The Ghostly Hallway

Several times over the past years, we have tossed around ideas for ghostly passages, say like the green slime guy in Ghost Busters, where the guest sees an apparition at the end of a hallway, it starts towards him, then vanishes. He still hears it coming, as the ghost sound effects pass by him in the hallway. To add to the effect, we've suggested a vortex cannon that fires a pulse of air, timed to coincide with the perceived sound passage, so the guest feels the "ghost" pass THROUGH him, as it vanishes behind him.

Basic hallway concept

The appearing-and-disapearing ghost seems like a good place to use a scrim, at the far end of the hall, with the model ghost (FCG on a straight track?) moving behind the false end of the hall. The vortex cannon has been pretty well refined, using either a mechanical or solenoid ram to pulse the drumhead, or a big speaker, with a shaped pulse driven to it. The sound effect would be easy in principle, but sweeping it down a long hallway would really require at least 3 or 4 channels of audio, preferably more - and those in stereo! 8 or 10 tracks, sent to 4 or 5 speaker pairs is a lot of gear, particularly in the multi-track audio system...

Hallway, with sound

I was trying to think of a better way of handling the sound system. We were talking it out a bit, and in describing the effect, I likened it to having a number of separate amplifiers, all fed the same effect sound signal, and the volume controls on each of them being swept up and down in a "wave", down the hallway. Sounds good, but it NEEDS to be automated!

Here's where the electronics gurus on the List can put their heads together, and maybe we can come up with a system that could be built fairly cheaply, and make a modular, extensible, sound sweeper for any kind of effect like this one.

A few ideas I had, to start the juices flowing:

Sound Sweeper block diagram
High-level Block Diagram

Gain-Envelope Stage
Gain-Envelope Stage

Envelope Generator
Envelope Generator

Inputs, counter suggestions, jeers, brickbats, all welcome!
Dave -