# [sdiy] IR Reverb

Mattias Rickardsson mr at analogue.org
Thu Feb 15 19:31:57 CET 2018

```And another way of thinking of convolution altogether is to ponder the
commutitative aspect of convolution. Just like a multiplication A times B
is the same as B times A, a convolution f * g is the same as g * f.

Just like the IR of the room is applied to all the samples in a song and
then summed up (when a song is played in a room), the song can be seen as
the IR and the room impulse response can be seen as the sound. The song is
then resonating for each of all the samples in the room, and the summed up
(the room is played in the song)!

(-:

/mr

Den 15 feb. 2018 16:09 skrev "Bruno Afonso" <bafonso at gmail.com>:

> Another way to think of IRs is as a way to capture the response of
> whatever thing that processes/affects sound at a given position. A reverb
> could be thought of a room processing a sound the same way a guitar pedal
> such as delay or distortion or a Woden case.  The reason IRs are not the
> end all be all is that a lot of interesting things are not time invariant
> or linear.
>
> There are indeed ways to speed up convolution, some of which are patented.
>
> On Thu, Feb 15, 2018, 05:17 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.
>>
>> -Richie,
>>
>>
>> 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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