[sdiy] IR Reverb
richard at wentk.com
Thu Feb 15 15:38:08 CET 2018
But only from one monitoring location.
To *fully* characterise a room you’d need to record responses from every possible location. Given the wavelength of a 20kHz sine wave in air, that would mean a grid of recorded responses on a 17mm spacing.
Of course no one does that in practice because a single response in the middle of the room is good enough for reverb simulation.
But it can be a problem if you’re using deconvolved impulse responses to measure room modes, like the Apple HomePod does, because the response at the device and the listening position will be different.
It’s possible Apple tried to compensate for this. It would be interesting if they did.
> On 15 Feb 2018, at 10:15, Richie Burnett <rburnett at richieburnett.co.uk> wrote:
> An impulse response takes energy from one point in time and moves it around in time. This "redistributing" of energy in time is actually the same as filtering. It just might not intuitively feel like it unless you were told it in a DSP class or something.
> If you have a very long delay then it is obvious that it will produce seperate audible echos. However we know that short delays of only a few microseconds or milliseconds produce something we call comb filtering, (think flanger). So depending on the amount of delay you might intuitively think of either echoes or filtering.
> A sufficiently long impulse response captures it all, and in fact there is really no distinction. If you do something to a signal in the time domain it will have implications for its frequency content, and if you do things to it in the frequency domain with a filter it alters the waveform in the time domain.
> In fact an impulse response is all you need to fully characterise a system, provided that it is linear and time-invariant. In other words, you can fully replicate all of the characteristics of a room's response by capturing and applying it's impulse response, as long as the room doesn't distort sound, and as long as the sound source, walls and listener are static, and there aren't any air currents in the room.
> Sent from my Xperia SP on O2
> ---- Tim Ressel 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.
>> --Tim Ressel
>> Circuit Abbey
>> timr at circuitabbey.com
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