Dolby Tech Page

2.3.4 Characteristic Curve

The Surround B-chain frequency response should conform to IS02969 from 125 Hz to 8 kHz, after correction for near-field response. The level difference between any two locations in the normal seating area, measured in ¹/₃ octaves from 150 Hz to 8 kHz, should not exceed 3dB. Below 150Hz it may not be possible to achieve this 3dB tolerance, depending on the equalizer in use. Care should be taken, though, to achieve the smoothest possible response at these lower frequencies.

Matching the surround characteristic to the target curve will invariably require equalization, either with an optional module available from Dolby Laboratories, or in the case of the CP50, use of an outboard free-standing equaliser. Achieving satisfactory uniformity requires a moderately large number of surround loudspeakers. Ceiling-mounted loudspeakers are unacceptable, as all films are mixed assuming a horizontal surround field; in addition, a very large number of ceiling speakers would be needed to achieve uniform seatto-seat response.

The bandwidth of the surround channel on a stereo optical film is intentionally band-limited to around 7 kHz, to avoid the risk of operational problems such as bad sound-head azimuth, and excessive impulse noise with worn prints. SR•D digital sound-tracks, though, and occasional 70mm magnetic prints have full bandwidth discrete effects on the surround track, and this will require analysis of surround loudspeaker response beyond 8 kHz. Surround equalization is more or less essential for quality theatre sound, as otherwise panned sounds cannot move smoothly around the theatre, or from front-to-back. For information on updating older Dolby cinema processors, see Section 5.0 below.

Several psycho-acoustic mechanisms combine to cause the perceived response from surround speakers to differ from that of the screen loudspeakers.

First, the surround information comes from a multiple array of loudspeakers, as opposed to a single source. Second, part of the signal comes from behind the listener, and the ear/brain combination reacts differently to sources behind the head. Finally, and probably of greatest significance, the average movie-goer selects a seat two-thirds of the way back in the theatre, and in a conventionally shaped theatre is thus normally much closer to the surround loudspeakers than to the speakers behind the screen. As a result, near-field response will be a far greater percentage of the surround signal than the screen signal, where far-field components normally dominate.

This large number of variables means that the ideal correction characteristic will be unique for each theatre. Figure 2.8 shows how the X-curve of IS02969 should be modified for surround use in a typical theatre, where most of the audience is closer to surround loudspeakers than to the screen.

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2.3.5 Measurement

Measurement microphones should be left in the when used to measure the screen channels. In addition, they should normally be left at the same orientation. A stereo surround set-up should be measured and equalized independently for the left and right channels.

Reference level (generated with a Cat. No. 85 Pink Noise Generator) is 85 dBC for each stage loudspeaker. A monaural surround channel should also be set to 85 dBC, a procedure described in each Dolby cinema processor manual. With a stereo surround installation, for Dolby SR·D or Dolby Stereo 70mm, the left and right surround chains should each be set to 82 dBC at the reference pink noise level.

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2.4 Sub-woofers

The installation must include sub-woofer(s), driven by dedicated power amplifiers--analog SR signals have to be derived from a bass extension module in the cinema processor.

Both Dolby Stereo 70mm magnetic and Dolby SR•D digital have dedicated low-frequency channels, requiring sub-woofers -- and one of the main benefits of Dolby SR with optical sound-tracks is the improvement in signal handling at frequency extremes. Figure 2.1 shows the relative peak level capabilities of an SR sound-track compared with those of mono and conventional Dolby Stereo. The significant increase in potential low-frequency signal energy requires the use of dedicated sub-woofers. Existing theatre bass bins (such as A4 units) are not acceptable.

With a Dolby digital sub-woofer track, the potential levels are even higher. Monitor levels are set for 10 dB of "in-band gain", as shown in Figure 2.9. This level setting procedure requires use of a real-time analyzer. Attempting to use a sound-level meter for sub-woofer level setting is extremely unreliable, for several reasons:

a) Different sub-woofers have different effective low·-pass filters, either caused by cabinet/speaker design, or by an actual low-pass filter. Even though the frequency range of interest is only up to 120 Hz with an SR•D track, the varying out-of-band components above 120 Hz can lead to variations of 3 or 4 db when read on a sound level meter.

b) There can be a substantial variation between meters at low frequencies, where C-weighting is not necessarily accurately followed.

c) Room nodes can affect the loudness perceived by a sound level meter, whereas the eye can easily see the effect of nodes when viewing the analyzer.

As real-time analyzers are always needed to adjust notch-filtering of room nodes, as described in Section 2.4.4, use of an analyzer instead of a sound-level meter does not affect installation time.

Many theatres which have been equipped with sub-woofers over the last few years have adequate relative low-frequency loudness when compared with stage channels at mid and low-levels. Some contain limiters such that if overloaded go smoothly into saturation without any clipping distortion--in such a case the signal may not sound distorted, but the peak levels are not correctly replayed. Badly designed sub-woofers, though, show significant distortion components at all levels (l) , and non-linear frequency response. The bandwidth of the digital sub-woofer channel on a Dolby digital sound-track extends from 5 Hz to 120 Hz. A linear sub-woofer acoustic response is desirable from, say, 25 Hz to 120 Hz. The 120 Hz sound-track cut-off is extremely steep, so a suitable sub-woofer need have little response above this frequency.

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2.4.1 Location

While sub-woofer location is not critical, a single unit should not be mounted on the centre-line of the theatre. If two sub-woofer cabinets are used, they should be mounted asymmetrically; ie they should not be mounted equally spaced either side of the centre-line as for channels 2(Le) and 4(Re) of a 70mm system. This asymmetric mounting reduces stimulation of standing waves derived from room dimensions.

To achieve maximum power, two sub-woofers should be mounted as close together as possible, thus achieving cross-coupling. Standing two sub-woofers to one side of the centre channel loudspeaker is probably a reasonable solution. On the other hand, if the target is to reduce the level of spot resonances, the two units should be separately mounted at, say, one-third of the way from the left wall, and one-fifth of the way from the right wall, though this, of course, will require more power.

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2.4.2 Notional Cross-over

With sub-woofers available to handle extreme low-frequency signals, it may prove benificial to lower the 40Hz band equalizer on stage channel Cat. No. 64 modules. The information is probably duplicated on the sub-woofer channel, and certainly so for an SR sound-track. Lowering the 40Hz band on L, C, and R avoids unnecessary stress to the stage LF units, and reduces risk of power amplifier clipping.

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2.4.3 Need for Bass Extension with Dolby SR

Modern main-channel loudspeaker systems have better low-frequency performance than systems designed a few years ago. However, extreme low frequency signal information requires special processing when derived from an optical sound-track, in order to suppress "streaking" noise and other processing artifacts. This circuitry, and a parametric equalizer to smooth out the primary room node, is contained on the optical bass extension module -- Cat. No. 160 or Cat. No. 560 in CP50 and CP200 units, Cat. No. 241 in CP55 units, and Cat. No. 441 in CP65 units. Also, see Section 5.0 below for information on retrofit modules for Dolby SR·D digital playback.

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2.4.4 Tuning

All cinema processor sub-woofer driver modules contain a simple parametric equalizer. This should always be used, as every room will have at least one dominant resonant frequency, which if not damped will lead to a characteristic low-frequency "ringing" every time the sound-crack contains extreme low frequency information. Instructions for adjusting the parametric equalizer can be found in each Dolby cinema processor manual.

2.4.5 Power Requirements

For playback of an SR•D soundtrack however, the power requirements for the subwoofer become more demanding. Two factors work together to raise the required amplifier size. One factor is the headroom available in the subwoofer channel. Like the boom channels on a Dolby Stereo 70mm mag print, the subwoofer channel is recorded 10 db lower than the other channels on a digital soundtrack. The cinema processor is then adjusted to playback 10 db higher, this providing 10 db more headroom to produce effects like explosions and stings at more realistic levels. This level requirement implies an amplifier power requirement 10 db or 10 times more than any other channel.

In addition, though, the subwoofers normally used for cinemas are less efficient than the stage speakers. Most of the models used are at least 3-5 db less efficient than contemporary direct radiator stage speakers. This means at least an additional 3 db or two times more power because of the lower efficiency of these speakers. Combined, the requirements become difficult - 13 db or 20 times the power rating of one of the screen channel amps. Figure 2.10 is an outline of the power requirments of the subwoofer channel depending on the size of the auditorium and can be used to determine the amps needed. As can be seen from the chart, it makes sense to use a model of subwoofer that has good efficiency since this will keep the power requirements smaller. However, be aware of claims of unusually high efficiency for subwoofers.

In loudspeaker design the laws of physics limit the maximum amount of efficiency improvement that can be achieved without a tradeoff in either low frequency response or enclosure size. In other words, for a given size of speaker enclosure the efficiency of a system can be increased but only at a loss to the low frequency performance of that system. This is why bass bins for stage speakers that are about the same size as subwoofers are more efficient but they cannot reproduce the deep bass that can be produced by a true subwoofer design.

Fortunately, there are ways to make the amplifier demands more reasonable. Sometimes it can be relatively easy to get increased headroom from the existing system. For example; if the system presently has two subwoofers each rated at 8 ohms and one stereo amp with each half driving one sub, it may be possible to wire the two subs in parallel and connect them across the amp running in a bridged mono mode. If the amplifier is of good professional quality with the capability of driving the equivalent of a 2 ohm load on each channel, it is possible to get as much as 4-5 db more headroom by simply rewiring the subwoofers in this fashion.

As mentioned earlier, multiple subwoofer units can be grouped to take advantage of mutual coupling. If the units are placed together ideally, doubling the number of subwoofers gives an extra 3 dB of output level due to efficiency gained by mutual coupling. Figure 2.10 shows the reduction in the amplifier power needed if more than one subwoofer is used, assuming that they are mutually coupled. As an example, if the system is as above with each half of a stereo amp driving a single 8 ohm subwoofer, two more subwoofers can be installed (mounted close to the other two for coupling) each wired in parallel with each of the other two, it can provide as much as 6 dB more headroom than before. Half of the gain is due to the efficiency increase of doubling the number of units and the other 3 db comes from the greater power output of the amplifier when driving a 4 ohm load on each channel instead of 8 ohms. Of course, one can always just buy more and/or bigger amps. This however, quickly becomes unwieldy if the system is something like 10 db deficient, and it is usually more practical to increase the efficiency of the system as well.

The placement of the subwoofer can also be critical if increasing the efficiency of the system is needed to keep the amplifier requirements practical. Generally speaking the units should be mounted as close as possible to as many boundary surfaces (read walls and floor) as possible. This means that the subwoofers should be installed (coupled together) at least on the floor and if possible next to the back wall and/or the side wall behind the screen. Unless the back wall behind the screen is appreciably greater than 10 feet behind the screen it is more desirable to have the subs back up against the wall on the door instead of up next to the masking. The increase in efficiency is preferable to the small delay in the bass signal because of the subwoofer being slightly behind the stage speakers. Corner mounting of sub-woofers in this way, however, may lead to a more pronounced primary room resonance. A further increase in efficiency would result from having the unit on the floor, and mounted in a baffle wall that extends to the dimensions of the screen, this of course would help the bass response of the stage channels in their proper locations as well.

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1 For background information see Engebretson, Low-Frequency Sound Reproduction, JAES May 1984, and Fielder and Benjamin, Subwoofer Performance for Accurate Reproduction of Music, JAES, June 1988.

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