How THX Works

Projector alignment with screen
Projector alignment with screen

Let's take a look at the THX certification process. THX looks at a number of theater qualities, which fall into four basic groups:

  • Physical structure
  • Projection system
  • Seating arrangement
  • Sound system

The auditoriums in many theaters do not provide adequate barriers to keep out unwanted noise. How many times have you sat in an auditorium listening to the gunfire and explosions from the movie next-door? You may also have been distracted by the constant hum of HVAC equipment. THX-certified auditoriums cannot exceed a noise criteria level of NC-30 per octave. Noise criteria is a measure of background noise in a room -- in this case, noise from outside sources. NC-30 is comparable to the level of outside noise you would hear from inside a church.

The THX team also measures reverberation and echoing in the auditorium. They do this by counting the number of seconds it takes for a 60-decibel (dB) tone to completely fade away. For THX certification, this time must fall within a certain limit determined by the auditorium's size. You should not hear any echoes in a THX theater.

The projection system and the screen's brightness level must meet guidelines established by the Society of Motion Picture and Television Engineers (SMPTE). The projector must align with the center of the screen within five percent, and preferably three percent, of the screen's width and height.

When watching a movie with a standard 2.35:1 aspect ratio, people sitting in the end seats on the back row must have a viewing angle of at least 26 degrees (36 degrees is considered optimal).

This theater has a viewing angle of 26 degrees.
This theater has a viewing angle of 36 degrees.

No seat can have a partially obstructed view. Additionally, the movie's sound must be clear and distinct from any seat.