100 metres, depending on the subject or subjects to be recorded. For instance, Long-tailed tits at 10m could best be described as medium range, whereas Black-headed Gulls at 10m would definitely be close range.
Let us now consider these two species more carefully. Assume the distance to be 10m for the Long-Tailed Tits, which will perhaps be flitting about a bush emitting a low sound level; and 100m for the Black-headed Gulls, which for the sake of convenience we will say are flying around in large circles emitting a high sound level.
The angle of pick-up to be covered will be about the same in each case and can be taken as 50. The sound level received will also be roughly the same in each case.
Narrow Angle Medium range
Is there, then, a microphone which will cover just the required angle, 50 and not much more, with true fidelity and good stereo, and adding very little noise in the form of hiss? Fortunately there is, the Sennheiser MKH805 or its successor the MKH813. These are capacitor RF microphones, which are 12V battery powered. They are very sensitive directionally for which reason they are called Gun microphones. Their self-generated noise is low and their output high; ten times that of the most sensitive dynamic microphones.
There are, however, two problems to be taken into consideration with these microphones. The first is the cost; even second-hand you could pay around £200 for a pair. In my opinion it would be £200 very well spent because with natural-history type work these microphones fulfil the majority of requirements. The second problem is a technical one; subjected to sound levels exceeding loud speech at half a metre, these microphones will distort the sound, due to the overloading of the sensitive RF system.
The MKH805s or 815s should be mounted as a pair, side by side, on a stand in their special wind shields. The angle between their axes should be 28 to 30 outwards. The results thereby achieved will be excellent in both quality and stereo.
Medium Range, Wide angle
Let us now consider recording a marsh or woodland atmosphere. The angle of pick-up to aim for with this type of work is around 140, which you will remember is about twice the angle of the listener’s loudspeaker system. There may be confusing directional information resulting from a greater angle of pick-up.
However, the two systems now described allow 360 pickup
The spaced pair
Omni-directional microphones are placed about 15 m apart. Sounds which reach the pair of microphones at the same instant ad level will appear to emanate from the centre, and will be from a position, if one thinks of a line between the two microphones of 90 or 270 from the midway point of that line.
Sounds from a small angle either side o the 90 and 270 position arrive at the two microphones at different instances, causing phase errors. It is by detecting these small phase errors that we are able naturally to tell the positions of sounds. However, the microphones will be greater distances apart that out ears and if we consider for a moment that for any stereo effect at all, the microphones will need to be more than 10m apart, sounds from the zero-angle in-line position will arrive at the second microphone, not so much as a phase error, but as a long time delay and will, when reproduced, be heard as a quick flutter echo.
The spaced microphone system also suffers from volume effects when a flying bird passes closer to the microphone than their distance apart, there will be a volume dip as the bird approaches the centre position, as well as the flutter echo effects when the bird is away past the microphones.
Crossed fig 8
A system which suffers from none of these problems but still offers 360 pick-up is the use of a pair of ribbon microphones such as the STC 4038, a large and heavy figure of eight polar diagram microphone, which due to the delicate nature of the ribbon. Must be protected against wind. The angle between their axes should be 60 for natural history recording. The polar response at the very highest frequencies is narrower than at lower frequencies where it is a true figure of eight.
A problem, which has to be overcome with this system, is the out of phase effects form sounds at the sides.
Looking at the diagram, one can see that the sound received is to the face of B and to the rear of A. Thus the phasing is B positive and A negative. For the mono listener A+B will cancel each other out and become zero, but the stereo listener will receive both signals and will be unable to locate the position of the sound, as it will not be
reproduced within the sound-stage (the angle between the loudspeakers) and the resulting effect will be unpleasant.
However, this problem can be overcome, as far as the middle and upper frequencies are concerned, by placing between the nearly co-incident microphones a felt-covered acoustic screen, about 9 inches square (22cm x 22cm). The sounds reaching the rear of the microphones from either side will be greatly reduced, but the screen will have little effect on sounds from the front and to the rear of the pair. This improved system is capable of very good stereo.
For less than 360 pick-up cardioid and super-cardioid polar diagram microphones are ideal for atmosphere recording. Cardioids should be set with their axes 50 outwards and super-cardioids at 65. If the angle is increased, the result will be bunching of sounds on the loudspeaker extremes.
Medium-distance narrow angle
Mounted in the same manner as discussed earlier, gun microphones will give excellent results. The forward angle of pick-up will be about 70 and discrimination against unwanted sounds or noise from the sides will be near 10:1. The rejection of unwanted sound is, in fact, where the microphone really shows its superlative characteristics.
Close range recording
The foregoing remarks in respect of medium range recording also apply here for close range, with the exception of gun microphones. They are also a little too large for very close work.
For recording "at the nest" a small pair of cardioid microphones will fit the bill, if you will pardon the pun. For example, AKG C451s are small, lightweight, high output microphones of very high quality and battery powered. Sennheiser MKH405s are small microphones of very high quality and battery powered and with very low inherent noise.
Other microphones I have used for close range work are moving coil type AKG D202s (rather large and heavy, but of good quality) and Beyer M88s, which are super-cardioid, of medium size and weight, but reasonably good quality by broadcasting standards.
How to phase microphones
The outputs from each microphone should be positive or additive and should be son on the recorded tape as will.
When a stereo tape is played on a full track mono machine, tracks A and B must add. If the phase of these is 180, the result on a mono machine will be zero; they will cancel each other out in the replay head. The microphones should be placed close together with the heads exactly in line and facing the same direction, and then a section of speech should be recorded. If the stereo machine is equipped with a monitor button for listening to the A + B signal, the result will be loud and clear for correctly phased, and weak and distorted for incorrectly phased microphones.
If you do not have this facility, but do have headphone monitoring on the machine input, an arrangement for paralleling the two leads to the stereo headphones should be made. This will then enable you to check the phase of any pair of microphones and cables. If the phase is found to be negative, it will be necessary to reverse the connecting wires from one of the microphones.
Having mentioned the importance of phase in stereo microphones and recording machines, I cannot emphasise too strongly the important of having a correctly phase loudspeaker playback system for listening.
For those fortunate enough to possess a tape machine with output amplifier combined and loudspeakers with marked terminals, all that should be necessary is to ensure that the marked connections on both amplifier sections and loudspeakers are identical.
If a full track mono tape is played on a twin track stereo machine, the output of the loudspeaker terminals will be in phase and there should be almost zero signal on a loudspeaker connected to (or across) the two live side terminals. A very old and reliable way of ensuring the loudspeakers are in phase is to apply a battery of 1.5 V or so to the loudspeakers and see which way the cones of the units move. The wires to the loudspeakers can then be arranged so that when the battery is applied to the same colour lead, both cones movie n the same direction. If listened to out of phase the effect is disturbing and unpleasant and the point of origin of the sound cannot be placed, as it will not be reproduced within the sound stage.
It is following a long stereo session, when one switches over to mono listening, that one realises just how narrow and dull mono sound is.
Wildlife Recording in Stereo Part 3- recorders
David J Tombs