| CES : Technical Guidelines for Dolby Stereo Theatres November 1994 Page 8 |
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4.0 Acoustics continued
4.2.5 Seats
Choice of seat design can have a significant effect on acoustic quality, especially in the small theatre. This becomes obvious when standing at the screen and looking at what the loudspeaker "sees" -- a major part of the sound field will be seats, or optimistically, audience.
First, a seat should be chosen which will be sufficiently damped as to offer no reflections to the screen. If the seats have fold-up bases (squabs), the undersurface should be damped. Never use seats with a plastic or metal reflective undersurface.
Next, an ideal seat has acoustic properties that do not change when a person sits in it; ie the absorption with respect to frequency does not change. In this way, regardless of what percentage of the house is filled with audience, the frequency response will be the same. Better quality seat manufacturers will provide information on this subject; contact Dolby Laboratories for information on testing procedures if you wish to carry out your own evaluation.
The most sophisticated seat selection will also consider high-frequency grazing at the top of the seat, and low-frequency aberrations caused by the potentially resonant chamber under a row of seats.
4.3 Background Noise
4.3.1 HVAC Design
General Layout
Mechanical equipment having rotating parts should not have any direct physical contact with the walls or ceilings of theatres.
Do not locate air-moving devices in the ceiling plenum directly above the theatre. This includes heat pumps, fan-powered boxes, variable air volume boxes (VAV), fan coil units CFCU), exhaust fans, and air handlers IAHU).
VAV boxes may be located above theatres only if the ceiling is comprised of at least one layer of 5/8-inch thick gypsum board. Other equipment should be reviewed for its radiated sound level if it must be located above the space. Avoid using fan-powered VAV boxes due to high levels of radiated noise.
Locate air-moving devices above storage rooms, corridors or the projection room. If for some reason they must be located over the auditorium, placement at a corner where they are supported by two load-bearing walls or a structural column will be of great benefit. Consideration could also be given to the use of stanchion supports on the roof. Noise from the air-handler is transmitted in two ways: direct transmission of rumble through the building structure, and airborne noise through the ducted air. If air-handlers have to be mounted on the roof at auditorium corners as described, extend the ducting as far as possible before a downturn to reduce airborne noise.
All air handlers and projector exhaust fans must be vibration isolated.
Again, never locate air-handling devices on the roof directly over the theatre.
Air Handling and Distribution
Review supply air diffuser selections and specify an NC rating five points less than required.
Lay out supply air ductwork and registers to have equivalent duct lengths between diffusers and the fan.
Only those ducts, pipes and conduits essential to serve a specific auditorium space should be allowed to penetrate its walls.
Supply Air Duchvork
Provide an exclusive duct branch and zone system to serve each auditorium/screen so as to avoid crosstalk.
Size ductwork mains over the theatre to have one-inch thick internal duct liner.
The discharge sound level of the supply fan being used should be reviewed in order to specify the appropriate duct length and/or sound attenuator.
Excess turbulence generated noise may be avoided by using 900 duct turns; provide long radius elbows. turning vanes. If unavoidable, specify air-foil
Dampers
Dampers should never be located directly behind the face of the air distribution device.
Plumbing and Piping Systems
Avoid locating plumbing and rainwater leaders in the walls or ceiling spaces of the theatre.
Any piping located in the walls or ceiling of the theatre space should be attached with resilient mountings.
Provide a clearance between pipes and gypsum board or other finish surfaces. Do not allow pipes to make rigid metal-to-metal contact between ceiling hanger wires, supports, framing, or other structure to which finishes are attached.
Penetrations
Ducts penetrating the sound-rated wall or floor/ceiling construction should be in an insulated sleeve packed with one-inch thick mineral wool fire safing and sealed on both sides using backer rod and acoustical sealant.
Piping penetrations less than three inches in diameter should be sealed using acoustic sealant filling a 1/4-inch clearance. Larger pipes should be treated similar to ducts.
Avoid any back-to-back penetrations, such as electrical panel boards, junction boxes or fire extinguisher cabinets. Offset penetrations two stud cavities.
4.3.2 HVAC Maintenance
Regular HVAC maintenance is essential to ensure the minimum noise level the system is capable of. Bearings should be lubricated, and belt tension and condition verified, at manufacturers' recommended intervals; most important, filters should be cleaned on a scheduled basis. Dirty filters have been found responsible for a theatre noise floor 10dB above the measured figure at the time of system installation.
4.4 Sound Isolation
4.4.1 Ceilings and Floors
A suspended gypsum board ceiling must be specified if:
a) Air-handling equipment is located above the room (a concrete floating floor may be necessary in the equipment room).
b) The floor above the theatre is uncarpeted.
c) The floor slab above is not at least six inches of regular weight concrete.
d) The Sound Transmission Class of the floor/ceiling construction will be inadequate using a lay-in tile (i.e., lightweight wood frame construction).Provide mineral wool or fiberglass insulation above all gypsum board ceilings.
The above analysis should also be conducted on the floor/ ceiling assembly below the theatre. Gypsum board ceilings are generally necessary in lightweight wood frame construction where concrete is not used.
4.4.3 Doors
Doors are among the most likely paths of noise intrusions. Vestibules are the most reliable means to obtain adequate noise reduction without extensive care in specification and quality assurance procedures during construction, including performance verification of the actual installation. The vestibule should be carpeted, have a sound-absorbing lay-in ceiling, and have full- height walls which are insulated and faced with wall carpet.
A very simple test for obvious sound leakage is to switch off auditorium lights and look for any visible light leakage around the doors.
Table 3: Typical STC Ratings for Sound-Rated Doors
STC 35: 1-3/4 inch thick solid-core wood door with double gasketing at the head and jambs and fixed threshold with door bottom.
Use only at vestibule or guarded entry conditions.
STC 42: 2-1/4 inch thick sound-rated wood door and adjustable neoprene head and jamb gasketing with flat threshold and automatic door bottom.
Use only at carpeted corridors which are designated as quiet zones.
STC 49: 2-1/4 inch thick insulated steel door having tandem offset magnetic gaskets at the head and lambs with cam-lift hinges and flat threshold.
Require manufacturer's authorized representative to supervise installation and prepare punch list.
While theoretically the CP50 could be updated for the playback of the Dolby SR·D sound-track, the cost of labour and material necessary approach the cost of a new Dolby CP65, and so such an update approach is strongly discouraged.
Many boards in cinema processors manufactured by Dolby Laboratories have been replaced from time-to-time with new versions with superior performance. Taking full advantage of the specifications of a Dolby SR and SIC•D film will require updating selected boards.
Matrix decoders. Significant improvements have been made by Dolby engineers over the years in the technology used to decode the four channels (L,C,R and S) from the two tracks on the film. Current modules provide stable inter-channel separation adequate to be indistinguishable from discrete sound-tracks with much program material. As "discreteness" is frequently quoted to be superior with 70mm magnetic when compared with simple matrix decoders used with 35mm stereo optical, updating to the latest technology is well worthwhile for SR playback.
Current (since 1992) production versions of the Cat. No. 150 decoder module are Revision E. Currently acceptable modules date back to Revision D, which was introduced at Serial Number 12000. In addition to superior decoding performance, the current modules have increased headroom for all channels, and a low-noise surround delay circuit -- both of benefit with the wider dynamic range of Dolby SR films. Cat. No. 150 modules of revisions A, B, and C should be replaced; old Cat. No. 116 decoders are certainly unacceptable for SR playback.
Optical pre-amplifiers. Later versions of optical pre-amplifier boards installed in Dolby theatre equipment exhibit significantly improved high-frequency response, improved stability and better linearity and phase response at high frequencies. CP50 and CP200 units should be updated with CN108 pre-amplifier cards of Revision C, which was introduced with Serial Number 11570. All CP55 and CP65 unit preamplifiers have adequate performance.
Dynamic range. Dolby SR and Dolby SR•D playback may also justify updating other modules in older CP50 and CP200 cinema processors. Some early boards have performance specifications more than adequate to play back conventional mono and Dolby Stereo films, but exhibit headroom and noise floor characteristics which could be improved for the new sound-tracks. Cat. No. 64 equalizer modules of Revision B, for example, have a significantly lower noise level than earlier variants.
Cat. No. 441. This surround/sub-woofer card provides two channels of surround equalization and the required capabilities for stereo surrounds and a sub-woofer channel for the playback of an SR•D sound-track. This module is for use in a CP55, where it replaces the Cat. No. 241 module.
Cat. No. 560. Provides the same functions as the Cat. No. 441 above, for use in a CP200, where it replaces the Cat. No. 160.
Cat. No. 517. Replaces the Cat. No. 117 in a CP2OO. Essential to handle the greater dynamic range of modern sound-tracks. Provides greater dynamic range, lower distortion and additional features for playback of Dolby SR and SR•D.
Cat. No. 137. Link card required for use with Cat. No. 517 output card -- replaces Cat. No. 137 in a CP200.
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