How to create explosion sound effects in audacity

So I find myself having to replace a few sound effects in a game I’m working on due to license considerations. (Fedora wants Creative Commons Attribution Share-alike 3.0, and the samples I was using are Creative Commons Sampling Plus 1.0, which is apparently too restrictive.) Some things, I can record myself, and in fact many of the sound effects I used I recorded myself to begin with, and for those, since I’m the sole copyright holder, it’s easy — just change the license.

But, explosions… not exactly something I can record — instead of “Two Turntables and a Microphone”, I’d have to have Two sticks of Dynamite and a Microphone. Not going to happen. Thought about trying to record firecrackers, trying to make explosion noises with my mouth, trying to hack something together with some loud slapstick bang plus crinkling up plastic… nothing was working.

Then, I remembered I used to make explosion noises on my old ti99/4a computer, ages and ages ago. That thing had a noise generator that could generate several “types” of noise, and a sort of “8-bitty” explosion noise could be made by starting up one of the noise generators, then lowering the volume over time to fade it out. Wasn’t great, but it did more or less sound like an explosion.

So that got me thinking about synthesizing explosion noises from raw data. At first I was thinking I’d have to write a program, to generate random numbers, cram them into a sample array, process them by algorithmic means, then save out a .wav file, and repeat this process, tweaking the algorithms each time, until I got some sort of explosion-y noise out of it.

Then I thought, hey, what if I get some white noise sample, read it into audacity, and start messing with it. First I thought, “no problem, I’ll just record my TV tuned to non existent channel. Well, hmm, my TV mutes when it gets no decent audio signal. Then I thought, hey, I’m being stupid, I can just find some white noise samples on the internet. And so I did, and started messing with that in audacity, albeit with some nagging copyright concerns. (The idea of someone copyrighting a particular sample of white noise is insane, but… still.) Then, I noticed Audacity contains a white noise generator. Tossed out my internet found sample.

So, to get down to it, how to create an explosion sound in audacity from nothing at all.

1) Generate a bit of white noise, a second or two’s worth.

2) Select the whole sample, and apply the “fade out” effect to it.

3) Duplicate this faded-out white noise, and apply the “change speed” effect. Make it slower. Repeat step three a number of times, making faster and slower copies. What’s happening here is the faster copies get raised in pitch, and made shorter, and crammed over on the left hand side (first part) of the sound, and the slower copies get made lower in pitch, and longer, and so the left hand side of the sound gets loaded with high frequencies, and the right hand gets loaded with lower frequences. Make an explosion noise with your mouth, and pay attention to what you do, and you’ll see that it starts out with high frequency noise that kind of fades to lower frequencies.

4) Depending how much “pop” you want the explosion noise to have, whether you want it to sound like a handgun, or like a volcano, you may need to front load the sound with a lot of high frequencies, and watch the length of the tail — for a handgun, it shouldn’t get too long, for a volcano, it probably needs to be several seconds.

5) Periodically select all your sounds, and “Mix and Render”, which will reduce them all down to one sound.

6) You can manually get a kind of low granularity “reverb” by duplicating the sound, shifting it over slightly to the right by inserting a bit of silence, applying negative amplification to get it down in the mix. Repeat this six or eight times, playing around with the amount of negative amplifcation, the delay of silence inserted. You may also want to knock down the attack of some of the echoes by judicious use of the “fade in” effect.

7) Try applying the “phaser” effect, it might do some good.

Try using EQ, boosting the bass, boosting some of the higher frequencies 3k and above, dropping some of the mids. Try the “bass boost” function of audacity. (Nice thing, you can “undo” if you don’t like how it works out.

You kind of have to play around a bit to figure out how to get a decent sound, and I say “decent”, but it won’t be as good as a real, well-recorded explosion. But, for most people, me included, recording a real explosion is simply not an option.

Update, Apr 8: You can hear some examples here, at freesound.

Update Apr 13:
I made a video tutorial…

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~ by scaryreasoner on April 6, 2008.