ScaryGuys Air Cannon Prototype

OK, here's a couple of room ideas and a prototype design for the air cannon. These notes were taken from an halloween-l contest entry from a few years ago. The goal was to produce a floating ghost that you could potentially walk through....

Ghostly Hallway:

Scene:
Guests enter a long hallway. They immediately hear ghostly sounds all around. Then, as they enter into the hallway, a loud ghost scream is heard from behind. A second later, the guests feel and hear the ghosts flying through the hallway. The clincher is a pepper's ghost at the end of the hall where the ghost(s) condense into humaniod forms, then scream/shout/point/interact at the guests.
An extension is where the ghost disappears, then the guests hear and feel them moving back down the hallway.

Layout:

The setup:
Ghost -o-matic, two sets of speakers, a stereo amp, a tape recorder, a tape loop, dimmable light, plexiglass, plywood, screws, paint, etc.
Air cannon: a joint compound bucket w/latex sheet spread over top and sealed (duct taped), a 2 inch hole is cut in the center of the bottom. The latex sheet is connected to the arm of an air ram (from the arm assembly of trash can trauma) with a serious glob of silicone caulking. The air ram is triggered by a momentary foot switch connected to an air solenoid. The footswitch also triggers a cd player and multi-channel haunt timer (see Automating Your Prop). When the timer counts down, it triggers a relay connected to a light that illuminates the pepper's ghost.

Guests enter a long hallway. They immediately hear ghost sounds all around (SP-1: a set of speakers across the hall from one another, placed just above ear level). Then, as they proceed a bit into the hallway, a loud ghost scream is heard from behind (from SP-1). A second later, the air cannon (GM-1) fires a large pulse down the hallway. A second or two after that, the screams move down the hallway following the ghost pulses as if the ghosts are flying through the hallway. Sound movement is done by panning the sound from SP-1 to a second set of speakers (SP-2). The guests feel the wind from the air cannon, and hear the ghosts pass by.
The clincher is a pepper's ghost at the end of the hall where the ghost(s) condense into humaniod forms, turn , then scream/shout/point, etc. The show could be extended to include another air cannon at the end of the hallway. After the ghost(s) turn (in the pepper's ghost illusion), they could enlarge, GM-2 fires, sound pans from SP-2 back to SP-1.

Enhancements:

Add light materials in the air stream to enhance the movement. Spider webbing, cheescloth, shredded cloth, etc. could be used to give visual effect to the air movement.
Add fog to the air cannon and use a blacklight to ehnance the puff.
Add a slide or video projector (Andy Oakland's idea) along the axis to illuminate the ghost.

Gary's Air Cannon Ideas

Here are Gary Ellison's air cannon ideas he devised.... He may be contacted at his e-mail address for more information.

My air cannon prototype

Version 1: is a standard air cannon design with a speaker acting as the air piston. Smoke is injected into the hole before testing.

Overview
This version doesn't produce a large volume of air (due to its size), but it does seem to produce a 'blob' of air instead of a smoke ring! The secret is in how the 'thump' is produced.

My guess
If I trigger the speaker with a quick pulse, the speaker cone goes out, then quickly in. Just the same as whacking a membrane. What this does is push air out the hole, then air is sucked back in immediately as the speaker cone moves back. The only reasonable explanation I can come up with is that on the 'push' stroke, there is a much higher air pressure along the edges of the hole than in the center of the hole due to the restriction. This higher air pressure moves the 'edge' air out much faster than the center air. On the return stroke of the speaker, air is sucked back in through the center of the hole also due to less resistance. This is what forms the smoke ring: a higher velocity/pressure along the edge of the hole being forced out, and a canceling effect in the center. Of course, this is armchair science and assumptions on my part. :)
Anyway, by applying steady current (i.e. DC) for a second or so, the speaker cone only moves out, the air is forced out from the center first (the least resistance), and since there isnt anything forcing the air to go the other way, you get a 'blob' of air.

Pulsing
By repeatedly triggering the unit, I was able to produce a series of smoke blobs following in a line (shown badly on the photo), almost like morse code smoke signals!

Details
I began using an 8" speaker and tube with a 2" hole, but this did'nt produce much of an effect at all. You could just barely feel the puff at 2 feet away from the hole. Next, I scrounged a 10" speaker from an old guitar amplifier and a tube from the center of a cable spool to build a slightly larger unit. On the hole end, I cut a piece of plywood with a 4" hole. I then cut out several pieces of masonite with various size holes. The hole size that gives the strongest puff is 1.5". It produces a puff you can feel up to about 5 feet. I used a 12 volt battery as the power source.
Increasing the size of the unit made a significant difference in the impact and distance, Im currently looking/scrounging up parts for a 15" version.

The Trigger
The speaker is connected to a DC power source (power supply, battery, etc), any source of switched DC voltage will work; a relay, X-10 momentary relay controller, momentary switch, etc.
Im using a 12 volt battery and poking the wire onto the speaker terminal (very unscientific, but very cheap). The speaker produces a puff of air, and unfortunately, an annoying click somewhat loudly. I reduced this significantly by adding a 1mfd 25v capacitor across the speaker leads.

Photos
Here's a few pix of the prototype I built...

The Air Cannon

Ghostly Hallway:

Gary's Air Cannon Ideas

My air cannon prototype