The Explodophone is an organ-like instrument that uses tuned impulsive sounds such as snaps, cracks, pops, or bangs, preferably from small explosions. Version 0.1 was made with a cap gun and a few "Pringles" potato chip cans. Version 1.0 utilized an improved cap gun with 16 stopped pipes (plastic and cardboard). Version 2.0 is intended to use explosions from an electrically ignited fuel/oxidizer mixture . Version 3.0 will incorporate a MIDI controller.
This idea was inspired by a Muppets skit with Jean Stapleton and "Crazy Harry".
In signal processing, the delta impulse (a.k.a Dirac delta or delta "function" characterizes a system's entire frequency response simultaneously (the "impulse response"). A snap, crack, pop, or bang approximates an impulse, exciting nearly all vibration modes of the resonator at once. Perhaps this gives a holistic sense of the acoustics of the resonator. The Explodophone would therefore provide some of the most frequency- and information-rich timbres of any instrument.
Here is a video of the first performance of the Explodophone 1.0.
When last we left our story, I was puzzled by bird cannons, a.k.a. bird scarers or bird scaring cannons. I was clueless as to the basic function, and I was having a lot of trouble finding info on the web. I had basically settled on using some kind of popper valve, resembling a sort of one-shot two-cycle engine or perhaps a one-shot pulse jet. Then, at last, I found this Instructable:
Perhaps a little underwhelming to some, but I was grateful for this game-changer. This Instructable was similar to another that was a simple tennis ball mortar. The photo clearly shows several tin (steel) food cans taped together. This is misleading; the device is actually-- well, no, it's several tin cans taped together. But it is NOT a simple pipe. Improving considerably on the illustration in the Instructable, I drew a diagram with a little more detail:
Why does the photograph show four cans and my illustration five? I have no idea. The description in the text makes it clear that five is the minimum number of cans. That bit about the "steepening" will be explained below.
Propane-air mixture is inserted into the combustion chamber through the hole on the right, then ignited soon after. The assemblage of cans then makes a sound like a shotgun blast. According to the Instructable, one can make it even louder by adding additional "baffles", which I have labeled "Baffle/ Iris/ diaphragm/ aperture". lasermaster3531 states:
"In my design, I alternate fully removing the bottom of one can and leaving half of the bottom on the next one, until I get to the muzzle end which has three can-lengths unobstructed. The can bottoms serve an important role, don't just tape cans together and think it will work if it's essentially a simple pipe. The "baffles" (can bottoms) cause the speed of the flame to increase every time it has to pass through a baffle, which greatly improves the loudness and pitch of the noise made. A simple tube closed at one end will usually just make a "whoosh" or "thump" sound, but a properly fueled and constructed noisemaker sounds like a shotgun blast."
This was precisely the information I was hoping for. But what was so critical about baffles for making noise? Drawing on my limited knowledge of fluid dynamics, it seemed like the baffle acted as a nozzle, accelerating flowing gases while reducing the pressure. Did this create a shock wave? I couldn't wrap my brain around it.
Then I found this paper, "A Gasdynamic-Acoustic Model of a Bird Scare Gun", by S.W. Rienstra, which helped immensely. (Warning-- link downloads a .pdf file.) While the Instructable gave an immediately practical way to get a SCPOB, the paper explained the theory. It did not mesh with the baffle/ nozzle idea. Apparently the paper was part of a series that attempted to mathematically model a number of industrial devices.
The paper is a tad frustrating in that bird cannons are hard to model. There are many approximations and assumptions, and the questions laid out in the introduction are not able to be adequately answered. Nevertheless, the paper was invaluable to me. Here is the excellent overview of how a bird cannon works:
"The mechanism is simple. A carefully-controlled mixture of air and propane or butane gas (stoichiometric1 mixture, or a little bit richer than that) is periodically (every 5 or 10 minutes) blown into a semi-open pot, which is the combustion chamber. This pot is connected via a small diaphragm or iris (a small hole in the wall of the combustion pot) to an exhaust pipe. After ignition, the gas burns quickly (but without detonation, i.e. with a subsonically moving flame front) so that pressure and temperature increase quickly. This high pressure drives the gas out of the pot via a hot jet, which issues from the diaphragm...
sudden material failure (dry stick, chalk in torsion, rocks cracking, etc. )
shipping cushions/ bubble wrap
Sparks (brief arcs)8
prilled black powder/cordite
caps, paper and plastic
Coloumbic explosion (electrostatic charge)12
air detonation by laser heating
cavitation by ultrasound/sudden heating (inkjet print head)13
There are notes below on entries that might confound.
Meditating upon this list and doing a little research, I came upon two basic insights: 1. A SCPOB is mostly caused by a relatively large volume of air forced to move quickly; and 2. Most explosives (including fuel/oxidizer mixes) need to be constrained in some way to make a SCPOB.
Remarkably, a finger snap or hand clap can sound surprisingly like a firecracker or rifle shot. Although I do not profess a thorough understanding of the physics of such things, my guess is that the sudden collision of skin to skin forces a volume of air out of the way very quickly, creating a brief, relatively high amplitude wave train. Is this a shock wave? I guess.
For the Explodophone 2.0, reliability and reproducibility are paramount. Although a mechanical "cricket clicker"3 would do the trick, it is a little dull. A cap gun provided a nice reproducible explosion for Explodophone 1.0, but it would be nice to up the ante, or at very least use something that was cheaper in bulk. I thought first of electric sparks8, which can make sharp and even loud cracking sounds. Electricity is relatively cheap and electric sparks are cool. With this in mind, I purchased 16 Ford Econoline ignition coil packs
and 16 spark plugs.
Ignition coil packs are designed to plug directly into spark plugs and to give a 7-15 kV spark from a 12V source. Sadly, these made no more than a quiet click, and the spark did not seem reliable or robust. I will have to deal with this eventually. Simple experiments with capacitors don't seem to help; I may need to get actual condensers or follow modern car design using microcontroller timers and IGBJT transistors to switch them on. At any rate, I knew that, failing a loud enough spark on their own, that the spark plugs could be used to ignite a fuel or explosive. Although some improvement such as a Marx generator or a free-air "flashbulb" could lead to a loud explosive spark, I decided to move on and use the sparks for electrical ignition of a fuel or explosive.
As near as I can figure, an "explosive" is basically a fuel that has an oxidizer combined with it. The most violent ones have the oxidizer on the same molecule as the fuel, such as nitroglycerin; these can rearrange into a huge volume of product...
So I decided to use the scale A1-C4 with only the "white piano keys" to start. This would make simple tunes possible without getting prohibitively expensive. The problem with that is the note number (e.g., -9 for C4) is not a simple progression. I had to keep the piano keyboard in my head the whole time I was making up the list. You can see the spreadsheet I used here. (Also included in Excel format in Files.)
In good ol' Physics 101, you may have learned how to calculate the resonant frequency of a pipe. Basically, the resonant frequency is the lowest note that the air in the pipe will naturally vibrate at. This is related to the wavelength of the frequency, which is dictated by the speed of sound in air. A pipe open at both ends will resonate with half a wavelength; a pipe stopped at one end, with one-quarter wavelength. The speed of sound in air is about 343 meters/second.
I decided to work with stopped pipes, because I could get low notes with shorter pipes(-> less material -> lower cost). Open pipes seemed to have a somewhat "sweeter" sound, but the sound was also more raw and less defined, because the resonance is not as strong-- the sound bounces between air masses at both ends as opposed to an actual solid barrier at one end. So I felt the quarter pipes also made the note more obvious.
Thus, to find the length of a pipe to produce a specific frequency, I used the equation:
where l is pipe length in meters, 343 is the speed of sound in meters/second, and f is the frequency in Hz (1/seconds). Again, see the spreadsheet.
The calculated diameters was based on traditional organ pipe diameters, which is extra confusing, but the math really isn't any worse than the rest of it. I ignored the diameters entirely in version 1.0. The "first and last" jazz was a crude attempt to find a way to avoid wasting material by pairing up short and long tubes. It really didn't help.
I had been collecting cardboard tubes of many sizes for some time. I used cardboard tubes of sufficient diameter to jam the cap gun into the mouth, about 2-1/2" to 3" in diameter. I didn't have enough cardboard tubes, so I also bought some ten-foot lengths of 3-inch diameter polyethylene drainage pipe with some end caps to match. To cut them to length, I marked the length to the nearest millimeter with a tape measure, then used a hand mitre saw and mitre box to try to get a nice perpendicular cut. The saw and box were cheap, and the box was not deep enough, so it didn't work too well. Moreover, those paper tubes are a lot harder to cut with a hand saw than you might think. The end result was not-terribly-accurate tube lengths and a surprising amount of damage to the mitre box. I labeled the tubes with their respective notes.
To stop the tubes, I used squares of double-thick cardboard hot-glued to one end. Some of the plastic pipe I just capped with plastic caps designed for the purpose, but I didn't have enough, so some plastic tubes were stopped with cardboard and hot glue as well.
To test them out, I would snap my fingers in the mouth of the tube, about 3-4 inches in. The notes were clear. I also got excellent sound just by striking the stopped end with a finger, tempting me to just make a set of tuned drums. But then there would be no explosions. True to the nature of sound, the longest pipes that sounded the lowest notes sounded the most out of tune, as the ear is very sensitive to small changes at the low end. The others sounded pretty good.
I practiced a simplified version of Beethoven's "Ode to Joy" (from his Ninth Symphony), considering it the most noble of melodies. I then performed it for extended family at a gathering. The tubes did a poor job of standing upright, so I decided to play them horizontally. I physically arranged most of the tubes on a large round patio table, largest on the left to smallest on the right, then...
Now for the other half. For maximum starting flexibility without taking the cost too high, I opted to go two octaves, using notes found on the white piano keys only, i.e., notes with only letter designations and no sharp or flat symbols. This means I could play any song that fits within two octaves and is in the key of C, or can be transposed to the key of C, and has no "accidentals" (sharps/flats/naturals not defined by the key signature).
Human hearing is logarithmic, meaning what we perceive as a linear increase in pitch is really a geometric increase in frequency. So when we go "do re mi fa so la ti do", that second "do" sounds like the first, only one "level" higher ( kind of like climbing up seven stairs to the next floor). BUT in reality, the frequency has DOUBLED. Another "floor" up, and you're really FOUR TIMES as high. This allows for a sort of signal compression-- on the low end, we are very sensitive to changes in frequency, but on the high end, we sacrifice accuracy for dynamic range. Here's a graph.
Here's a blog that I grabbed the picture from. I haven't read it but it looks nice .
Following is what I learned about music.
Now the numbering on this graph is not very helpful. There's a better way. We currently use the" even-tempered scale" where there are 12 half-steps in each octave, which are evenly spaced in an exponential sense. There are other tunings, such as "just intonation" or the "well-tempered scale", but they're not used very much, despite having a few passionate boosters. We humans love to hear frequency ratios like 1:2, 1:3, 2:3, 4:5, or 4:5:6. The even-tempered scale is a compromise, approximating these sweet-sounding ratios while making instruments easy to tune and able to play in several keys. For example, instead of 5:4 ratio (1.2, a "perfect major fifth"), the even-tempered scale gives us the cube root of two, about 1.26.
The even-tempered scale is easy to model mathematically. An octave is a frequency doubling, and there are twelve steps in it. All we need now is some note to be defined in terms of a frequency. The people who decide such things picked "concert A" to be 440 Hz. (Hz is hertz, or cycles per second.) There is a passionate and genuinely crazy faction out there that insists that this definition is a Nazi plot to destroy the world, and that 432 Hertz is the "right" concert A. Personally, I think middle C should be defined as 256 Hz, which is 2^8, which would make repeated halving or doubling much easier.
Thus, to figure out the frequency of a note, I used
( f=440*2^(n/12) ), where f is frequency in Hz and n is the number of half steps from concert A. Concert A itself is n=0, notes lower than concert A are negative, and notes above it are positive, and it all works. For example, middle C is nine half-steps below concert A, so n=-9. This gives a frequency of
, which is about 261.63 Hz.
So... about all those letters. I'm not touching staff notation here, but each note has a name, which is a letter from A to G and then some modifiers. Each octave has eight letters in it. Now right away you know something's up, because if each letter represents a full step, and there are 12 half-steps in an octave, then there should be six letters in an octave. Well, I don't know all the reasons, but the interval between the letters is not always a full step. Cheating is involved. When you sing "Do Re Mi Fa So La Ti Do", you are singing C D E F G A B C (with the second C an octave higher). It seems like a straightforward linear progression, but in reality, there is only a half-step between E and F, and also between B and C. Where the half-steps are placed in a scale constitutes the "mode". The ancient Greeks used several modes in their music, but modern Western music settled almost exclusively on the "Ionian" mode. (More about modes here.)
This explains why there are "missing" black keys on a piano. Playing only the...
After my Fourth of July "concert" for my siblings (see previous log), my youngest brother suggested I get a REAL cap gun that wouldn't misfire so much. When the cap gun didn't fire, instead of a lovely note, I got an awkward silence and an interruption of the song, along with a derpy repeated pulling of the trigger until I did get a bang.
Most cap guns are semi-automatic, but he suggested one that took plastic strip caps, which could be hooked together into a continuous string. Although the caps are not too hard to find, the guns can get relatively expensive for cap guns. An example of the kind of cap gun I obtained can be seen here. (I am not recommending or promoting any particular sales site-- you might find it cheaper on another site.)
Apparently it's a Parris brand replica of a 1911 pistol cast in avocado-green plastic. There really aren't many other guns that fit my specifications.
I obtained one, but experimentation (drilling holes to get more sound out) and my kids sealed its doom. I resurrected the idea a few years later and got another pistol. To get slightly more sound out of it, I cut off some of the barrel, which is pretty much for show anyway. I would like to take it to pieces and get a better idea of how it works, but it has no damn screws-- plastic is completely welded. As I am on a shoestring budget, I opted to keep it together. The caps I obtained from a local ranching store (my area is a rapidly developing semi-rural area), which really likes guns of all types. Walmart* used to carry the caps, but apparently stopped a few years back when they were outlawed in certain cities. The caps aren't cheap-- it costs roughly 1.5 cents for one note.
The strip caps look like this.
For my first performance (video in a later log), at the last minute, I decided to use a drop of hot glue at each junction to hold the strip caps together. There were still misfires (nonfires), but I don't know whether to blame that on the glue, mechanical misfeeding, or on the differences between the standard gap between caps and the gap between strips. At any rate, the glue did the gun no favors.
In the beginning, God created the heaven and the earth. Sometime later, as a kid, I was watching The Muppet Show when I witnessed a skit with actress Jean Stapleton (of Archie Bunker fame) and the Muppet named Crazy Harry. In the skit, Harry claims to be playing an "instrument" called the Explodophone. Ms. Stapleton sings "I'm Just Wild About Harry" and dances about the Grecco-industrial set while Harry pushes plungers on dynamos, setting off pyrotechnic charges at specific times with the music.
Part of the joke was the doubtful legitimacy of the "Explodophone", which seemed an excuse to blow stuff up, Harry's favorite pastime. But the Explodophone was actually used as a percussion instrument to punctuate pauses in the music. The charges were also choreographed to create visual interest, framed in each shot that followed Ms. Stapleton's dance.
I also recall the wonderful description of the fictional band Disaster Area in The Hitchhiker's Guide To The Galaxy, which was best heard "from within large concrete bunkers some thirty-seven miles away from the stage, whilst the musicians themselves played their instruments by remote control from within a heavily insulated spaceship which stayed in orbit around the planet - or more frequently around a completely different planet."
Years later, while watching an experimental music exhibition at the University of Utah, I was discussing a performance with a friend. Eight performers knocked pieces of lacquered wood together in rhythm. I struggled to enjoy the piece. My friend said he found it boring, because he was bored by percussive instruments that had no variation in pitch.
While an undergrad at Brigham Young University, I enjoyed listening to lunchtime concerts at the Carillon Bell Tower. This fermented with these other experiences to lead me to brainstorm about other massive instruments. The primary idea to come out of this was the Nuclear Steam Calliope, a steam whistle powered by a uranium reactor. To save money, instead of organ-like pipes, it would use a single vertical concrete tube that had holes at intervals like a large tin whistle. The holes could be covered in different combinations for different pitches.
Yet later, I was watching a video (sorry, can't find it) by Bay Area performance artists Survival Research Laboratories that blew stuff up with a device called The Shockwave Cannon. This used focused shockwaves from propane explosions to blow apart dolls, windows, a model house, and various other things, and it was remotely controlled by people on the Web. It came to me that tuned explosions could strike recognizable notes, turning the "boring" Explodophone into a tonal percussive instrument that would be interesting.
Scaling it back umpteen times, I hit upon using toy caps for controlled explosions and empty Pringle's potato chip cans for tuning. I cut the cans to a few different lengths, creating the Explodophone 0.1. I then inflicted an impromptu concert on my siblings for the Fourth of July one year. I arranged the cans upright on a table, then fired the cap gun into the mouth of the appropriately-sized tube for each note. The tune was something very simple, like "Mary Had A Little Lamb" or something, and there were many misfires. Afterwards, I sought feedback from a stunned audience. One remarked that they didn't expect an actual tune, but there it was. That was all the encouragement I needed. I had created a tonal percussive instrument using explosions. It was no longer "boring".
UPDATE: Since publishing this log, I have become aware of few related ideas out there. I failed to acknowledge the inspiration of SRL's hovercraft, which was propelled by a pair of twin pulsejets that are slightly detuned to create a very loud, musical throb when fired together. I also recently learned of the pyrophone, a concept similar to the Explodophone. Also, I stumbled across an issue...