THX Tech Pages

What THX Needs From Architects

A. Updated Drawings

1.	Floor Plan
	An indication of seating area allows us to ensure even sound coverage of the audience through speaker placement and aiming.
2.	Interior Elevations
	Elevations are needed for room volume calculation and coordination of surround speaker placement with interior details.
3.	Common Wall Construction Details
	THX checks for appropriate isolation between theatres.
4.	Roof Plans - HVAC
	THX checks placement of roof top units and ducting.

B. Early Warning

1.	Bid Set Date
	We understand the importance of avoiding Change Orders and will work with you to meet Bid Set deadlines.
2.	Opening Date
	This helps us plan our design and field certification test schedules.
3.	Changes
	a) Floor Slope b) Ceiling Height c) Exits d) Screen Sizes e) Corridors Behind the Screen f) Interior finishes g) Mezzanine Overhang h) Anything Else

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II Environmental Criteria

Note: The recommendations made in this are general. An acoustical consultant should be retained to make site specific recommendations so that THX criteria established in our Architects's and Engineer's Manual are met.

A. Isolation Walls

1. Suggested minimums

A minimum of Sound Transmission Class (STC) 75 construction is recommended. (Please refer to the wall construction details in the Isolation Section of our Architect's and Engineer's Manual,) A more specific recommendation of acoustic noise reduction between theatres is listed in the table below:

Octave band		Noise reduction
31.5 Hz		38 dB
63 Hz		48 dB
125 Hz		52 dB
250 Hz		54 dB
500 Hz		66 dB
1000 Hz		66 dB
2000 Hz		66 dB
4000 Hz		66 dB
8000 Hz		66 dB

2. Constructions

Avoid penetrations in theatre common walls. Perimeters of an isolation walls should be well caulked using acoustical sealant. Resilient mounting of the wall to the roof deck should be employed with constructions using gypsum. If it is not employed, deck deflection could deform the gypsum providing a flanking path for sound.

B. Reverberation

An acoustical consultant should be brought in to calculate the absorption needed to meet the reverberation specification supplied by THX.

1. Specific

a)Sidewall

If the sidewalls are too reflective, a "flutter" or side to side "slap" echo will be heard. The absorption needs to be brought down to below seated ear height, as close as possible to the level of the finished floor in the

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seating area. A minimum of 1" (2.5 cm) of fiberglass must be used, with 1" to 2" 12.5 to 5 cm) of air space behind the absorptive treatment preferred.

To avoid side to side reflections, the design of non parallel sidewalls is recommended although a "crawling" flutter echo may still occur.

b) Projection Wall

Much heavier absorption needs to be used on the projection wall to avoid delayed reflections of dialogue reaching the audience from the rear. A minimum of 2" (5 cm) of fiberglass should be used here also with an air space behind it. All of the available area of this wall should be treated, as it is otherwise the potential source for Long delayed reflections for listeners in the front of the house.

2. General

a) Low Frequency 120 Hz - 250 Hz)

Excess reverberation in the low frequencies makes the theatre sound "boomy." Generally, low frequencies are absorbed by thick material, up to B" (15 cm) thick, spaced off of a surface by 2" to 4" 15 to 10 cm). Because of the long wavelengths involved, large cavities or air spaces are also needed for low frequency absorption. Lay-in tile ceilings with a large air plenum above can be very useful for absorbing low frequencies. Six inches (15 cm) of batt insulation above the suspended ceiling absorbs low frequencies well. In climates where pipe freezing is a possibility, this technique should be limited to the front (screen) half of the auditorium. The use of fiberglass tile instead of mineral fiber tile is also good for absorbing low frequency, However, fiberglass allows sound to pass through it more easily than mineral fiber. Thus sound from the area above the tile will tend to pass into the auditorium more easily. Therefore when using fiberglass tile, extra care must be taken to reduce breakout from the ducts or other sources of unwanted noise above the lay-in tile ceiling.

b) Mid Frequency (250 Hz - 2000 Hz)

Excess reverberation in the mid frequencies lessens dialogue intelligibility. Absorption is easily dealt with by i" to 2" (2.5 to 5 cm) of fiberglass mounted on wall surfaces. For the lower part of this frequency range, spacing the absorptive treatment off the wall is recommended.

c) High Frequency (2000 Hz - 16000 Hz)

Excess reverberation in the high frequencies makes the dialogue too sibilant. In most theatres, absorption in this range is usually not a problem due to the absorption from air in large volumes and the high absorption coefficients fat high frequencies) of typical acoustical panels.

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C. Background Noise

1. HVAC as a Noise Generator

a) Roof Top Units (RTU's)

1) Structure

Low frequency noise can be transmitted structurally in the form of vibration through walls or the roof deck. Avoid this by providing isolation (spring isolators), short-circuiting vibrations to ground (mounting RTU's over a load bearing wall), or distance (mounting RTU's as far away from the theatre as possible or, in the worst case, over the projection booth). If possible, avoid locating any mechanical equipment directly above the theatre. When using spring isolators, attention must be paid to spring deflection. Deflection is determined by frequency of the vibrations, stiffness of the roof deck, and weight to be mounted on the spring isolator. It is best to request an acoustical consultant or mechanical noise specialist to specify the exact isolation springs for a given mechanical unit.

2)Roof

Avoid mounting RTU's mid-span over an auditorium. This causes the roof deck to act as a diaphragm creating low frequency noise problems which are difficult to reduce. The more the deck deflects, the more severe the problem.

3)RTU Duct Exit

A side exit from the Roof Top Unit allows a large amount of the airborne sound to dissipate in the first turn (more if the duct is lined).

b) Ducts

1) Turns

In general, the more turns between the Roof Top Unit and the diffusers, the more noise attenuation. Turns also produce turbulence so the last run up to the diffuser needs to be straight. The longer this straight portion is the less turbulence will contribute to the noise.

2) Lining

Fiberglass duct lining absorbs airborne noise (especially at turns). Ten to sixteen feet C3 to 5 meters) lined ductwork is a good minimum for each auditorium's feed.

3) Breakout

Sound leaving the duct through its walls can be avoided by boxing in the duct with a frame and gypsum board which does not touch the ductwork. This enclosure may also be lined with fiberglass to provide even more duct breakout absorption.

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