As broadcasters move into 5.1 and surround broad-casting, the importance of remote audio has increased greatly. It's no longer adequate to hand a couple of mics out the press box window and call that adequate stereo sound.
There are several issues to consider when recording for surround sound. Not only does the engineer need to optimize imaging and image location, but also he or she needs to ensure the smooth and even distribution of the reverberation around the listener. In addition, there should be a cohesion of the front and back components of the sound field. The main idea behind a 5-channel microphone technique is to capture the entire acoustic sound field, rather than to simply present the instruments in the front and the reverb in the surrounds.
When recording, consider first the listener's environment. Remember that the response of the front and rear loudspeakers are different at the listening position. For example, there is far more interference at the listener's ears between the signals from the center and left or left and left surround loudspeakers than there is between the signals from the left and right, or left surround and right surround drivers. The principal result of this interference is a comb filter effect caused by the lack of attenuation provided by head shadowing.
In order to reduce or eliminate this comb filtering, the microphone array must ensure that the signals produced by some pairs of loudspeakers are different enough not to create a recognizable interference pattern. This is most easily achieved by separating the microphones, particularly the pairs that result in high levels of mutual interference. At the same time, however, the engineer must ensure that the signals are similar enough that a coherent sound field is presented. This means the microphones must be in relatively the same location.
After all, if the microphone separation is too great, the result is five completely unrelated recordings of the same event. The result is little sound image continuity or fusion between channels. With optimal microphone spacing, the reproduced sound from the five loudspeakers works together to form a single coherent sound field.
The proposed microphone placement in Figure 1 provides adequate separation between specific pairs of microphones to prevent interchannel interference. At the same time, the configuration relies on the response of the loudspeakers at the listening position to permit closer spacing and, therefore, a smoother distribution of the sound field for the rear pair of microphones.
The 5.1 recording configuration consists of three front-facing subcardioids and two ceiling facing cardioid microphones, as shown in Figure 2. The approximate dimensions of the array is 60 centimeters between the center subcardioid and the left and right subcardioids, 60 centimeters between the front microphones and the surround pair, and 30 centimeters between a pair of cardioid microphones aimed upwards. If desired, the center microphone can be moved slightly forward of the left and right mic's to an approximate maximum of 15 centimeters.
A subcardioid microphone is theoretically equivalent to coincident cardioid and omnidirectional microphones whose signals are mixed at equal levels. By using subcardioid microphones, the result is a wider pick-up than is typical with cardioid mics, but with a higher directivity than omnidirectional mics. In this way, the microphones can be placed further away from the ensemble than omnidirectional microphones for an equivalent direct-to-reverberant ratio.
The key is to provide a certain amount of diffused, reverberant sound in the front channels that blends with the direct sound and with the reverberant signals produced by the surround channels. The direct-to-reverberant ratio can be adjusted by changing the distance between the microphone array and the sound source.
The width of the front array is determined by the size of the ensemble being recorded or by the desired level of inter-channel coherence. For a larger ensemble, a wider array (up to 1.8 meters) is likely necessary. A narrow spacing (1.2 meters) is appropriate for a small ensemble. A wide spacing will reduce the amount of coherence between the front three channels, thereby reducing the image fusion between the loudspeakers. The amount of coherence between the front and surround images can be partly determined by the spacing between the front and rear microphones.
For the surround channels, aiming the surround cardioid microphones to the ceiling has two advantages. First, the direct sound from the ensemble is attenuated because it is arriving near the null of the polar pattern. This is also true of audience noise in the case of a live recording. That being said, any direct sound that is picked up by the surround microphones helps to create some level of coherence between the front and surround channels. The front-back coherence provides an even spread of the sound image along the sides of the loudspeaker array. The level of front-to-back coherence can be adjusted by changing the angle of the microphones and, therefore, controlling the amount of direct sound in the surround channels.
Second, the often-ignored vertical dimension of an acoustic space provides diffuse signals that are ideal for the surround channels. For instance, when listening to live music in a concert hall, we hear sound arriving from all directions, not only the horizontal plane.
The microphone array allows for a large listening area in the reproduction system. Even when a listener is seated behind the sweet-spot, the front image of the direct sound will remain in the front and will not be pulled to the rear, despite the listener being closer to the rear loudspeakers.
Editor's note: This article was adapted from a presentation by Jason Corey, University of Michigan and Geoff Martin, Bang & Olufsen, Denmark. The complete paper, along with a wealth of other microphone application notes, is available at the DPA Microphones Web site at www.dpamicrophones.com.