Spaced Microphone Techniques

Spaced microphone techniques were the first methods employed to relate a stereo image to the listener. Bell Labs did a lot of work on these methods. They experimented with two and three spaced microphones, adding the third microphone to "fill in the center image". This method has the microphones facing forward along a line perpendicular to the sound source. The polar pattern , spacing and distance from the sound source are all variables that need to be considered using this technique. Unlike the other methods described here, this method creates a stereo image as a result of both time and amplitude differences between channels.

Two significant problems can arise using this method. They are : 1) Vague center imaging and 2) low- frequency comb-filter effects on sounds sourced to the extremes of the soundstage. Another concern, to some, is erratic mono compatibility. If possible, a phase monitor oscilloscope should be used with these methods to assist in microphone placement.

Spaced Microphone Techniques -Omnidirectional Method

Spaced omni-directional microphones are best suited to this technique. Typical spacing is from 2 to 10 feet, centered on the midpoint of the sound source. The spacing is determined by the width of the sound source and the distance from it.

For indoor use the general rule of thumb for microphone placement is to space them one third to one half the distance from the center to the outer edge of the soundstage. For outdoor use the lack of reflective surfaces allows for more distance from the sound source.

Here we are more concerned with the "hole in the center" effect. To avoid this problem, keep the mics between 6 and 10 feet apart. Omnidirectional microphones offer an excellent sense of acoustic space and the most natural, uncolored sound of any pickup pattern. However, they require the closest placement to the sound source for a given direct to reverberant sound ratio and are more likely to pickup unwanted sounds in the acoustical environment. The sense of perspective they develop is very natural and "relaxed" sounding. They are also very immune to wind noise, where directional microphones require the use of a wind screen outdoors, the omnidirectional does not. When used indoors, special care must be taken to avoid recording air- conditioning and low frequency traffic noises.

Omnidirectional microphones can be either flat to an on axis sound or flat to a reverberant sound field. For the latter high frequency response will be tilted up. This type of response is the best choice for outdoor use. Under the best conditions, outdoors or upclose indoors (see distance rule above), they are unequaled in their performance.

Spaced Microphone Techniques -Pressure Boundary Method

Pressure boundary microphones are very similar to omnidirectional microphones, except they pickup a hemisphere about the boundary to which they are mounted. When baffles are used to shape their response they can be treated as spaced directional microphones.

Spaced Microphone Techniques -Directional Microphone Method

Space directional microphones include the cardioid, hypercardioid and bidirectional (figure of eight) pickup patterns. While similar to spaced omni techniques, directional mics require even more care in placement than spaced omni's. Audience response, reverberation signature and other off axis sound will suffer from significant coloration. Therefore their placement and orientation are more critical than the omnidirectional pattern.

Of all the directional microphones available to use for this method, the bidirectional ones perform the best. They have better reach than the hypercardioid or cardioid mics and due to the rear facing lobe being out of phase, but equal in amplitude output, the reverberant signature and audience response will have a more natural characteristic.

As a rule, we advise against the use of spaced directional microphones for any application, except, on stage facing into the audience, for audience response.