Help me understand convolution reverbs and impulse responses

[albertcroxton]

I understand what they do and I use them regularly. I don't quite get how they do what they do, or how I would go about creating my own IRs.

In my collection of impulse responses, I notice that many start with a clap type of sound, but many start with a sine sweep.

How I thought it works from google searching is the plugin has a source audio (clap, sweep, etc) and also has that source recorded in a reverberant space. It subtracts the original source from the version recorded in a space, and uses the result to determine what happens to a sound in that space so it can recreate it with other sounds.

I suspect I am wrong in my understanding. Mainly because many of my impulse responses have different source sounds, they all include the reverb, and I don't have to load a dry version into the plugin for it to work.

So how does the plugin separate the original signal from the reverb? And if I used a long (3+ seconds) sine sweep in a room with a very short reverb, how would the plugin handle it? The reverb of the first frequencies to occur in the sweep would be done before the sweep is done.

I want to make my own impulse responses, but do I just have to play a clap or sweep and record it in a room? Or is there more to it? Yes, I have googled it and don't really understand the results I have gotten, so I hoped maybe some of you expert professionals could dumb it down for me.

Thanks!

[LostRobotMusic]

An "impulse" refers to only a single frame of audio, with everything surrounding it being silence. If you send a single impulse through a convolution effect, the resulting audio will be 100% identical to the impulse response. If you send other audio through it, you can think of this as sending one impulse through per frame at various volumes, which results in the impulse response being played back several times at those same volumes. The phase cancellation resulting from all of this results in filtering, and if the impulse response is long enough, you'll get reverberation as well.

Here's an audio file of an impulse so you can test this yourself. Load any audio you want as the impulse response, and if you play this impulse through it, the playback will be identical to the impulse response (assuming the plugin isn't doing anything extra to it). Inversely, if you load this impulse as the impulse response, playing audio through the effect will have no impact whatsoever and just pass the dry signal.
https://drive.google.com/file/d/1OXLOMx ... sp=sharing

When recording an impulse response, you want something as close to an impulse as you can get (since you're recording the room's response to an impulse). A clap is commonly used because the sound is extremely short and has a sharp transient, and is therefore relatively close to an impulse.

A sine sweep is generally for measuring the frequency response of things, not the impulse response (though it may be possible to derive one from the other?). It's possible some plugins out there are able to work with that sort of information, but I'm not personally familiar with any.