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Nizo Super-8 Cameras that have four digits in the model number are considered by many to be among the quietest Super-8 cameras ever made.
Examples of these quiet Super-8 Nizo cameras include the Nizo 6080, Nizo 6056, Nizo 4080, Nizo 4060, Nizo 4056, Nizo 4036, Nizo 4020, Nizo 3056, Nizo 3048, Nizo 2056, Nizo 2048, Nizo 2036, and perhaps a couple more model numbers I am not aware of. These Quiet Nizo Super-8 cameras are belt driven which is why they were among the quietest Super-8 cameras ever made.
If you do not have access to a quiet Super-8 camera you can create a scripted soundtrack in studio and then have the actors lip sync to the pre-recorded scripted soundtrack dialogue while on location. This can be called a variation of music video lip sync for actors. Newbie actors, or part time actors might enjoy this type of lip sync and treat it as a challenge. This technique does require use of an amplified sound track that must come from a crystal sync based source, such as a CD.
However more experienced actors may not like on location lip syncing to pre-recorded dialogue because it forces the actor to do the scene with the same cadence over multiple takes. The on location lip sync technique can work and it makes it possible to shoot in locations that have loud noise levels in the surrounding area. Even if there is a marching band performing nearby or a nearby noisy neighbor mowing their lawn, it can be possible to film anyways.
Elmo 1012 XLS & Chinon
Not quite as quiet as the Nizos mentioned above, the Elmo 1012XLS and certain Chinon Super-8 cameras can be surprisingly quiet in their own right. Chinon's don't seem to be as reliable as the Elmo or the Nizo's already mentioned.
I recently was shown a Canon 1014-XLS that Spectra Film and Video had serviced and then placed in their rental department. The camera was shockingly quiet, perhaps the quietest Canon 1014-XLS I had ever heard. What Spectra Film and Video did to make their Canon 1014-XLS rental camera so quiet remains a mystery, a super-8 mystery. (cue the appropriate music) but it did involve their on site technician working on the camera to make it happen.
Louder Cameras can still work for dialogue scenes shot outdoors in open spaces.Please note that outdoor scenes can allow one to use a louder camera than when shooting indoors. The more wide open the outdoor space, the less chance for the sound of the camera to bounce around and reflect off of walls. Outdoor sound dissipates quickly so just because a camera is "too loud" to shoot indoor dialogue does not mean that it is too loud to shoot dialogue with outdoors.
Interior locations cause any camera noise to "bounce" around the room and may actually sound louder to the camera than the human ear. Always check out how you plan on recording your location sound before your actual shoot day. Bring your camera and run it, preferably with film in it to see how loud it sounds. Old, outdated film cartridges make perfect testing cartridges for testing the loudness of a super-8 camera.