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 box generated a pseudorandom noise signal with energy between 17 and 120 Hz. 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. When receiving the noise signal, the amplifier and subwoofers responded with sound pressures ranging from 100 dB to 120 dB. The resulting rumble could be felt by audience members as well as heard. For the film Earthquake, Sensurround was activated during the quake scenes to augment the conventional soundtrack. In addition, portions of the main soundtrack were redirected to the Sensurround horns to create a partial surround sound effect. The control tones recorded on the film's optical or magnetic track triggered the rumble or surround sound effects as well as controlling their volume and the overall blend of the main soundtrack and low frequency noise effects. 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: 35mm 4-Track Magnetic with Optical Track (magoptical): This type of Sensurround print was not usable as a mono optical print. Unlike standard magoptical prints, in which the optical track was a mono version of the magnetic stereo tracks, the optical track on magnetic Sensurround prints contained only the control tones. If played as a standard optical print, only the 25 Hz and 35 Hz control tones would be heard. To control the Sensurround System and its volume levels, control tones recorded on the optical track varied from −14 to −6 dB below 100% modulation and activation of the