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Is it possible to create an IR Impulse Response WAV file (FIR speaker cabinet frequency response profile) to simply remove top end from the frequency response of a guitar amp modeller? It requires a 'chirp' frequency sweep WAV file.

I know an equalizer would be a simpler solution, but that can't be used in this case.

The frequency response would need to be flat up to the 3db cutoff frequency.

I've tried to edit an actual speaker IR file to flatten it out in the lows & mids, but the results were not very good, so that's not an option.

I've also tried simply equalizing a flat FIR WAV "chirp" sound file and loading that. Also sounded terrible for some unknown reason.

If it's possible - what software would be recommended to create the file?

Ideally, I'd be able to create an IR from scratch, and determine the cutoff frequency.

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  • You said you have used an "actual speaker IR file", postprocess it and it was terrible. Was the IR file unmodified OK ? How have you postprocessed it (Audacity ? other tool ?).
    – Frédéric LOYER
    Apr 22 at 15:35
  • The unmodified IR was good. I used SoundForge for the LP filter. My OP question has been difficult to describe or clarify, I'm going to tackle the problem some other way & may post a re-phrased question. I tried all the suggestions & appreciate the interest, it is indeed an intriguing problem.
    – Samuel Barns
    Apr 23 at 18:06

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There are typically 2 types of digital filters:

  • Finite Impulse Response filter, where you give the filter a WAV file (the room response of a bang if you want to emulate the room reverberation), and

  • Infinite Impulse Response filter where you typically use few numbers to build typical filters (IIR filters have a retroaction which permits this).

Assuming you have a FIR, the Fourier transform of the WAV file gives you the frequency response of the FIR filter. You can search some FIR filter design on the Web. You will find some articles or presentations like Design_of_FIR_Filters.pdf.

Note: SciPy is a handy Python library where you can find Fourier transform, inverse Fourier transform, WAV export.

You should be more precise about the filter you use (whether you give it a FIR WAV file, or a profile it tries to compensate… which would be strictly the opposite).

I can give you two ways of computing the FIR.

  1. Inverse Fourier transform of the desired frequency response filter. But you will have a filter with samples before the peak.

  2. Designing an IIR filter, and pass through this filter a Dirac signal.

Or the most straightforward way, use FIR software like rePhase.

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    REW (Room Eq Wizard) can generate a chirp. But your purpose is hard to understand. Usually a chirp is send to speakers and recorded in a mic in order to deduce the room frequency response, its reverberation… The chirp is not the FIR coefficient. Then I don’t know how it can help you since you want a lowpass filter response.
    – Frédéric LOYER
    Mar 22 at 17:04
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    sweetwater.com/sweetcare/articles/… tells you how to have an IR. Just snaping fingers and record it. Now, take ownhammer.com/store/… from OwnHammer. It is only a 208ms WAV file. You can't put a chirp sound in such a tiny file. Making such a file with an impulse sound recorded is challenging. The impulse has a huge dynamic. Then, one way is to record the amp you want to emulate with a chirp sound, and process the file like REW does. rePhase compute directly an Impulse Response.
    – Frédéric LOYER
    Mar 23 at 19:25
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    The idea behind chirp is that the IR can be computed by IR = IFFT( FFT(chirp_recorded) / FFT(chirp_emitted) ). What you want for the AMP is the IR. Either you compute it directly with rePhase (and select your low pass filter), either you measure it with REW which handle the whole thing (emit a chirp, record it, does the math). The chirp is only useful for measurements and when used you need 2 chirps : emitted / recorded.
    – Frédéric LOYER
    Mar 23 at 19:31
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    Have you tried the WAV produced by rePhase ? As I have written, chirp are used to measure an IR, but it is not mandatory if you can make the IR from scratch. See z2dsp.com/2018/07/10/impulse-response-modelling you will see that the IR is not the chirp. It is something different computed here from the chirps by a convolution. (I am a bit skeptical about this method... but if it works). The article tells you how to use the IR before feeding your amp. And a processed chirp is strictly unusfull if you have not its reference (unprocessed chirp).
    – Frédéric LOYER
    Mar 24 at 14:15
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    According to this article, you can create a processed chirp with the octave freeware in the Using the impulse response section. You will need a chirp as an input WAV (REW can produce one : tools/generator/sweeps), an IR (rePhase can produce one). Then you will have a unuseful processed chirp. Unuseful because alone it won't be permit the amp to deduce your low-pass filter.
    – Frédéric LOYER
    Mar 24 at 14:25

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