Sensurround
The low-frequency sounds were more felt than heard, providing a vivid complement to onscreen depictions of earth tremors, bomber formations, and amusement park rides.
Sensurround involved the installation of large, low frequency, horn-loaded speakers which contained specially designed 18-inch Cerwin-Vega Model 189 ES drivers in custom black wood cabinets.
They came with special extenders used to widen the mouths of the horns and take advantage of the theater walls to further increase low frequency extension.
[3] Two low frequency control tones were printed on the film's mono optical or magnetic track; from the projector, the tones entered a control box in the projection booth, which fed low frequency pseudorandom noise to 1,600 watt BGW 750 audio amplifiers driving the speakers.
The control track method was employed because there was no way to accurately record bass lower than 40 Hz on an optical or magnetic film soundtrack at the time.
[5] In addition, portions of the main soundtrack were redirected to the Sensurround horns to create a partial surround sound effect.
Two tones of 25 Hz and 35 Hz were used: these two tones allowed different effects to be activated when desired: the rumble could be turned on or off in the Sensurround horns, the main soundtrack could be sent to the Sensurround horns, the level of both the rumble and the soundtrack coming from the horns could be varied by varying the level of the control tones and the theater's main audio system could be increased in level by up to 8 dB at select points to create a greater dynamic range (MCA called this the "Step-Gain" function.)
For the original version of Sensurround, prints were available in the following formats: Even before Earthquake ended its theatrical run, MCA began a program to re-engineer the Sensurround system to enable the recording of the deep bass on a standard 35mm optical soundtrack, with no external rumble generator being required, as well as improve fidelity and the overall effect and to simplify the system.
The addition of dbx Type-II noise reduction and its attendant increase in useful dynamic range created a high-fidelity sound from a conventional optical print.
While Warner Bros. and Paramount seriously considered adopting the system, in the end only Universal Studios ever released films using the process.
Thus, scenes could have dialogue and other audio at standard levels, plus the effect of movement from the infrasonic rumble, creating sensations that had not been possible in earlier Sensurround films.
MCA made these changes due to the success of the system and because the shipping costs of the heavy Sensurround horns were becoming very expensive.
Because control tones were used to trigger the Sensurround horns, theaters could not generally use the system with their standard films unless a rewiring was done.
The success of Star Wars over Rollercoaster in the middle of 1977 demonstrated to theater operators the limits of Sensurround as a guarantor of audience draw.
The alternative pop/rock band They Might Be Giants released a song titled "Sensurround" that was incorporated in the soundtrack to Mighty Morphin Power Rangers: The Movie and (in a different, faster-paced recording) their EP S-E-X-X-Y.
