How To Make (And Record) Explosive Sounds Using Common Household Chemicals

By Peter Drescher
September 17, 2012 | Comments: 0

Sound designers love to blow shit up, for the same reason DJs love big thumping bass beats. When the frequency of a sound is low enough and loud enough, it ceases to be heard with your ears, and becomes something you feel in your gut. It moves the molecules of your body the same way a speaker moves molecules of air, transmitting energy waves through the medium of your blood and bones. Literally, big booms are exciting!

This is why audio guys love doing recording sessions of machine guns, hand grenades, and cannon fire. When sound designers post photos of these sessions to their websites and Facebook pages, they always have big goofy grins on their faces ... and not just because explosions are fun to (safely) experience up close and personal. It's also because they have to utilize a wide array of microphones and recording gear, usually in some remote mountain location, to accurately capture the crack-thud-wham of the explosive shock wave. It's an audio guy's dream scenario.

Once you have multi-track recordings from 25 different microphones at various angles and distances from source, you then have to edit, mix, and implement the sounds so they can be used in your game. Console audio developers spend an enormous amount of time and energy layering sounds, adding interactive effects, and testing on enormous 5.1 Genelec surround speakers in floating floor, acoustically perfect, soundproof studios. They also pay particular attention to that .1 subwoofer -- because that's what produces the sounds you can't hear, but only feel.

But what if you're working in mobile game audio? How do you get that butt-wrenching thump out of an iPhone speaker?

The answer, of course, is: you can't. Phones barely have enough power to move air molecules, let alone body fluids. Certainly, there's no way a ten millimeter speaker can produce a sound wave 20 feet long; it's a physical impossibility. I usually describe the situation as a "fastest gun in the west" joke (here's the sound of a big boom played on a cell phone speaker ... wanna hear it again? :)

BUT you can hear the high-frequency crack of the initial explosion, the sizzle of burning black powder, even the mid-range whoosh of expanding gasses. I wanted to find out just how many different kinds of explosive sounds you could coax out of an iPhone, and so wrote a "target practice" game containing a wide variety of things that go bang!

Download the game for free from the App Store
Bang Bang

I started simple: balloons. The party store stocked balloons in different sizes, from five to twelve inches, and I tried popping a few in my spare bedroom with a Sennheiser MKH416 and a Neumann KM184 set up in the corners. The sound rebounded off the walls of the box-shaped room too much to make useful recordings, so I cleared out the garage and tried again. Better, but still a fairly common, not very interesting, sound.

So then I tried making nitrogen tri-iodide, an unstable, low-powered, explosive compound. I'd read about the substance years ago in "The Curve of Binding Energy" by John McPhee, a scientific biography of physicist Ted Taylor, designer of the Orion nuclear powered spacecraft, and the Super Oralloy Bomb (the highest yield fission device ever exploded). As a precocious child, Taylor would make the compound by combining tincture of iodine and ammonia, which produces a dark sandy precipitate. Filtered out and left to dry, the chemical is completely stable in regards to heat ... but not to vibration. Tickle it with a feather, and it explodes with a sharp crack, releasing purple clouds of (poisonous) iodine gas.

I'd always wanted to try that science experiment, but discovered that the tincture of iodine antispetic you get from the drug store has too little elemental iodine in it to be useful. It also turns out that iodine crystals have recently been classified a schedule II controlled substance, because apparently, it can be used as a precursor ingrediant for cooking crystal meth! Fortunately, very small amounts can still be purchased on EBay, so I didn't have to try to explain to the DEA that I wasn't making drugs, I was making explosives! :)

nitrogenTriIodide.jpg Drying the percipitate took longer than expected, because the entire mound of black sand in the coffee filter has to be completely free of ammonia to get a good reaction. A radiant space heater helped speed up the drying time, particularly in the cold wet climate of Redmond, Washington. I also placed the experiment platform on a cement block resting on a foam pillow, to reduce stray vibrations.

Even so, sometimes the compound would spontaneously combust during drying, so I pointed the shotgun mic at the platform, and set my digital recorder to Pre-Record mode. This continually fills a 30 second audio buffer, so if I was in the house and heard a sudden bang from the garage, I could run in and capture the sound. Usually, however, I'd wait a couple of hours, then carefully tap the mound of nitrogen tri-iodide with a toothbrush attached to the end of a ten-foot pole -- BAM! bits of compound flying everywhere, plus a puff of purple smoke ...

Although it produced a good sound, it was also a fairly messy procedure. Debris from the explosions crackled underfoot, and stained the floor with purple dots. Most of it came out when washed with ammonia, and the rest sublimated to nothing after a few weeks anyway. One interesting and unforeseen side effect of producing quantities of iodine gas -- seems like all the spiders in the rafters of my garage have died ...

Tulalip Go Bang Bang

In mid-June, I drove up to the Tulalip Indian Reservation to get something with a little more kick to it. Fireworks are illegal in Washington state, but apparently, indigenous people can sell whatever they want to the white man, including high stakes gambling and high powered explosives. Behind the casino, there was a gravelled acre of temporary plywood shacks, each filled with an amazing array of bangers, spinners, rockets, shells, and firecrackers of all sorts. I explained to the dealer that I wanted "things that go bang, but nothing that flies, shoots fire, or might burn down my house!" He was very helpful in picking out what I needed, and along with an assortment of whizzers and sparklers, I got a bunch of firecrackers I called the little red ones, the medium ones, and the really big ones.

The little red ones were pencil-thick, and about 2" long. They produced a sharp crack, but not much power or heat. I figured each one contained about as much gunpowder as a bullet (in fact, during test recordings, my tolerant neighbor thought I might be firing a weapon in my garage). I did get some good sounds by blowing up a few in a large metal pot, which produced a nice ringing clang. Even better was dropping one into the cavity of a cement block and covering it with a 2x4. The woody clonk produced is one of my favorite sounds in the game.

The medium ones were too powerful for the garage, and too loud even for my understanding neighbors ... and the big ones, well, let's just say, they packed a frighteningly potent punch. So I loaded my Sennheiser, Tascam HDP2, fresh batteries, mic cable, and duct tape, into a messenger bag, and went off for a nice long hike in the woods. In a secret location, far from the works of man, on a forested hillside, overlooking a valley, I duct-taped the mic high up on a tree trunk, turned on the recorder (no limiter or filters), and moved away about 50 yards.

Bam! Boom! Pow! Even that distance wasn't quite far enough for the big ones, and I picked up some ground thump, and clipped the initial shock wave to a degree. But wow, you literally cannot buy reverb like that! The long smooth tail of the explosion echoing off the hills provided excellent source material for the game. In fact, I like those recordings so much, I am providing the full resolution (24-bit, 48kHz) files for download, free of charge. Check them out, the echo really is a thing of beauty ...

Fourth Of July, 2012

Of course, the one time you're allowed to shoot off firecrackers in Redmond is on Fourth of July; in fact, it's practically mandatory! I have a fairly large backyard, and had cleared out an overgrown-with-weeds section on the side of the house for use as a bombing range. I set up the mics on the opposite side of the yard, and the recording gear on a table nearby. An extra long extension cord provided power, and a Mackie 1202 mixer with outboard RNC Compressor from FMR Audio provided control of the sound going into the Tascam. I set the compressor's Attack and Release to their fastest settings, the Ratio to max, and adjusted the Threshold to limit the initial transient and preventing clipping ... and then started blowing shit up!

But not just fireworks. The day before, I'd gone down to the local Fred Meyer and picked up a block of dry ice, and some six-packs of water and soda ... not to drink, but to make dry ice bombs. Frozen carbon dioxide sublimates to gas under one atmosphere of pressure, so when you drop it in warm water, it bubbles up copious amounts of cold white mist, an effect familiar to haunted house enthusiasts. The volume ratio of dry ice to carbon dioxide gas is ~850 to 1, which means you can stuff dry ice into a soda bottle, and it quickly expands to the breaking point of the plastic.

This produces an astonishingly loud bang; the bigger the bottle, the louder the noise. Liter bottles designed to hold carbonated beverages under pressure, require a lot of dry ice to burst, which has to be chopped up and stuffed through the one inch spout. Small 12 oz bottles of Diet Coke blow up reeeal nice with only a little dry ice, as do the larger, more standard shaped 20 oz bottles. Thin Arrowhead water containers work well too. Add dry ice, some water, cap it, toss it, and wait (video of dry ice bomb explosion).

Of course, you have to be sure to use enough dry ice to get an explosion; otherwise, you end up with a highly pressurized, fairly dangerous, UXO. I probably should have gotten a pellet gun to take care of duds, but instead improvised by leaning a magnesium sparkler against the side of the distended bottle, and running. When the flare burned down to, and through, the plastic, the gas was released in a sometimes comical phssssssssst-ploof!!!

During the day, I also blew up little, medium, and big firecrackers, along with some other whizzers and snakes I'd gotten in Tulalip. But when the sun went down, as expected, the entire neighborhood erupted in a cacaphony of bangs, booms, and screamers. I turned up the input to the mics, pointed them at the sky, and recorded about an hour of "war zone in Redmond". This serendipitously captured a number of surprisingly useful booms, like rippling thunder, reflected by the hills and houses, creating interesting reverb effects. Some of those samples are included in the game as "explosive sweeteners".

Hydrogen Balloons

The game features four firing modes: Cannon, Machine Gun, Mortar, and Lazer. The "space cannon" sound is a classic Slinky zap, the machine gun plays a series of little red ones mixed with other sounds, and the lazer effect is derived from some magnetic balls I picked up at Project BBQ years ago. For the mortar launch sound, I wanted a tubular BOOF! that would fire a spark-spitting projectile.

At Home Depot, I got a large carboard tube, a 2" diameter steel pipe and cap, a PVC fitting with nozzle, and some toilet bowl cleaner containing 20% solution of hydrogen chloride. Combining aluminum with hydrochloric acid creates an exothermic reaction (meaning, it gets really hot!), and hydrogen gas is produced, along with other toxic chemicals with a distinct aroma that reminded me of Newark, New Jersey.

Attaching a balloon to the nozzle captures the resulting gas. I knew the reaction had worked because when the balloon stopped expanding, I tied it off and let go ... and sure enough, it rose quickly into the rafters! I had to get it down using the ten-foot pole with some duct tape wrapped around the now-purple-stained toothbrush. I made a number of balloons, discovering that if I washed out the chamber with water each time, I'd get a better reaction, and more hydrogen.

I brought the tube out into the back yard, tied a hydrogen-filled balloon to a steel washer, dropped it down the tube, and tossed in some flaming paper ... KA-BOOM!!! Holy crap, that was louder and hotter than I'd expected!! Although the balloon wasn't very big, hydrogen is so light, it compresses very easily, and there was a lot more flammable gas in there than I'd realized (video of hydrogen balloon explosion).

Surprisingly, uncompressed hydrogen hardly made any sound at all. I tied up a cardboard mailing tube vertically, removed the bottom cap, filled it with hydrogen, and struck a match underneath. Result: a barely audible pop. Lighting from the top didn't do much either. Another thing I noticed was that the hydrogen didn't stay in the balloons very long. After a few hours, they'd fall to the ground; the light gas escaped through the rubber quickly (or perhaps the impurities weakened the balloon).

Miscellaneous Sounds

Additional sounds used in the game include metal pipe clanks for the aiming mechanism, chain rattles for the machine gun, large steel washers dropped on concrete for shell casings, 8x10 pieces of picture frame glass being smashed, some synthesized horn blatts, and more. All of the sounds are original recordings, because the game isn't just a game, it also serves as an interactive sound design demo in the form of a game.

In the next article, I will describe how all these myriad sounds were edited and implemented using FMOD for iOS. Stay tuned ...

   - pdx

<shameless plug> Note: if you'd like to include this kind of interactive audio soundtrack in your mobile app, I am available for contracts and job offers at Twittering Machine </shameless plug>

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