Innovation Week: Just a few short years ago, impulse responses (IRs) were something only studio engineers had ever really heard of. Now, they’re in the lexicon of every modern guitar player. Everyone’s downloading IRs, but precious few actually understand how to properly use them, let alone what they actually are.
I’ve heard so many guitar players complain about the sound of their modeled guitar tone. Or just outright claim that amp modelers are crap. This is why understanding what an impulse response is and how it works is all part and parcel of getting the best possible sound when using your amp modeler or multi-effects pedal.
I’ve been recording guitars digitally for over 20 years now, and I was running a multi-effects unit with amp modeling directly into PAs when I was touring with my band in the early 2010s. So, safe to say I’ve been there and got the T-shirt when it comes to this stuff. We’ll start with what IRs are and how they’re made, then we’ll get onto how to use them properly.
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What is an impulse response?
An impulse response is a snapshot of an audio device or space, captured in order to recreate the way it sounds. The first part, the impulse, is a burst of sound we feed into the device. The impulse contains every frequency in the spectrum of sound, delivered all at once, and all at the same level.
If you run this impulse into a guitar cabinet and record what comes out, you get a snapshot of what that cab is capable of in terms of audio. That includes the frequency response, phase response, and the decay of the sound over time.
In the modern age, we use a logarithmic sine sweep to capture the sound of a guitar cabinet. This lasts about 10 to 20 seconds and helps deliver more energy to really move the speaker and capture what it’s doing more accurately. This is then collapsed down to create the short impulse response.
An impulse response is the recording, but the process of applying it is called convolution. Convolution combines your dry signal and the impulse response, then sums the whole thing together.
It might sound complicated, but if you bring an impulse response into your DAW, it’s just a simple WAV file, and it will appear as a tiny spike of sound that disappears very quickly.
A single snapshot
Impulse responses are a really accurate way of capturing the sound of a guitar cabinet, but they’re linear. That’s why we can’t get impulse responses of very dynamic sounds, like fuzz pedals. They’re also linear over time, which is why we don’t have impulse responses of modulation effects.
Impulse responses can capture things like cone breakup on an amp, which is why they sound so convincing. They just can’t capture how that behavior changes with dynamics or over time.
Amp speakers can get hot, resulting in power compression and resistance changes that occur when you drive them hard and play for a long time. The IR is taken at one level, so you can’t replicate that when using an impulse response with your amp modeler.
Despite that, this linear behavior is what we want. A speaker is typically a pretty linear thing, unless it’s being pushed especially hard. If an amp model feels lifeless, it’s far more likely to be down to the model rather than the cabinet impulse response.
Cabinet impulse responses
So what are you getting when you buy an impulse response? Well, it’s not quite a full 4×12 cabinet. An impulse response typically only captures one speaker, with one microphone, in one room. It picks up the construction of the cabinet, whether it’s open- or closed-back, and the preamp and converters used in the recording process.
The one-microphone part is really important, because the placement of a microphone on a guitar cabinet drastically changes how it sounds. Dead center of the speaker delivers a much more fizzy and direct sound. Move it sideways from the center, and it gets warmer and darker. Angling the microphone away from the cabinet also takes away the top end. Engineers call this ‘off-axis’ placement.
You also get a change in sound when you pull the mic back from the cabinet. Close up, you can take advantage of the proximity effect, resulting in a thicker low-end. Pull it back, and you start to capture more of the room that the cabinet is in.
This is why you can get two impulse responses of the same make of guitar cabinet, but they’ll sound nothing alike. When you’re downloading a cabinet IR pack, you’re getting that engineer’s take on a particular sound, and their preference for microphone, positioning, etc.
In impulse response packs, you’ll often get loads of different mic positions, so it’s worth auditioning them to find out what you prefer. Some guitarists like a 57 right on the cone; others prefer a 421 on the seam of the speaker angled at 45 degrees. Some prefer a condenser mic. The devil’s in the detail, and you’ll need to do a lot of auditioning to get the most out of your purchase.
You might have hundreds of IRs in a pack, but you’ll probably only end up using three of them!
Convolution reverbs
Guitar cabinet impulse responses are around 20 to 50ms, so they’re discernible as a tone for the human ear. Once you get to 80ms it becomes more like an echo. Past 200 ms, the human ear hears it as an actual space. This is where we get into convolution reverbs.
It’s a similar process, but this time you play a sine wave sweep into a room or space and capture the result with a microphone, usually a stereo pair. Doing this gives you a linear response of how the room or space reacts, and you can then play your own sounds through it to make it sound like it’s in the same space without having to physically be there.
Like our guitar cabinet impulse responses, convolution reverbs are linear over time. That means you can’t properly change things like the decay time, size, diffusion, etc. It’s why convolution reverbs are less popular for guitarists versus the more common algorithmic reverb in your pedals, because they’re nowhere near as flexible.
They’ve been used in studios for a long time, however, because they’re useful for creating spaces for other instruments like drum kits or vocals.
Impulse responses versus captures
Impulse responses and captures do a similar thing. Where impulse responses are linear recordings of a system’s reaction to sound, captures do much more complex processing.
Captures use a test signal that trains a neural network to recreate the relationship between the input and output. It can capture the dynamics of an amp in terms of input level, but is static at the particular settings the amp was recorded at.
An amp simulation or model mimics all of the components, knobs, transformers, tubes, and anything else going on underneath the surface. Captures tend to sound better than models, but models offer more flexibility as you get a more realistic response when you turn the knobs.
The reason that impulse responses are still the standard for speakers is that, as we mentioned before, speakers are very linear in their response. It would be a waste to use processing power to simulate this using capture or modeling technology, so for now, impulse responses are still the way to go for guitar cabinet modeling.
How to use an impulse response
So, now we know everything we need to know about impulse responses, let’s get to actually using them. These are my top tips you need to know to get the best out of an impulse response.
1. Turn off the cab block
If you’re running an impulse response, make sure you’ve not still got the original cab block running. Stacking two responses after another is a recipe for muddy tone. You want an IR or cab block if using an FRFR speaker or headphones, but turn off the IR or cab block if you’re running into an actual guitar cabinet.
2. Signal chain
There are a couple of places to put your IR block in the signal chain. Generally speaking, we want it after your overdrives, fuzzes, and your amp block. Where things get interesting is the placement of reverbs, delays, and modulation. You can experiment with putting them between the amp and cab block, or making them the last parts of the signal chain.
3. EQ adjustments
Most IRs let you adjust the EQ, and you’ll want to remove any sub frequencies. High-passing around the 60-70 Hz mark will help tighten up the low-end of your tone. Just be careful here because a low E is at 82 Hz, so you don’t want to be cutting off the fundamental of your low strings.
We also want to cut at the high-end, with a low-pass filter somewhere between 5 and 8 kHz to get rid of unwanted fizz. You might want to extend this out for a clean tone. If you find you’re cutting a lot here, sometimes it’s better to choose a darker impulse response rather than taking away all your high-end.
4. Use mic blends
Many IR manufacturers include mic blends, which for me are the best kinds of impulse responses. These have two different microphones blended into a single response, which is what is often done during professional recording of guitars.
You can do this manually with two separate IRs, but you need an impulse response loader with a time alignment control to get rid of any potential comb filtering, which can have a negative effect on your tone.
5. Audition at volume
Loads of guitarists create their dream tone at home with headphones, then when they go to play with their band it gets lost in the mix. If you want to ensure your impulse response is great no matter where you play, you need to tweak it at performance volumes. This will allow you to get a tone that cuts through when playing with other instruments.
6. Check your levels
If you’re using IRs from different manufacturers, it’s important to make sure your levels aren’t jumping up and down. Different engineers normalize at different levels, so you could be shifting the balance of your sound up and down by a few dB when switching patches. This also affects anything downstream of the IR, so make sure those levels are correct.
7. IR format & length
Certain impulse responses are designed for use with different modelers. Most IR manufacturers label these, but not all of them do. Some IRs are done at 44.1 kHz, 48 kHz, or 96 kHz, and different modelers are compatible with different frequencies, so make sure to check which one yours is designed to work with.
Some amp modelers and IR loaders are also capable of working with longer impulse responses. Most will truncate an IR if it’s the wrong length, so this won’t have a huge impact on most users. It is worth checking what your modeler is compatible with, as there’s a slight improvement in quality with a longer IR.
IR devices
So now we know what impulse responses are, what they’re not, and how to use them; here’s a brief list of the devices you can use.
1. Multi-effects/modeler
Probably the most common use case. Pretty much every amp modeler and multi-effects unit has an IR block nowadays, even the budget ones. It’s a great way to massively upgrade the sound of your models on budget multi-effects pedals by replacing the stock cab block with a 3rd party IR.
2. Reactive load box
If you’re hell-bent on using a real tube amp but need more flexibility, a reactive load box could be the way to go. Many load boxes now feature a way to upload your own impulse responses, allowing you to bring the sound of any cab you want with the flexibility of playing or recording at lower levels.
3. IR loader pedals
There are also quite a few impulse response loader pedals nowadays, so you can bring the power of IRs straight to your pedalboard. Some of these also come with amp models, or you can use them with power amp pedals if you want to keep everything on your ‘board.
4. In the DAW
There are plenty of free and paid IR loaders that can be used inside the vast majority of DAWs. If you’re recording with guitar plugins, you can simply disable the cab block inside the software, then add the IR loader as the next plugin in the chain to load your cabinet response of choice. There’s plenty of software that also allows you to load third-party IRs, too.
