Since this theatrical subwoofer system debuted years before many of HTF's members were born, I thought it would be interesting to post about this theater-based subwoofer system. And if anyone was there when it debuted - I was! - go ahead & post your own impressions of it for this trip down memory lane.
I was reminded of this groundbreaking /img/vbsmilies/htf/smiley_wink.gif"> system when I was at a music store recently & spotted a dvd version of Earthquake
, one of the many disaster movies which came out in the 70s. It's not too bad to watch if you have a couple of spare hours on your hands and the effects, while not CGI, are decent (though one scene in an elevator contains the cheapest & incredibly fake-est "special effect" I have ever seen in a major-studio release). About seven years ago I bought the dvd version put out by Good Times Video but was disappointed to find it lacked the extremely low frequency bass I heard as a kid in the theater. So I was excited, temporarily at least, to see that the Universal version I saw included a Dolby Digital format I had never seen before - 3.1
- and figured aha! this must include that fun rumbling stuff I so vividly remembered. But I wasn't sure so left it at the store & when I got home did some research on it. Looks like the Universal version still
doesn't include that fun rumbly stuff, but I did learn more about the Sensurround system and how much it affected future movie soundtracks.
The following wikipedia page is crammed full of information on the Sensurround system, so I'll just include some of the highlights contained in it.
What it is:
Sensurround is a process developed in the 1970s by Universal Studios to enhance the audio experience during the presentation of theatrical movies. Specifically developed to showcase the 1974 film Earthquake, the process was also used in three subsequent films, Midway (1976), Rollercoaster (1977) and in the theatrical version of Saga of a Star World (1978), the Battlestar Galactica pilot.
Eventually there were improvements made, and Sensurround II and III came into being.
Sensurround involved the installation of large, low frequency, horn-loaded speakers which contained specially designed 18-inch Cerwin-Vega Model 189 E 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.
I most definitely remember these very large & rather ominous horns looming in the darkened corners of the theater. I also remember seeing the "clean" patches where the several rows of seats had been removed to make room for them.
The following is probably why neither dvd I mentioned above contains those extreme low-frequencies*:
The original Sensurround design used for "Earthquake" employed a pseudorandom noise generator, designed by D. Broadus "Don" Keele, Jr., to create the low-frequency rumble, with the waveform matching that of the 1971 Sylmar earthquake. 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 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.
Despite no computer technology being used, analog systems can be useful if their design is clever enough:
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.
FYI: later in the article, it does mention that other soundtrack versions (there were several)included a true surround sound soundtrack.
With such large subwoofers and so many of them, the following was bound to happen!
The excessively loud Sensurround caused additional disruptions for theaters playing Earthquake, including structural damage in some cinemas. At Mann's Chinese Theatre in Hollywood, theater management stretched a safety net over the seating to catch errant pieces of plaster after a test screening revealed the process had actually cracked the ceiling. In Germany, Sensurround movies were only allowed to be played in single screen cinemas.
BTW maybe an experienced Californian can comment on this: one thing that bothered me about the rumbling is that the rumbling sequences would immediately start and stop i.e. there was no "fade in" or "fade out" of the bass effects. I've never experienced an earthquake firsthand, so am not sure if that is what actually happens with the real thing.
* there is more about this issue in the article section labeled "Remastering for DVD"