Introduction: The Psychoacoustic Bucket From Hell
What is this you are probably asking yourself from the title (and the fact that the bucket in question looks just plain normal) ?
Most certainly, you were expecting some kind of mega Halloween hell bound caldron or at least a mean looking trick or treat bucket that produced some kind of light show.
Well, kind of.
This is without doubt, one of the scariest props that you can build, not because it in itself is scary, but because it will induce anxiety, uneasiness, fear, chills down the spine in about 30% of the people who are in the same room as the bucket.
How does it do that?
Inside the lid of this bucket is a DIY infrasound driver that can produce high pressure sound waves up to some 18hz.
Psychoacoustics is the scientific study of sound perception. More specifically, it is the branch of science studying the psychological and physiological responses associated with sound (including speech and music). It can be further categorized as a branch of psychophysics
It turns out that while we can't actually hear infrasound, we can still feel it, and some frequencies cause some odd (and some creepy) reactions to our body.
For example, 4hz very high pressure transducers are banded from sales, because at 4hz, the air in our lungs starts to resonate and you can't breath.
Some people experience nausea at 10hz.
And some, about 30% according to some of the only papers I have found of the subject, feel afraid or "a presence" at around 17Hz.
Between 17hz and 18hz some people's eyes start to resonate, and a slight visual hallucination is produced, like the light you see if you press you eyes with your fingers.
This, combined with the strange unease and your killer Halloween murder ghost story, and you will have some very scared guests.
If you're curios, and want to know more about the science behind this phenomena, download the papers included in this Instructable.
Now, this actually took some 6 months of research to come up with, while the build in itself will take only some 6 hrs.
The first build question is the most obvious: Why not use a subwoofer and amp?
Well.....it turns out that few woofers can go safely down to 15hz, and they are EXPENSIVE. Add to that an amp capable of driving it, and you are looking at close to $500 specially if you need (as we do) high sound pressure levels.
ouch... back to the drawing board.
Next I researched into subsonic whistles (driven by compressed air) but the tube length needed made it both expensive and impractical.
I finally came to an old sub woofer design called the ServoDrive, that used a servomotor coupled to the cones.
This was perfect since the old ServoDrives were close to 80% efficiency due to the servomotor's high efficiency (95%) and the direct mechanical coupling.
And I could cheaply build a simple single frequency version.
Something I learned while doing this project is how inefficient voice coil speakers are. Most commercial woofers are 1% to 2% efficient, meaning that the 300 watt woofer and amp are giving you some 3 to 6 watts of equivalent sound level pressure (over simplified but its close)
I'm feeding my driver some 20watts (at 50% efficiency) so maybe I builded the equivalent of a 500 to 1000 watt woofer and amp.
Now, my design is limited to a single output frequency.
Step 1: What You Need
- One plastic 19L bucket (I bought mine at HomeDepot, so it would be a Halloween orange :D )
- One cheap 12 volt car portable air compressor (that you will destroy)
- Some scrap plywood strips
- JB Weld or Liquid Nails
- 4 wood screws
- 1 wire hanger
- Variable DC power supply
Step 2: Prepare the Lid
Saw a hole in the bucket lid one inch from the inside lip.
Save the left over, since it will be your "cone"
Step 3: Disasemble Air Compresor.
Once you remove all the screws from your pump housing, you will find something very much like what you see in the picture; a motor plus cylinder.
Remove the cylinder, and the piston head, so you are left with the motor, the crank and the piston arm (like in the last pic)
Step 4: Start Building the Spider.
Use the plywood scraps to build the spider that will hold the motor and crank.
The piston arm must be dead center in the lid. Use little plywood blocks to lift the spider 1" to 1.5" from the lid bottom so that your "cone" will move only on the inside of the lid.
Now be careful with the blocks, since they must be parallel to each other. (see pic and next steps)
Step 5: The "cone"
I glued some plywood to the leftover lid to give it rigidity.
The distance between the two parallel sticks is the same as the thickness of the spider legs (the small plywood blocks). This is important because I glued some bits of wire hanger on the sides that inhibit the cone from spinning inside the spider. Please see next step so this will be clearer .
Step 6: Assemble the Driver.
Drill a hole dead center in the cone.
Attach a piece of 4" wire hanger to the piston arm.
Place the cone inside the lid as in the picture. The parallel pieces of plywood on the cone will line up to the spider's wood blocks. Glue the 2" pieces of wire hanger to the cone´s plywood. This will avoid the cone from moving sideways.
Turn the driver upside down and clip the extra wire coming from the piston arm, fold it and glue it.
Drill a small hole in the lid and pass the motor's wires to the other side of the lid.
Place a screw through the lid in each of the spider's blocks to firmly secure them in place (see picture)
Place driver on bucket.
Step 7: Turn It on and Calibrate.
Since room acoustics are really important in sub sonic drivers, I put the bucket, a variable DC power supply and myself into a small bathroom and closed the door and bathroom window.
I hooked up the bucket to the DC power supply and started to experiment with different voltage settings.
Since the DC voltage is more or less proportional to motor speed, the higher the voltage, the higher to output frequency.
At 2 volts, the prototype driver is not completely noiseless, as the motor makes noise and the cone rubbing the sides, but it really moves air!!
At 6.1 volts, I had to turn it off, as I got really nauseated.
At 9.4 volts, Success!! I managed to creep myself out!! I could "feel" someone in there with me, it was a really odd sensation, not pleasant at all and even tough I knew what was going on, the creepy factor was there.
I can´t wait for Halloween next year!!
If you don't want to keep your DC power supply connected to the bucket, you can use a DC power wall supply and the appropriate resistor network to supply the correct voltage, or if your a bit handy in the electronics department, build a regulated DC power supply using a LM317 or similar.