THX Tech Pages

c) Diffusers

1) Air flow Recommendations:

a) Supply should be 350-425 CFM (10-12 cubic meters per minute) per register maximum.

b)Return should be 420-5 TO CFM (12-14 cubic meters per minute) per grille maximum.

2)Rattles (There should not be any.)

This can be easily tested by generating a slowly varying sine wave signal through the sound system. As the frequency is increased, different parts of the theatre construction will rattle. Identify the sources and eliminate the rattles by tightening, damping, or fastening them down. Common rattles occur from lay-in ceiling tiles, HVAC grilles, and loose lighting fixtures.

2.Reciprocating Equipment (e.g. Compressors, Pumps, and Related Ducts)

Spring isolation mounting should be used. Box enclosures should be used in extreme cases. All ductwork connected to vibrating equipment will also transmit vibrations along its path. The use of flex duct is recommended at the beginning of the duct run and isolation hangers should be used wherever the ducts need to be suspended or supported to a nearby wall/ceiling construction. Avoid mounting any mechanical equipment and ducts directly on demising theatre construction Avoid locating concession stand equipment directly adjacent to the theatre.

3.Environmental Sources

Special steps may need to be taken if the outside noise threatens to interfere with the quiet of the theatre. Resiliently mounted suspended ceiling isolation, exterior wall isolation, and sound rated doors with neoprene head and jamb gasketing are recommended.

a) Trains
b) Subways
c) Airports
d) Traffic
e) Discos
f) City Streets
g) Lobbies

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D. Projection

1. Projection Port

Projection booth noise (e.g. projector, speech) should not be audible anywhere in the auditorium.

a) Double Glass

Two panes of glass help isolate the noise of the projection room from the auditorium. The frame construction should have neoprene perimeter gasketing to create a tight seal and eliminate sound leakage around the edges.

b) Angled Glass

Angled glass on the projector side avoids a reflected image from being sent back into the projector lens. Our general recommendation is a 7 degree outward tilt from the perpendicular plane of the projector.

Glass angled up slightly on the auditorium side avoids sound from the screen being reflected to the audience.

c) Access Hole

A pass-through, sealable hole of 1.5" (4 cm) in diameter is necessary to provide access for microphone test cables and communication between the booth and the auditorium. Another option for test cable access is to use removable and sealable glass. (Please note that removable glass or access holes must be sealed before presentations.)

2. Projection Angle

a) THX Standard

Image distortion due to the horizontal or vertical projection angle should be less than 3%, the maximum preferred, with 5% distortion the maximum accepted for THX Certified Theatres. (Please refer to SMPTE EG 18-1994 for further details about the formulas.)

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b) Formulas for Determining Projection Angle Distortion

H = Image Height
W = Image Width
PA = Horizontal Distance from Projector Aperture Plane to Screen (or chord if curved screen)
CA = Vertical or Horizontal Offset

Examples of projection Distortion Calculations:

H = 20 feet (image Hieght)	W = 47 feet 10 inches (image Width)
PA = 80 feet	PA = 80 feet
CA = 4 feet (Vertical Offset)	CA = 1 foot (Horizontal Offset)
PC = 80.10 feet	PC = 80.01 feet
alpha symbol = 2.86 degrees	alpha symbol = 0.72 degrees
Percent Distortion = 1.25%	Percent Distortion = 0.75%

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3. Viewing Angle

a) Horizontal

The angle subtended by the left and right edges of the Cinemascope image and the farthest seat in the auditorium is recommended to be 36 degrees. The minimum acceptable angle is 26 degrees for THX certified cinemas.

Horizontal Viewing Angle=

2 x ARCTAN[(0.5 x Width of Scope Image)/(Distance from Screen to Farthest Seat)]

b) Vertical

The vertical viwing angle is measured at seated eye height from the front row center seat to the top of the tallest projected image. SMPTE notes that

Rev. 1/98                                               Page 8

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