Dolby Tech Page

2.2.5 Stage Loudspeaker Characteristic Curve

The B-chain frequency response of the Left, Center and Right screen channels should conform to the wide-range characteristic defined in IS02968. The response should extend smoothly from 40 or 50Hz at low frequencies to significantly beyond 10kHz, and ideally as far as 16kHz. The level difference between any two locations in the normal seating area, measured in ¹/₃ octaves from 150Hz to 10kHz, should not exceed 3dB (1). See Figure 2.3.

The quality of a theatre's B-chain can be assessed in two areas: first, how closely the curve matches the required frequency response; and second, how uniformly the same response is maintained throughout the seating area. Matching the required response almost certainly requires use of bi-amplification and an active cross-over. The required uniformity of response will normally make use of constant directivity high-frequency horns mandatory.

A complete discussion of 8-chain equalization techniques can be found in each Dolby cinema processor manual.

Note: the USA national equivalent standard to IS02969 is ANSI PH22.202M, and the British Standard is BS5550:7.4.1.

---

2.2.6 Measurement

There is a discussion of B-chain measurement techniques in each Dolby cinema processor manual. Until the last few years, it was normal practice to use a single calibrated microphone, placed in a "normal" seat location, asymmetrically located with respect to the theatre's centrelines, and set approximately two-thirds of the way back in the theatre. See Figure 2.4. A conscientious installer would then move the microphone to an alternative location, and "average" the equalisation for the best overall results.

In recent years, microphone multiplexers have come into increasing prominence, typically with four calibrated microphones. Measurement of four locations in the theatre results in a much more even equalisation throughout the seating area. Practical experience suggests that best results are achieved with the microphones placed substantially in the reverberant field, in a layout such as that shown in Figure 2.5. Again, care should be taken not to place microphones on each central axis of the theatre, where standing waves can cause aberrant conditions.

In mixdown, dubbing theatres, and small review rooms used for print quality control, where the listening/viewing area is small with respect to the size of the room, the microphone locations should be located within the area of interest.

The microphones should normally be mounted at listener's head height. However, if the seats have high backs, the microphones should be raised up so that they are at least 9 inches above the top of the seat, thus avoiding any grazing effects.

---

2.3 Surround loudspeakers

2.3.1 Number and Location

The first step in determining the number, type and location of surround loudspeakers, is to consider the likely power handling requirements. Dolby SR, for example, can require a peak level in the middle of the auditorium of a minimum of 92 dBC with normal program, and as much as 6dB more if the sound-track were used to its full low-frequency limits. For an SR•D digital sound-track, the equivalent level is 103dBC for a mono surround playback, or 100dBC for individual left and right surround strings of a stereo surround installation. Assuming no assistance from reverberation (ie the maximum peak level is that required to deliver a transient sound, see section 2.1.1 above), the dimensions of the theatre can be used to calculate the total loudspeaker power required.

The first thing to do is to calculate the total electrical power required. In some cases the proximity of the surround speakers to a wall may contribute to their efficiency. However, this factor has been omitted from the present calculation since it is only valid for low and middle frequencies and only if the speakers are against a wall and not spaced away from the actual hard surface.

The desired maximum rms sound pressure level at the listeners' ears is 100 dB per surround channel for a stereo surround configuration. The total electrical power required from each side's power amplifier is given by

Watts= 10^{(Lp-Ls+20logR}/₁₀⁾
where

Lp = desired SPL (100 db in this case)
Ls = speaker sensitivity, dB SPL at 1 meter distance for 1 watt input
R = distance from wall to centerline of theater in meters
See Figure 2.11

Having determined the total electrical power required per side, we must now find out how many speakers are required to handle this amount of power. The number of speakers N is calculated from

N= electrical power (calculated above) divided by the power rating per speaker

This is the minimum number of speakers per surround side required to handle the necessary power. A greater number of speakers may be required to secure good uniformity of coverage of the audience area. In practice, the number of speakers required is the laruger of the two numbers derived from coverage requirements and power handling ability.

The speakers should be connected in series/parallel so that they all receive equal power and the impedance presented to the power amplifier is around 4 ohms. Most well-designed modern amplifiers will drive 4 ohm loads with a somewhat higher power output than they will a 8 ohm load, but as this ability is a function of the details of each amplifier, the manufacturer's data should always be consulted. Some amplifiers will drive impedances lower than 4 ohms; again, consult the manual or manufacturer.

It may be desirable in some installations to arrange the series/parallel connection so that the rear-most speakers receive slightly less power than the front ones. This is done to match the lower sound level heard from the screen speakers in the rear of the auditorium. In general, this practice is most appropriate in long rooms with short reverberation times.

Next, consider that this power has to be shared by a given number of loudspeakers, which should be spread about the back wall, and the two rear side walls of the theatre. Optimum sound balance between channels dictates that surround loudspeakers should be evenly spread from half-way back from the left side-wall, through the auditorium back wall, to a point half-way up the right wall. This configuration takes account of the ratio of screen to surround sound pressure levels, and also seems subjectively optimum when the visual dominance of screen activity is taken into account. (See Figure 2.6). Avoid placing any surround speakers further forward than 50% or 60% of the way from the rear to the front of the house. Placing speakers too close to the screen results in surround sound blending into screen sound for audience in the middle part of the house (especially when the "draw" of visual screen action is taken into account -- see Figure 2.7).

---

2.3.2 Loudspeaker Type

Manufacturers' literature should be consulted for the power handling of a given surround loudspeaker, and this determines the number of speakers required.

Selection of a suitable type of loudspeaker, though, demands assessment of more than just power handling capability. Diffusion is also a major requirement of surround channels, meaning that surround signals should never appear to come from a point source. This means that a large number of loudspeakers are always preferable to a few, regardless of power handling.

Avoiding localization to a local speaker will also be assisted by selecting a speaker without too wide a dispersion, as an excessively wide dispersion will cause a domination of high-frequencies at the seat closest, or directly under, a given loudspeaker. For this reason, threeway bookshelf-type units should be avoided, as wide dispersion is one of the intended design parameters of these units, primarily designed for the home for music listening.

---

2.3.3 Mounting Angles

Some types of loudspeakers intended for surround use are mounted in a box with a built-in down angle. Care should be taken not to accept this fixed angle as correct for any given auditorium. Depending on the mounting height, the angle should be set to achieve the most uniform

response across a lateral row of seats. In cases where a low ceiling results in the surround speakers being mounted lower than would be desirable, any downward cant would make a bad situation worse, enhancing localisation to the nearest loudspeaker for those seats closest to the walls. In such a situation the loudspeaker drivers should be aimed horizontally, getting a percentage of the dominant direct signal above the heads of listeners in the closest seats.

---

1 In most theatres, the reverberant field dominates in most of the normal seating area. In small rooms, however, with a seating capacity of less than, say, 150 seats, that sector of the audience seated closer to the screen receive a signal dominated by the direct field. In such cases, inverse square law losses can cause a noticeable fall-off in energy over the first few rows of the theatre, and may make it impossible to sustain the quoted 3dB figure. In these small theatres, the installer should verify a smooth fall-off with distance from the screen, and an even distribution laterally across the seating area.

---

Back | Index | Next