Dolby Tech Page

2.2 Screen Loudspeakers

2.2.1 Location

The Left and Right screen loudspeakers should be mounted at the left and right extremes of the screen width when the masking is fully opened for projection of a 2.35:1 (Cinemascope) aspect ratio anamorphic picture. If screen masking covers these loudspeakers when a 1.85:1 picture is being projected, high-frequency attenuation due to the screen masking should not exceed 2dB at 8kHz.

The obvious intention of many elements in a stereo sound-track is that a sound should seem to emanate from the same location as the related picture image. When an actor closes a door at the left of the picture, the sound of the closing door should come from the same location; when we see a trumpet player close to the right edge of the screen, the sound should come from the same side of the screen. The objective of stereo sound is to place the apparent sound source sufficiently close to the image of the trumpet, in a way such that sound and picture together seem "real".

Listening to music in the home places no specific demands that stereo width be accurately defined -- there is no picture to which the sound should match. Typically, though, the two loudspeakers will subtend a total included angle of around 60 degrees to the listener. In the home the distances from the loudspeakers are short, and the room surfaces (furnishings) are absorbent -- as a result the listener will hear dominantly the direct signal from the loudspeakers.

In a commercial theatre, however -- no matter how small, and no matter how absorbent the materials on the walls and ceiling -- the path lengths are so much greater that what most of the audience hears with non-transient material is dominantly reverberant information coming from many directions, reflected from many room surfaces. This is why with a typical theatre layout, measurements show that at best only in the first row or two of seats does the near-field direct signal dominate.

As a result, from a prime seat where the screen subtends an ideal projection angle of 45 degrees, the listener may hear an acoustic width of only 25 or 30 degrees from loudspeakers typically 40 degrees apart, mounted at the screen ends. Further back, dominance of the reverberant field increases, and acoustic width therefore narrows still more. Indeed, in the back rows of most theatres, so much directional information is lost and the sound becomes so diffuse that few, if any, spot effects can be directly associated with the action on the screen.

This progressive attenuation of stereo width towards the rear of the house explains the requirement for maximum possible width in loudspeaker placement. As films are mixed to match picture in the dubbing theatre, it is difficult to conceive of situations where the screen is so large, and the reverberation so short, that the sound image is too wide for any of the audience not sitting in the front one-or-two rows.

This requirement for maximum stereo width holds equally true with the narrower screen image of a 1.85:1 movie, and to the maximum width of a 2.35:1 anamorphic print. Even though the masking has moved in to sharp-matte the 20% narrower picture image, the widest possible audio image should be retained -- in this way, the sound/picture match will "work" for the largest possible percentage of the audience.

Some years ago, narrowing the masking and covering the left and right loudspeakers when projecting a 1.85:1 picture caused major audio problems. (Not surprisingly, when considering the high-frequency attenuation resulting from black felt!) Happily, new techniques and materials have been developed to answer the problem. Black muslin, or acoustically-transparent, loudspeaker grille cloth (as used for highfidelity loudspeakers) stretched over an "open" frame can be used for an insert covering the small area of the masking obscuring the highfrequency horns. For new theatres, masking cloth (Harkness 2000M) has been developed which appears matte black, is acoustically virtually transparent and is only slightly more expensive than the black felt it makes obsolete. (1)

But even if acoustically transparent masking cloth is used, care should be taken that the hard mounting edge (typically plywood) which supports the cloth does not cover any part of the horn mouth. Care should also be taken that cloth folded back on itself at the mounting edge does not attenuate the high-frequencies, and that "bunching" masking cloth in front of a loudspeaker can cause severe highfrequency attenuation.

---

2.2.2 Loudspeaker Type

Figure 2.2 can also be used for guidance as to suitable loudspeaker types. Manufacturers' literature should be consulted as to the maximum power handling capability of a given loudspeaker. While there are several manufacturers offering a wide variety of loudspeaker types, most quality theatres today are installing bi-amplified systems, with a constant directivity high-frequency horn and a direct radiator cone diaphragm-vented box low-frequency unit.

---

2.2.3 Loudspeaker Walls

Since the earliest days of loudspeaker design, it has been recognised that low-frequency response can be assisted by mounting the transducer in a plane baffle, An infinite sized baffle puts the transducer in what is termed a 2 pi space. This better bass efficiency is why the classic cinema loudspeaker for many years, the Altec A4, was provided with "wings", plane wooden sheets mounted on either side of the LF unit. This concept was extended in the late seventies, when three or five A4 units would be connected with a wooden frame with a solid plane baffle connecting each unit. A similar idea was used by Tomlinson Holman with the THX loudspeaker system (2), where the loudspeaker system is mounted in a wall, covered with sound absorbent material. This baffle gets close to the theoretically perfect 2 pi baffle, but if the wall extends from floor to ceiling it can also effectively cancel transmission of rear screen echoes, as discussed later.

---

2.2.4 Cross-overs

A bi-amplified system, with active cross-overs, is an essential requirement of any high-quality contemporary theatre sound system. The most important reason for bi-amping is the capability of power handling of bass and treble simultaneously. In addition, an active cross-over enables a smooth characteristic over the cross-over region, with minimum phase discontinuity. Active cross-overs also make possible a signal delay to the low-frequency unit, improving coherency of arrivals of HF and LF signals in the seating area, and further improving signal phase continuity around the cross-over region.

---

1 Harkness Screens, The Gate Studios, Station Road, Boreham Wood, Herts WD6 IDQ, England, Tel: 081 953 3611, Fax: 081 207 3657.

2 THX is a registered trademark of Lucasfilm Ltd.

---