11th March 2003 - Stereo Microphone Techniques

Mike Skeet is probably most famous for his garage door recording which involves him talking in his garage, opening and closing the door and banging on it as loudly as possible. It has been said, probably rightly, that this recording is responsible for many a dead tweeter. It is fitting then, that Mike chose to use this recording as the introduction to his talk on microphone techniques at his lecture on Tuesday 11th March.

The first question Mike asked was why use spot microphones, when carefully placed stereo arrangements can do a much better job of portraying the ambience of a room, giving a feeling of the front-to-back depth of an orchestra. Mike felt that when listening to a stereo recording, you should get a feel of the image starting in the centre and gradually, smoothly moving outwards towards and even beyond the speakers. However, when striving for the perfect stereo recording, one also has to be aware of mono compatibility.

Mike started by giving a quick overview of different types of microphones and used part of his extensive collection to show examples of each type.

The first type of microphone was a moving coil microphone where the diaphragm moves a small wound coil within a magnetic field to generate the electrical signal. This type of microphone is very simple, but can suffer from very low output level and hence be rather noisy when amplified to a suitable level.

Ribbon microphones work in a similar way to moving coil microphones, but this time it is the diaphragm, or ribbon, that moves directly in the magnetic field. Microphones seen on old television footage are good examples of Ribbon Microphones.

Capacitor or Electret microphones require a power source to operate. They polarise the diaphragm which modulates power source when vibrated. The disadvantage of having to power these microphones is often cancelled out by their higher output level and better signal-to-noise ratio properties.

Polar Patterns

One of the key differences between different microphones, is the polar pattern. Mike demonstrated this by using a Pearl 4 capsule condenser microphone with switchable polar patterns. As Mike talked and turned the microphone, it was possible to hear how the sensitivity of the microphone changed with the angle it was held at.

When demonstrating the omni- directional microphone, you could hear Mike's voice at the same level as he turned the microphone. When he turned the microphone on its "figure-of-eight" setting, you could clearly hear the null at each side of the microphone's response. Mike described how this null can be very useful when trying to mic various instruments in a band. Careful positioning of the microphone can place unwanted instruments in this null so they are not picked up by the microphone. Due to the properties of a figure-of-eight microphone, the depth of this null is very deep, giving very good rejection to off-axis sounds.

Flicking another switch, the microphone now had a cardioid, or heart-shaped, polar response. Turning the microphone, it could be heard that the sensitivity reduced as Mike moved to the back of the microphone. The depth of the null is not as pronounced as that of the figure-of-eight response, but is nonetheless useful in many situations.

Other response patterns are Hyper cardioid which has two nulls at 130 degrees from the front, and the more directional supercardioid. The differences between the different polar patterns are not just limited to the sensitivity in different directions. Mike demonstrated how figure-of-eight microphones are very susceptible to handling noise (and wind noise), whereas omni-directional microphones are much more immune to them. Figure- of-eight microphones also suffer from the proximity effect where the bass increases the closer you use a microphone - although many microphones are actually designed to take advantage of this effect.

Stereo Arrays

The purpose of microphones is of course to record sound. The most common microphone arrangement is a stereo pair. Mike handed round numerous examples of stereo configurations he had used and played examples of each for people to compare.

The first example was using coincident microphones, where the capsules are placed as close together as possible. The microphones used for this are normally two cardioids or two figure- of-eight mics placed at 90 degrees to each other. Cardioid mics are more directional, but the figure-of-eight mics can be processed later to vary the stereo width as required. The other advantage of figure-of-eight mics is their inherent mono compatibility.

Another popular microphone technique is near coincident, where microphones are placed between 10cm and 1m apart. Again, Mike had many examples to show. The first example was an ORTF layout where two cardioid mics are placed head width apart at an angle of 110 degrees. The short demonstration that Mike played showed this arrangement suffered from a slightly diffuse centre stage, but had better off-axis listening compared to a coincident placement. Reducing the angle between the two microphones lowers the amount of side pickup at the expense of width of the stereo image and worsens the off-axis listening.

Another popular microphone technique is using spaced omnis. Here there are many different techniques, ranging from the Decca tree which has two microphones one metre apart with another half a metre in front, to the Jecklin Disc which has two microphones separated by a 28cm foam-covered disc. The Jecklin Disc has been used on numerous Classic FM recordings.

Mike has also spent a lot of time trying various dummy head microphone techniques. A classic dummy head consists of two omni microphones placed at 180 degrees from each other in a ball or more sophisticated representation of a human head, sometimes even with torso and ears. The technique has been used very well for binaural recordings which are to be used with headphones. However, Mike prefers to use two omni directional mics pointing more forwards with a baffle in-between somewhat like the Jecklin Disc. The idea being that both microphones can see the centre image producing a recording more suitable for replay on speakers. Mike showed several examples of dummy head arrangements he had made out of wood, foam, sieves and tights. One example even had a minidisc player in the back which supplied the phantom power required for the microphone capsules creating a truly portable recording device.

Mike has been using this technique to good effect for several years, however recent experiments have shown that there may be colouration of the central image and Mike is currently looking into ways of removing this colouration.

Finally Mike showed a couple of dummy heads he has made for making 5.1 or 5.0 recordings. One of these consisted of five omni directional capsules placed in a shaped wooden block surrounded in a foam wind shield.

Throughout the talk there was active discussion with the audience with many people confirming Mikes observations. Thanks were given to Philip Stokes for assisting in the audio demonstrations.

Howard Farrar