[sdiy] IR Reverb

rsdio at audiobanshee.com rsdio at audiobanshee.com
Thu Feb 15 09:52:29 CET 2018

On Feb 14, 2018, at 11:42 PM, Tim Ressel <timr at circuitabbey.com> wrote:
> (note: laughter is understandable and probably mandatory)
> So I just did a cannonball into the murky waters of impulse response reverb. My tenuous grasp of DSP coupled with my sketchy math skills is making this, well, interesting. So far I have acquainted myself with linear convolution, which looks suspiciously like an FIR. Since it says "impulse response" on the box, that seemed to make sense. But then I stepped back and tried to imagine a reverb system as I understand it and got confused.
> Reverbs have two things going on: time delay and filtering. The time component gives the reverb time and overall thickness of the reverb, while the filtering can make the effect warmer or colder (yes, oversimplified). So I am guessing the impulse response is a room characterization used to color a reverb. However that seems incomplete. Unless the impulse response is really long or is sets of impulse responses over time.
> I suspect that gurgling sound is me in over my head.

Now for a shorter response:

Don't try to think of IR Reverb as recreating the synthetic reverb electronics that you know and love. In the synthetic world, you can pitch shift, distort, and amplify the regeneration to the point that it's no longer LTI. This isn't to say that rack-mounted reverbs with pitch shifting, distortion and infinite regeneration don't sound cool - they sound great! - but they can't be totally recreated with IR Reverb technology. I'm thinking of Eventide here. The fact that they can't be completely recreated with IR just means you're more likely to get lost and confused.

Instead, think of IR Reverb as a way of recreating natural reverbs that occur in halls, rooms, caves, car interiors, and even forests. None of those can pitch shift (unless something is moving at high speed); They don't create distortion in themselves; and they can't sustain infinitely. All that happens is reflection and absorption of sound waves. Those limitations, with an understanding of how an FIR can become a filter without really trying, will probably explain IR Reverbs.

Again, products are free to combine synthetic reverb components with IR & convolution - they don't literally have to be pure technologies (especially if a product needs to be unique in the market).


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