Left and Right

Left and right front speakers are located at ±30° from center, when viewed from above, in plan. This makes them 60° apart in subtended angle. Sixty degrees between left and right, forming an equilateral triangle with the listener, has a long history in 2-channel stereo, and is used in 5.1-channel stereo for many of the same reasons that it was in 2-channel work: wider is better for stereo perception, while too wide makes for problems with sound images lying between the channels. While the center channel solves several problems of 2-channel stereo including filling in the "hole" between left and right (see Chapter 6), it has nevertheless been found by experimenters that maintaining the 60° angle found commonly in 2-channel work is best for 5.1-channel sound (Fig. 2-4).

There is a further consideration in left/right angles when sound is to accompany a picture described on page 42.

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Fig. 2-4 In the standard stereo setup, a = b = c and an equilateral triangle is formed. The listener "sees" a subtended angle of 60° from left to right.

Surround

The surround loudspeakers in the plan are located at ±110° ±10° from front center, that is, between 100° and 120° from front center. This angle was determined from experiments into reproduction of sound images all around versus producing best envelopment of the listener. More widely spaced (further from the front) surrounds produced better rearward sound images at some expense in envelopment, while further forward surrounds produce better envelopment at the expense of rearward imaging. Also, 110° reportedly better represents the likely home listening situation where the principal listening position is close to a rear wall of the space, rather than in the middle of the space if the loudspeakers were more widely placed.

Surround loudspeakers height is often elevated to avoid control room equipment, doors, and so forth, and this is permissible for many kinds of surround presentations. In cinema usage and in many home systems, surrounds are elevated with respect to the audience. However, in certain types of presentations, such as music with a "middle of the band" perspective, the elevated surround may cause a curious tilt in the resulting sound field, so may not be as well liked. Applause reproduced with some kinds of live programming is also anomalous as it does not correspond to what is actually found in halls. Thus surround elevation is a question the answer to which depends largely on the program material being made in a particular studio destined for a particular reproduction situation.

Subwoofer

The subwoofer in a bass managed system carries the low-frequency content of all 5 channels, plus the 0.1 LFE channel content; this is a consideration in the placement. Among the others are:

• Placement in a corner produces the most output at low frequencies, because the floor and two walls serve as reflectors, increasing the output through "loading"; the subwoofer may be designed for this position and thus reduce the cone motion necessary to get flat response and this improves low-frequency headroom.

• Making an acoustical splice between the subwoofer and each of the channels is a factor that can be manipulated by moving the subwoofer around while measuring the response.

• The placement of the subwoofer and the listener determine how the sound will be affected by standing waves in the room; moving the subwoofer around for this effect may help smooth the response as well.

• Multiple subwoofers placed differently with respect to room boundaries can help smooth the modal response through multiple driving points and thus differing transfer functions between each of them and the listener. Where a room may produce ±12dB variations in response measured with high-frequency resolution equipment, that variation can be reduced with multiple subs,

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