Sensurround System occurred at −16 dB. For program sounds applied to the Sensurround Horns, Track-2 (Center Front) was used, with its level and timing controlled by the 25 Hz control tone. For magnetic playback in mono, Track-2 (Center Front) could be used because Earthquake was mixed with mono dialogue, unlike most stereo films up to that time. For 4-Track equipped theaters that wanted to run Earthquake without Sensurround, the standard 12 kHz CinemaScope switching signal was recorded on Track-4 (surround). Because no noise reduction system was used on magnetic prints at the time, and due to the lower technical performance of the surround channels magnetic track, this switching system was implemented by 20th Century Fox during CinemaScope's development to reduce the hiss and system noise heard from the surround speakers. For Earthquake run in the United States, the surround speakers were disconnected and the Sensurround system was relied upon to produce all the surround sound effects; Track-4 was not used nor even connected to the Sensurround Integrator. In Europe, the surround track was used in the normal manner, being controlled by the Sensurround Integrator like the other magnetic channels. 35mm Mono Optical: These were dual-purpose prints and could be used either with or without Sensurround. The optical track was a composite mix of the program and the control tones and were compatible with any optical playback system. All mono mixes of the Earthquake soundtrack contain the control tones. To control the Sensurround System and its volume levels, control tones recorded on the optical track varied from −28 dB to −20 dB below 100% modulation. Activation of the Sensurround System occurred at −30 dB. For program sounds applied to the Sensurround Horns, the composite mono soundtrack was used, with its volume and timing regulated by the 25 Hz control tone. 70mm 6-Track Magnetic Stereo: The Sensurround Control Tones were recorded on Tracks 2 and 4 and are 100 Hz instead of 25 Hz and 35 Hz. This was because the magnetic system could not reproduce the very low-frequency control tones reliably. The 100 Hz tone on Track-2 controls the program effects applied to the Sensurround Horns and the 100 Hz tone on Track-4 controls the rumble effect applied to the Sensurround horns. Like the other systems, the level of the 100 Hz tones varies the levels applied to the Sensurround horns and when both control tones are present, the Step/Hi-Gain function