What is an impulse response and what do you use it for?

The impulse response (IR) is a relatively new technology that can simulate a specific environment—a hall, room, studio space, the body of an acoustic guitar, and so on. At first, impulse responses were used for reverb effects, such as plug-ins in a digital audio workstation (DAW). These days, they’re also used for other applications. In this blog, you’ll read how impulse responses can be genuinely practical in the studio or on stage for both sound engineers and musicians, with special focus on the guitarist!

Wat is een impulse response en waarvoor gebruik je deze?

How does an impulse response work?

The impulse response (from now on referred to as ‘IR’ in this blog) is used for signal processing of a ‘dynamic system’. A dynamic system is a scientific term for a situation or environment from which the IR is created. The IR consists of a short input signal, called the impulse, and then functions as an output signal. In the case of, for example, a reverb, this is the result—in other words, the reverberation that was present in the room while the IR was being created. So the output is the reverb effect in your DAW.

Applications of impulse responses

Originally, IR was used in, for example, the popular convolution reverb called Altiverb. This reverb plug-in is known for its true-to-life simulations of all kinds of spaces, such as concert halls, studios and cathedrals. You can then use these environmental effects on any instrument you like in your DAW software. But IRs aren’t only used for reverbs. You can sample virtually anything you run sound through, such as a speaker cabinet with a microphone setup.

Impulse responses as a replacement for speaker cabinets

IRs are particularly interesting and practical for guitarists and studio engineers. As well as applying a certain EQ curve to the output of, for example, your guitar amp, you also immediately apply the specific sonic characteristics of the mic used, the speaker, cabinet, cables, the room and even the preamp used for the microphones! There are countless IRs available online, with a wide variety of microphones, microphone combinations and speaker/cabinet pairings. The possibilities are virtually endless. This powerful technology saves you a lot of work, since you no longer have to do all the physical work involved yourself. And that’s without even mentioning the phase problems that crop up when you place multiple mics at the same time! On top of that, you can save your favourite mic-and-cab combinations as a preset in your DAW software and load them back in instantly when you need them.

So how do impulse responses work with guitar amps?

Because every valve amp normally uses a speaker cabinet, you need to replace it with what’s known as a load box (see the blog about that). The load box ensures that the energy from the amp—which is converted into sound and would normally be sent to the speaker—is now loaded into a line level signal. If you then send this signal straight to your interface, you’ll get a very flat, static sound. After all, this is the signal coming directly from the amp itself, without an impedance curve—something the speaker normally provides. This is where the IR comes in. The next step is to use what’s known as an ‘impulse response loader’ into which you load the IR. This ranges from (free) software IR loaders to hardware versions, such as the Mooer Radar. Now you’re ready to load the IR into the IR loader of your choice. Another major advantage of IRs, by the way, is that you can use them for so-called ‘silent recording’. After all, you can set your amp to its sweet spot by turning the volume right up, without anyone noticing. This way, you can get the very best out of your valve amp.

Do impulse responses also work in live situations?

Absolutely. In fact, it’s recommended. You go straight from the amp output into a load box and then into the venue PA system. You also save the sound engineer a lot of frustration, because they no longer need to spend time on perfect mic placement, fighting feedback and other time-consuming hassles. It’s also a solution for you as an artist: you always have your own sound with you—every time, regardless of the setting.

How do you use impulse responses? Leave a comment!