Impulse Responses (IRs)

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The information on this page supplements the official manuals.

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Contents

About Impulse Responses (IRs)

An Impulse Response (IR) is a collection of data representing sound measurements taken from a speaker cabinet or system. A test signal is played through the actual speaker, recorded, and used to generate a profile.

The Axe-Fx II and AX8 use IRs in the Cab block to reproduce the measured response and emulate a particular speaker cabinet, as well as for modeling of microphone types.

The terms "cab", "user cab" and "IR" are often mixed up.

Fractal Audio:
"An IR stands for "Impulse Response". In mathematical terms it is the time response of a system to a Dirac delta function (also known as an impulse). An IR can be used directly as the coefficients for an FIR (Finite Impulse Response) filter. In the modeling world IRs are obtained from real speakers and when processed using an FIR filter produce extremely accurate results. In essence an IR is a "sample" of the speaker and microphone and uses very similar principles. However the quality of any IR is subject to the talents of the individual(s) capturing the IR. Mic placement, preamp choice, etc., etc. are important as you are essentially recording the speaker. In the old days modelers used EQ to emulate speaker response but I don't think there are many left that still use that technique. So the quality of the IR is really the issue here. The original Axe-Fx pioneered this technology which has since become almost ubiquitous." source

Wikipedia

Impulse Responses: supported by which Fractal Audio products?

Cabinet block and IRs

This wiki page is about Impulse Responses in general. For information about the Cabinet block and its parameters, visit the CAB block page. The Cab block is the container that processes IRs (built-in and external ones) and provides additional parameters.

Near-field and far-field IRs

Most speaker IRs represent the tone of a speaker that was recorded with the microphone close to the speaker, aka "near-field" or "close-miked".

"Far-field" IRs represent the sound of a speaker that was captured at a longer distance. There are a couple of far-field IRs among the stock cabs, created by Jay Mitchell ("JM"). These IRs are better suited for recreating the "amp-in-the-room" sound of a traditional guitar speaker.

IRs and FRFR amplification

Read this: FRFR and close miking (versus amp-in-the-room).

Length or resolution of IRs (Ultra-Res, Hi Res, Normal Res)

Fractal Audio devices and software support IRs of various lengths, measured in number of samples (points) and milliseconds:

Ultra-Res speaker IR processing is a proprietary technique that enhances the spectral resolution of an IR without adding CPU burden or storage requirements.

Ultra-Res IRs do not support size warping, which is why the Cab block's Speaker Size parameter is unavailable in Ultra-Res mode.

Ultra-Res lRs are displayed in italics in the editors.

Fractal Audio (source):

Fractal Audio:

(about Tone Matching and Ultra-Res)

Fractal Audio (source):

(Ultra-Res 2.0):

Fractal Audio:
"1" from the speaker is the near field. The response of a speaker in the near field is very different than the response in the far field. In the near field the response changes (drastically) across the face of the transducer. Even moving the mic a fraction of an inch will result in a very different sound. 10 ft. from the speaker is the far field and the response changes smoothly as you move across the field. If the near field were the same as the far field then the sound wouldn't change as you moved the microphone and you could place the microphone anywhere on the face of the speaker. Anyone who has mic'd a speaker knows that this isn't the case." source

Minimum Phase and Auto Trim

If desired, the phase of an IR can be manually adjusted or automatically. There are two ways to have the software do it:

Getting the phase right is important when mixing IRs together. When neither Min Phase or Trim has been applied, the IR is "raw", containing the original phase details.

Create your own Impulse Responses with IR Capture

Read this: IR Capture.

IRs compared to Tone Matching

Fractal Audio:
"Tone Matching is a nifty feature and certainly useful but you'll get far more satisfaction by concentrating on capturing good IRs. The single most important aspect of recording guitar amps is micing the amp. Therefore the single most important aspect of using your Axe-Fx is the IR. People are too hung up on "matching" or "profiling" an amp but fail to realize that when you are doing that you are basically capturing an IR. If you capture the IR separately now you have an IR that is fully separated from the amp and therefore can be used with all models. Matching and profiling cannot mathematically separate the amp's frequency response from the cabinet frequency response. Once you do this you'll be surprised at how accurate the amp models are. I do this all the time and find Tone Matching is unnecessary now (in fact many of the amp models have had their built-in matching data removed in the latest firmware). Any differences between the model and the real amp are so minuscule as to be immaterial. A little tweak of the tone stack or EQ is usually enough to remove and differences. Besides, once you get into mixing you'll realize that you'll be applying EQ anyways so tiny differences in EQ are irrelevant. Moving the mic just a small amount drastically changes the sound. The best producers have mastered micing. You can only fix so much via EQ since EQ is essentially painting with a broad brush where mic technique is akin to using a fine-point brush." source

IRs of acoustic tones

To emulate acoustic instruments (acoustic guitar, cello, violin etc.), an IR of an acoustic body can help. You can find some here.

Acoustic sounds benefit from long IRs, so Ultra-Res IRs are preferred.

Differences between IRs for Axe-Fx II Mark I/II/XL/XL+

Fractal Audio:
"The XL has a different sysex ID and therefore requires different cab files. The only difference in the files is the sysex ID.” source

Commercial and free IRs

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The Axe-Fx II and AX8 include a lot of stock cabs (factory cabs). You can also create your own IRs or get additional ones from the sources below:

Importing IRs

Axe-Edit's and AX8-Edit's built-in Cab-Manager lets you load IRs into the hardware.

Fractal-Bot saves and loads entire banks.

IRs for Axe-Fx Standard / Ultra

IRs for the Axe-Fx Standard/Ultra must be converted to be able to use these with the Axe-Fx II. source

It's no use converting 1024-point IRs to 2040 points because they don't contain the necessary data. You need an original WAV-file of sufficient length to create a 2040 point IR.

Information about guitar speakers

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