Acoustics in cinemas

History


·       The first cinema was built in 1907.
Early UK and USA cinemas developed ornate, escapist interiors, the amount of absorptive finishes not being critical when silent movies were accompanied by orchestra or organ.

·       Later on the size of the movie theatre increased; the 1927 Roxy in New York housed 6000 people.

·       Sound systems were initially primitive, and then loudspeakers replaced simple, single speakers.

·       In 1950s, wide screens became necessary.
The aim was greater sound-in-space effect from surround sound systems.

Design criteria
Design is considered under the following headings:
1.      Development
2.    Size
3.    Isolation
4.    Seating
5.    Materials
6.    Ventilation

Design criteria
The elimination of reflective surfaces in a cinema hall is necessary not only because of low RT ,but also to preserve the directional effect of sound ,since the sound should appear to come from the projection screen
So the loudspeakers are located behind the screen
The rear wall,the balcony parapet ,the wall behind the screen must be highly sound absorptive.
Interior concave shapes,which lead to sound concentration or focusing should be avoided.
Parallel side walls,if used should be treated with sound absorptive material to eliminate echoes.
The volume of theatre should be as small as possible,consistent with hall geometry.
The floor rake must allow for clear sight lines on screen .
Usually ,the floor rake in a cinema hall is much hallower than a lecture hall,since cinema screen is higher than the height of stage in a typical auditorium.

Size
·       Cinema auditoria as currently designed have diminished in size.
·       A form of subdivision is used for conversion of large cinemas.
·       In considering such conversations, it should be remembered to tailor the volume as well as the floor area to smaller units.

Isolation
·       Cinemas tend to be planned in groups within a complex, so isolating lobbies and buffer zones are sometimes necessary to provide satisfactorily low levels of noise within the cinema.
·   A composite wall consisting of solidly grouted masonry and a freestanding metal stud wall is recommended as a partition wall between two adjacent halls.
Doors of opposite cinema halls should be staggered.

Seating
    Seating rake is less critical than for concert halls or theatres because there are few rows, view to the screen is elevated, and the sound comes from overhead loudspeaker sources rather than from stage level.
· Front rows may be tilted up for a clear view of the screen.
Seating can be well upholstered, so the cinema does not sound empty at low occupancy.

Projection Room
The sound levels in the projection room are very high , particularly in a multiple cinema complex where a common projection room is used for all halls.

The projection room walls must be of heavy constuction to reduce noise transmission from projection room to theatre.

Use a minimum practicable projection port opening for viewing by the projectionist –the viewing port.
The projection port as well as viewing port should be doubly glazed with sound absorbing material.

Materials
    Finishes of ceiling, floor and walls.
generally should be sound absorbent
   This will permit clarity of sound track and soak up noises.
·  Wall absorption can
be effected by panels or
by medium-weave curtains
hung in folds. Or even by
permitting the floor carpet
finish to continue up to the walls, at least up to dado height.
·      
Materials
  Usually 50-100mm thk fiberglass spaced 100mm from the backup wall is required to achieve low frequency absorption.

Carpet to aisles as well as under seats absorbs footfall sounds.

A protective finish over fiberglass, such as wood slat screen,brick screen or perforated metal panel us required.

 Ventilation
·       A suitable ambient noise control standard for motion picture theatres is NR 30-35.

Where a multi-use auditorium includes a requirement for cinema, the need for sound track clarity demands a 20-25 percent reduction in reverberation time.

Design
  Determine size of cinema-box office occupancy.

Set auditorium volume-volume per occupant could be as low as 1.25 cu.m. per person for a large cinema, in order of 5 cu.m. per person in case of small studio cinema.

Design roof and walls for a high standard of isolation from external noise.
     Determine sound absorptive finishes to achieve a reverberation time of 1.0 seconds or less, perhaps as low as 0.5 seconds for a studio cinema, which is necessary for good audibility.

Set seating rake for good sight and sound- use sound absorptive seating.
  Design in sound lobby arrangements- ensure that sound-absorbing finishes are designed into the foyers.

Design doors and projection windows to be of an adequate standard.

Design air-handling / central plant and services away from hall.

Design suitable sound reproduction and amplification systems.

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