An equalization method standardized by the Academy of Motion Picture Arts and Sciences in 1938 and applied to the standard monaural track then used for sound in film, also known as the N (normal) curve. The response is flat between 100Hz and 1.6kHz. Response is reduced 7dB at 40Hz, 10dB at 5kHz and 18dB at 8 kHz. The severe reduction of the high end of the spectrum was specifically designed to conceal the high frequency crackling noise inherent in early film sound production.
The intent of this specification was to ensure that the sound quality of motion pictures was consistent from theater to theater. Prior to the establishment of the Academy curve specification, thousands of movie houses had limited amplification and loudspeaker configurations and relied on custom equalization to shape their sound projection, which was less expensive. An expert engineer sat in the auditorium with a set of variable tone controls rather like a graphic equalizer. When he was satisfied that the best results were being obtained from the samples, the frequency correction was measured and built into the amplifier system. This method was not entirely satisfactory since many theaters contained acoustic defects, which could not be corrected by equalization.
The Academy of Motion Picture Arts and Sciences set up a committee to study the standardization of theatre sound equipment. A test reel was prepared containing samples of dialogue and music from the major studios, and after repeated running in several theatres the final characteristics were established. Amazingly, despite consistent improvements in amplification and loudspeaker technology, the Academy Curve remained in effect almost 50 years.