Sound Design Microtorial #2: Explosions With A Crack

A common task of a sound designer is to create a fat sounding explosion. Wikipedia defines explosion as a “rapid increase in volume and release of energy in an extreme manner” (by that definition, gunshots are nothing different and the following applies for those, too).

There are many ingredients that make up a good explosion but today, I just want to talk about the very first part of an explosion: the initial bang. It’s quite hard to make it sound right and unfortunately, I don’t have a magic recipe. But there’s one thing that helps sometimes: giving it a crack.

Imagine you have a generic explosion sound. It should start with an extreme transient where the amplitude goes from 0% to 100% in a very short time. First, make that transient even more extreme by trimming the beginning of it. Cut to a high amplitude sample so that our explosion starts with 100% right away. Now it should sound something like this:

Explosion with trimmed beginning (tap here for iPhone/iPad version)

Now for the crack: chop off the first 5ms portion of the explosion and nudge it to the left by about 30ms (times may vary, use what suites you best). Make sure you chop during a maximum amplitude to get a maximum cracking effect.

Explosion with crack (tap here for iPhone/iPad version)

That’s it! Experiment with that a little and optimize it. It works best on explosions with strong low frequencies.

I came up with this by just fooling around, but I also have a theory about what happens here: by chopping up high amplitudes and creating a little silence in between, we basically simulate low-frequency distortion. Imagine a very strong, very low (like 60Hz) sine wave. Distort that very much and you get something like a 60Hz square wave. Now square waves are nothing more than no change of amplitude (silence) followed by extreme change of amplitude (slope). And one half of a 60Hz period is about 33ms. So we created a super-distorted half period of 60Hz in the example above.

So why does that sound fatter? My guess is this: we have an intuitive knowledge about distorted sounds. Distortion happens in every signal chain: in the air, in amps and speakers and even in our own ear. And usually it happens with strong signals. So that little bit of distortion we created implies a strong signal and makes us believe that the explosion is a big one.

That’s it for this time, hope you could benefit from it!

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