3-3
Many loudspeakers used in theatres are far from ideal and require boosting of the low- and high-frequency extremes in order to produce an approximation of the standard reference response curve. Bass and treble controls—centered on the turnover points of typical loudspeakers—lift the ends of the spectrum without the need for large amounts of narrow- band boost from the third-octave controls in the Cat. No. 64 cards. The third-octave controls are used for minor adjustments that are required to smooth the frequency response curve.
The final factor is masking of the screen. Most stereo films today are shown in a wide-screen format. The masking curtains of the screen must be drawn back sufficiently to clear the left and right speakers before any adjustments or measurements are made. The treble horns should clear the screen frame and be mounted as close as possible to the screen. Conventional black felt side masking can severely curtail high frequency response. Consequently, there would be severe losses if the left and right loudspeakers were equalized with the masking open as for a 2.35:1 film, and then the masking were brought in for a 1.85:1 film, thus obscuring the outer speakers. To avoid this problem, some theatres have installed acoustically transparent masking cloth, and others leave the masking open whenever they are showing a 1.85:1 film with a stereo soundtrack. Moving the speakers towards the center of the screen so as to clear heavy masking is not a good solution, since the stereo sound width would be degraded.
Repainted screens cannot be used for quality sound playback, since the perforations which allow the high frequencies through the screen become clogged with paint.
