voici un extrait des recommandations techniques de Dolby, concernant les salles d'exploitation.
Est-il selon vous toujours d'actualité, ou ont ils revu certains points ????
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
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).
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