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.