Use of Surround Arrays

In motion picture use, surround arrays are commonplace, having been developed over the history of surround sound from the Fantasia onwards.The AES/ITU recommendation recognizes possible advantages in the use of more than two surround loudspeakers, in producing a wider listening area and greater envelopment than available from a pair of direct radiators. The recommendation is made that if they are used, there should be an even number disposed in left and right halves in an array that occupies the region between ±60° and ±150° divided evenly and symmetrically placed. Thus if there are four surround loudspeakers, the pairs would be placed at ±60° and ±150° from center; with six speakers, the pairs would be placed at ±60°, ±105°, ±150°, etc.

However, use of this rather sparse array, with just four or six speakers depends on something vital: uncorrelated signals available for each of them. The original experiment on which the recommendation was made used four surround channels, all the way from different diffuse-field dominant microphones to the loudspeakers. I have found that an array of four surround speakers, with two driven in parallel on each side as one would do with 5.1-channel sources, is particularly poor at coloration because the comb filter fingerprint is so very audible. In small low-reverberation-time rooms for film sound mixing I prefer to use a large number of smaller loudspeakers. In one installation we have 16 surround loudspeakers, six on a side and four across the back, and this works well. These are quite small two-way designs you can hold in your hand, but the array of them sounds better, and the sound is more uniform from front to back, than any point source large surround speaker would be.

Surround arrays have some advantages and disadvantages but are commonplace in large theater spaces. Since they are used there, they also appear in dubbing stages for film, and also for television work that is done on the scale of film, such as high-end television post-production for entertainment programming. The advantages and disadvantages are:

• In large rooms, an array of loudspeakers can be designed to cover an audience area more uniformly, both in sound pressure level and in frequency response, than a pair of discrete loudspeakers in the rear corners of the auditorium.This principle may also apply to smaller control rooms, some of which use arrays.

• In addition it is possible to taper the output of the array so that the ratio of the front channel sound to surround sound stays more

constant from front to back of the listening area (i.e., putting more sound level into the front of the listening space than the back, in the same proportion that the front loudspeakers fall off from front to back helps uniformity of surround impression, the fall off being on the order of 4dB in well-designed theatrical installations). This is an important consideration in making the surround experience uniform throughout a listening space since it is the ratio of front to surround sound that is more important than the absolute level of each.

• In the context of sound accompanying a picture, it is harder to localize an array than discrete loudspeakers due to the large number of competing sources, thus reducing the exit sign effect. This effect is due to the fact that when our attention is drawn off the screen by a surround effect, what we are left looking at is not a continuation of the picture, but rather, the exit sign.

• A drawback is that the surround sound is colored by the strong comb filters that occur due to the multiple times of arrival of each of the loudspeakers at listener's locations.This results in a timbral signature rather like speaking in a barrel that affects the surround sound portion of the program material, but not the screen sound part. Noise-like signals take on a different timbre as they are panned from front to surround array. It turns out to be impossible to find an equalization that makes timbre constant as a sound is panned from the screen to the surrounds.

• Another drawback is that pans from the front to the surrounds seem to move from the front to the sides, and not beyond the sides to behind. Discrete rear loudspeakers or 7.1-10.2 channel systems can do this better.

The standards referred to above were written before either matixed 6.1 or discrete 6.1-channel systems separating the back from side surrounds was conceived. However, it is worth noting that in Cinerama, the surround array switch between the two available tracks between left-right surround and front-back surround. This was done manually by the theater projectionist, presumably between the segments in travelogues like This Is Cinerama. Surround arrays in cinemas today are divided into four groups: left, right, left back, and right back. The left and left back are driven together for 5.1, as are the right and right back, and the back is separated out for those mixes that have a back channel so that the array becomes left, back, right. The advantage this has is particularly in pans from screen to off screen. With conventional left-right only surround and an array the mixer will find that the sound only images from the screen to the sides of the listener; it can't go behind him.

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