6. Cab Impulses
Traditional hardware speaker simulator are usually not much more than a series of filters and notches, designed to simulate frequency response of a cabinet they try to mimic. Some may take it a step further and add a bit of clipping to simulate cone distortion, some may use the actual speaker coils to bring the inductance into the picture, but that’s about as far as you can go with traditional hardware speaker simulators. However, there’s much more to the sound of a cabinet that traditional simulators cannot capture. It’s a complex electrical-acoustical-mechanical system and simulating all aspects and interactions between all parts of it is not very practical, if possible at all.
So, What is an Impulse Response (IR)?
Rather than trying to simulate some aspects of the cabinet, IR is a very short recording of how the cabinet responds when presented with a sample input signal. So, technically, it’s a very, very short WAV file. That way we get the information of what the cab does rather than trying to comprehend what the cab is and then simulate some of that. Since we need to somehow reproduce signal into the cabinet and then capture the output, an IR captures the sound of the cab and the sound of the mic, but also the sound of every piece of gear used to reproduce and capture the sound – power amps, preamps, AD/DA converters, cables…the full circle. Also, any flavor that room may have added to the sound, phase cancellations between the speakers, acoustic properties of the cab, it all goes in. So rather than emulate parts of the system, now we have enough information to tell what that system does to the sample input signal, and we may use that information to apply the same transformation to any signal. Typically, the target of making an IR is a combination of cab and mic, and for everything else, we hope that the gear used is as neutral as possible. That’s why it’s important to get a decent IR made with a good, quality rig.
However, there’s a limit in what can be captured by an IR. It cannot capture any non-linearities like distortion or compression produced by the system. That’s nothing to worry about bad because we usually have plenty of distortion produced by amp and pedal models and we can always add a compression effect. It’s much more important to capture the feel the of the cabinet and that’s what IRs do very well.
It’s worth noting that IRs are used not only for cab emulation, but also for emulating mics, reverb and even rooms. It’s the same principle – get the most neutral sounding rig around the piece of the chain that you are trying to capture.
Where Do I Get an IR?
Assuming you don’t have the gear and time to make your own IRs, there’s plenty to choose from, whether you are looking for free or payed options.
Mixing Multiple IRs
As you will see, most of IR loaders can take at least two, if not more IRs at the same time. That gives us so many different combinations for experimenting, just as we dream of doing in a real studio, with a bunch of different cabinet, mics, mic placements. All that assumes that you have enough different IRs to experiment with. When mixing multiple IRs, there is an important thing to keep in mind: phase. When mixing IRs from different vendors, they won’t have their phases aligned, so there’s no guarantee how they’re going to sound together. Or when mixing two mics at different distances from the cab. It will take longer for the sound to get to the second mic, so it will not be in phase with the first one. When mixing IRs, try playing with phase switch or delay settings on one of them (no need to change both, it’s only important how they interact with each other). Even when phases are perfectly aligned it may be worth trying to delay one of the IRs for very short period of time (like 0.1ms). It can make the sound thicker.
I usually start with Royer R121 placed about 1″ from the cap and Shure SM57, close to the cap edge, then pan one slightly left, the other slightly right and then take it from there. R121 has great neutral sound that you can easily use on its own. SM51 adds more bite, so it’s one way to accentuate the highs. I usually blend more of the R121 if the sound is too shrill and then play with delay to get it to my liking. Some like adding a room mic to the mix to add to the depth, so that’s worth trying too.