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In video games there are audio and sounds playing when e.g a gun is fired, or when a sword is smashing something. Sometimes, when two identical sounds, (e.g the same sword-sound) are played at the same time, we can hear some sort of 'artifact', like a swooshing sound. What is this called, and why does it occur?

Edit: This might be a question for a science forum, but I figured some sound designers/enthusiast would have been taught/come over something about this.

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It's comb filtering. This happens because the two sounds are played not exactly at the same time, but with a tiny delay between, yet unlike in the real world the sound events are exact copies, not just very similar. As a result, significant portions of the sound spectrum will cancel in both copies, while other frequencies always add up; this uneven (but static) frequency response makes this weird sound sensation.

The artifact can be avoided by using not always the exact same sample but some random choice of basically the same sound, perhaps recorded with slightly different microphone position. This "shuffles" the phase relations, so frequency cancelling doesn't happen at consistent points but only randomly for a short time, like in the real world.

  • Thank you very much! That was the 'artifact' I was looking for. However, it feels wrong to call what I experienced a filter. When researching comb filtering, I gather that it's a common filter to be used for effects and songs. Do you happen to know what an unintended comb filtering could be called? (I'm trying to find an example on youtube from a video game or something like that (not with a microphone close to a speaker with a plate slightly further away on the other side)) – Sti Aug 18 '14 at 15:07
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    It is still called comb filtering. Actually I think it's mostly encountered unintended, being one of the major issues in room acoustics. The reason you don't find any video game examples on YouTube is that gamers are unlikely to know that this is called comb filtering. If you want to demonstrate the effect to someone, you can easily do that in any DAW (e.g. Audacity): simply drop the same sound on two tracks and move one of them a few milliseconds to the right in the time line. – leftaroundabout Aug 18 '14 at 20:06

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