Dolby Tech Page

4.1 Criteria

4.1.1 Noise Floor

The steady-state theatre noise floor should preferably be below NC25, with NC30 the worst case acceptable. Intermittent increased noise floors should not exceed NC35.

Dolby SR and SR•D sound-tracks can contain very quiet sounds, as well as louder peaks than conventional film sound-tracks. Playback of these subtle components requires extra attention to background noise levels in the theatre.

Background noises can be broken into two types: steady-state noise, caused by HVAC equipment, refrigerators, projector noise and distant traffic rumble; and intermittent noise, caused by adjacent traffic noise, aircraft noise, footfall and adjacent screen breakthrough.

Figure 4.1 details the frequency characteristics of a family of NC curves in the range of interest. It should be noted that these curves show the NC figures for noise measurements made in whole octave bands, as conventionally used for background noise measurements. Figure 4.2 shows a family of curves for use in third-octave bands.

Normal techniques for background noise measurements are intended to quantify steady-state noises, and may not adequately define the annoyance of "chatter" noise, such as running projectors. Such noises should be subjectively inaudible in the seating area.

Reference: SMPTE RP141 -- Background Acoustic Noise Levels

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4.1.2 Reverberation Time

The reverberation characteristic for a theatre should be within the ranges shown in Figures 4.3 and 4.4.

Certain acoustic parameters differ depending whether a space is intended for music performance (a concert hall, or film sound-track reproduction (a cinema). The most obvious of these is reverberation, which in the cinema should be effectively as low as possible, and in the concert hall may consciously be extended in the design, to improve the subjective loudness of the music, and to make a more pleasant sound. In the cinema, the prime requirement is a more accuuate sound; reverberation needed to make the sound more pleasant is added during the mix during sound-track production. As most dubbing theatres are now moderately small, with short reverberation times, the mix will add adequate reverberation for all replay theatres.

Within reason, the reverberation characteristic of a theatre should be as short as possible. Excessive reverberation results in coloring of the sound and reduced intelligibility of the dialog. Assuming a theatre is built with sound absorbent material on all surfaces, the resultant reverberation characteristic will increase with room size, in consequence of greater reflection time delays caused by increased path lengths. Figure 4.4 shows the acceptable range of reverberation time change with frequency. This is a scaling curve, and the value at a given frequency should be multiplied by the optimum reverberation time at 500 Hz found from Figure 4.3 (1)

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4.1.3 Reflections

Optimization of reverberation time, though, is not enough to ensure good acoustics. A good theatre design will also avoid resonances and reflections. Good practice dictates that the front of the loudspeaker wall should be heavily damped with sound absorbing material, and even more important, that the rear wall of the auditorium should be heavily damped. Any theatres still using A4 type loudspeakers with wings should apply sound absorbing material to the front surface. Acoustically absorbent material can be added to an existing theatre, but new theatre designs should also consider issues such as minimum port glass size ( see below), as too large a glass area in the projection room wall can cause both picture and sound-reflection problems. Other sound reflection problems can come from converted old theatres with proscenium arches which face the screen, and ceiling beams and vertical column faces reflecting sound from the screen.

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4.1.4 Early Lateral Reflections

Another difference in acoustic requirements between cinema and concert halls relates to the desirability of early lateral reflections (sounds that reflect off the side walls at the front of the auditorium). In a concert hall, with a music performance, these reflections can be attractive, adding to stereo width, and giving the music more "body". But the same effect with dialog in a cinema can be disastrous to speech intelligibility, as the central speech image becomes diffuse, and there are multiple delayed reflections. For this reason, the side walls at the front of a cinema should be as absorbent as possible, and the loudspeakers should have a spatial response tailored to minimise the amount of signal which can hit the side walls (especially at frequencies above, say, 500Hz). The controlled directivity from use of horns is the only practical way this can be achieved. Direct radiator cone loudspeakers are not suitable for stage loudspeaker use, as not only will energy be reflected off the side-walls, but signals will also be reflected off the ceiling, further muddying dialog clarity.

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4.1.5 Rear Screen Damping

No behind-screen acoustic reflections should be audible in the seating area.

Behind screen echoes have historically been responsible for many of the intelligibility problems with cinema sound. The most effective method of achieving screen front/back isolation is to mount the loudspeakers as integral elements within a well-damped wall; this will block all but the lowest frequency back-screen audio. The front surface should be covered with acoustic absorbent material, damping any front/back reflections in the auditorium.

A wall also creates a perfect plane baffle, as described in classic loudspeaker design literature, thus significantly improving extreme low-frequency response and linearity. This is one of the reasons that a loudspeaker wall is one of the major elements of the THX loudspeaker system.

Without an isolation wall, attenuation of behind-screen reflections becomes much more difficult. The first and most obvious requirement is that the high- frequency horns should be mounted as close as possible to the rear of the projection screen, minimizing acoustic reflections off the screen surface itself. (The front of the horn should never be more than an inch or two from the screen.) Next, each loudspeaker assembly should be draped with substantial acoustically absorbent material, wrapping the entire assembly up against the screen. Finally, as much of the cavity surface area behind the screen as possible -- rear wall, side-walls and ceiling -- should be covered in absorbent material.One further consideration relating to systems without a loudspeaker wall is that the front surface of any large area bass bins, (and even more important, if fitted, the speaker wings), should have absorbent material mounted on front surfaces with cut-outs for the woofers. Without such material, significant reflective ''ping-pong" echoes can build up between the screen and the parallel rear wall of the theatre.

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4.2 New Theatre Design

Interior acoustics are Excess reflected sound of most importance for dialog intelligibility. can result in flutter echoes or reverberation which diminishes dialogue intelligibility.

It is not necessary to provide specific sound-reflecting surfaces in motion picture theatres. Most of the surfaces can be sound-absorbing. Some might argue that it would become difficult to sustain adequate loudness; suitable modern power amplifiers and loudspeakers, however, can easily be selected which provide enough power. Experience indicates that sound-absorbing rooms promote excellent speech intelligibility provided they are reasonably quiet.

Sound-absorbing material can be used to reduce reverberation and control echoes. Standing waves can result in low-frequency room resonance's which accentuate a "boomy" quality. Standing waves can be controlled using sound-absorbing material with an air space behind, such as a lay-in ceiling.

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4.2.1 New Theatre Location

Select a quiet location to reduce the costs of construction to prevent noise intrusions. Areas and adjacencies to avoid:

a) Next to window glazing.
b) Building service areas such as toilets, mechanical rooms, electrical rooms and elevator equipment rooms.
c) Other noise generating adjacencies.

Remember to review the use of spaces above and below the theatre for potential noise generation. Avoid locations beneath equipment rooms, and dance and exercise studios, or above parking garages or subway train lines. Airport flight paths, truck loading areas, and busy traffic intersections should also be considered during site selection, as the increased cost of adequate sound isolation may be significant.

Never locate a theatre below a curb-mounted air handler with direct bottom inlet and discharge, unless the ductwork is fury enclosed in special sound attenuation construction.

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4.2.2 Ceilings

In order to avoid excessive bass, specify a lay-in ceiling with soundabsorbing tiles having an NRC rating of 0.90 or greater. The tiles are typically comprised of 1.5" thick fiberglass with a painted glass cloth facing.

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4.2.3 Floors

Unless absolutely impossible, aisles and floors should always be carpeted.

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4.2.4 Walls

Sound Transmission Design Criteria

Walls, doors and floor/ceiling constructions are rated for their sound transmission properties according to ASTM Standards E90, E336, and C413 which result in a single figure of merit rating system known as Sound Transmission Class, or STC.

The selection of appropriate STC ratings needs to be made on the basis of the background noise criteria selected in the theatre and the level of noise anticipated in the adjoining spaces. Continuous background noise can play an important role in perceived sound isolation by masking transmitted sound. The sum of the STC rating plus NC rating should always equal or exceed 95 at common walls between theatres. Other sources of intrusive noise should be evaluated.

All sound-rated partitions must incorporate full height slab-tostructure framing containing fibrous insulation and gypsum board sealed airtight at the head and sill with a bead of acoustic sealant. All penetrations must be sealed airtight and recessed boxes fully enclosed. Four-gang and smaller junction boxes can be sealed using sheet caulking on the back and sides, as shown in Figure 4.5. Larger boxes can be effectively sealed using one-hour fire-rated gypsum board construction. Comply with the standards outlined in ASTM Standard E447, "installation of Fixed Partitions of Light Frame Type for the Purpose of Conserving Their Sound Insulation Efficiency.

Figure 4.6 shows minimal and typical multiplex demising wall designs necessary to achieve acceptable auditorium isolation.

Table 2 below presents suitable STC ratings. Note that higher numerical STC ratings transmit less sound, and higher NC ratings permit louder background sound due to the ventilation system.

Table 2: STC Ratings for Common Walls Between Theatres

Minimum in-situ STC Rating Noise Criteria Description STC 60 NC 35 Minimum Standard STC 65 NC 30 Typical STC 70 NC 25 Desirable

In order to avoid sound "flanking" the walls, continuous metal roof decks are discouraged without gypsum board ceilings, and independent floor slabs with an elastic joint are required at STC 65 and greater walls.

Figure 4.7 shows a more sophisticated wall design, used to ensure isolation between projection rooms and auditorium -- a construction of this type can be a major element in isolating projector and machinery room noise.

Sound-Absorbing Wall Treatments

The wall behind the audience should be covered in sound-absorbing material entirely.

Typical sound-absorbing panels are comprised of 1.5" thick glass- fibercore wrapped in porous fabric having inherent flame-resistant properties. Panels are available with fabric edge wrapped conditions and with resin hardened edges, metal, or wood frames. Complete prefabricated sound-absorbing panels are available.

If the side walls are not angled, avoid hard flat parallel gypsum board surfaces facing one another across the room, particularly in the audience area at ear height, in order to avoid flutter echoes.

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1 Reverberation time measurement techniques are discussed in IS03382.

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