Effects For Violin: Impulse Response

If one of most commonly asked questions we get from first time electric players is "how to I make my electric violin sound like an acoustic violin?" Another of our most frequently asked questions is, "how do I make my violin sound like an electric guitar?" It seems counterintuitive, but one of the best answers to both questions is to use a special type of effect called 'impulse response', also known as IR.

What is Impulse Response?

Impulse response is exactly what it sounds like: a response of a space or a system to an impulse. For example, if we were to stand in a cathedral (a space) and clap our hands (an impulse), we hear an impulse response. Since no two spaces/systems are exactly alike, an IR is like their unique DNA.

IRs are essentially an audio recording of the moment after an impulse. They measure a system's specific EQ curve and are captured as extremely short .wav file.

There are three types of IR: speaker cabinet IR (aka cab simulations), instrument IR, and room reverberation (reverb IR). We'll revisit the reverb type for another article and focus on cab sims and instrument IR's here.

Speaker Cab IR's

Pickups used on electric instruments typically produce signal captured directly at the source: the vibrating strings. When amplified directly, either through headphones or an amp head, the sound can be extremely forward, in-your-face, and even harsh, as they contain every artifact of your playing, from attack, to bow noise, to thumps, to offensive frequencies.

In acoustic music, however, we don't listen to violins (or guitars) with our ears a millimeter from the sound source. We hear it after its been filtered through the bridge and body of the instrument and the air in the space between the instrument and our ears.

This is where cab simulation IR's are extremely helpful. Speaker cabinets color tone by damping the parts of the EQ curve that contribute negatively to tone, while enhancing the positive parts. Any electric tone will tend to improve with the use of a speaker cab simulation. You'll also find that the effects you apply sound better as a result. So, if you "want to sound like an electric guitar," then make sure you're running your signal through a cab simulator!

There are just about as many speaker cab sims and IRs available as there are actual speaker cabs, so you'll need to experiment with which one suits your tone the best (and it may be different for different effects).

Instrument IRs

How can I make my electric sound more acoustic?

Whereas speaker cab IRs will bring your final post-effects signal to life, instrument IRs are where the rubber hits the road for your electric violin's own tone. As mentioned above, direct pickup sound can be forward and unforgiving--quite different from what we're accustomed to hearing from acoustic violins. This is where instrument IRs come into play.

The job of an acoustic violin body is two-fold: to act as a speaker, amplifying string vibrations acoustically, and to filter the vibrations from strings through bridge, top and body in a way that's pleasing to the ear. The qualities of the wood, the craftsmanship, and the design all come together to make a unique EQ response that acts as a filter between strings and the listener's ear.

When you play amplified, your pickup is capturing a lot of direct string vibration--way more than we hear unfiltered when playing acoustically. Whether you're playing a solid-body instrument (or even an acoustic-electric) directly through speakers, you're hearing a fairly direct response of string vibrations with a relatively small amount of body filtering compared to hearing an acoustic.

So, what if we could put an effect in between your pickup's output and your speaker that adds in the "missing" wood body EQ response? We can, by using an instrument impulse response!

There are a couple of ways to get an instrument IR:

You can acquire a pre-existing IR, taken by someone else using an acoustic instrument (could even be a very fine acoustic instrument, e.g. a Stradivari!)
Creating your own custom IR by recording your acoustic instrument. With enough expertise you can do this using audio software, however, there's a pedal that's designed to do it for you. The LR Baggs Voiceprint DI pedal measures the acoustic response of your acoustic-electric instrument (or acoustic instrument with pickup) by leveraging the processing power of your iPhone to accurately capture your instrument’s one-of-a-kind voice. (Voiceprint can't host other IR files, so it's only good for storing your own custom made IRs.)

The more practical option for most players will be to simply acquire an existing IR (or a collection of IRs) from a number of sources, of which here are a few...

3 Sigma Audio has a nice selection of acoustic instrument IRs that can purchase and downloaded from their website.
University of Parma (Italy) has a few nice instrument IRs that are public domain and may be downloaded for free here. (You may come across other postings of free or shared IR files by searching violin forums or elsewhere on the web.)
Signal Wizard Systems, maker of the Prosody pedal, has proprietary IRs taken from very high-end acoustic violins and cellos, including Stradivarius, Guarneri, Tononi and Catenari instruments, which come with the purchase of their Prosody pedal.

How To Apply An IR To Your Signal

After all this, you may be wondering, "that's great, but how do I actually go about using an IR?" You simply need what's known as an Impulse Response Convolver. Essentially, it's an "effect" that lets you load up an IR and insert it into your signal chain. Here's where you'd find and use it:

As a channel strip effect within your DAW (For example Logic Pro's IR Convolver is called 'Space Designer' and is found in the effects tree under Reverbs)
Using a purpose-made IR stomp box pedal
As a block within an Amp Modeler effect (e.g. as within Line 6 Helix multi-FX pedals)

Be aware that some units that can load IRs are designed for speaker cab IRs and are not really any good for loading *instrument* IRs. Much of the gear available to string players is designed for guitarists, who mainly make use of Speaker Cab simulations. Speaker Cab IR Loaders, such as the BOSS IR-2 or the the one found in the BOSS ME-90 mutli-FX pedal, typically don't perform as expected when used to load instrument IRs, so best to use an IR convolver that's purpose-built for instrument IRs.

Blending

Blend is the mix between dry signal (the sound coming from your pickup) and wet signal (the sound from the IR). It's helpful to be able to blend these two, as the dry signal is what we're trying to improve upon, while the wet signal can sound a little generic and lifeless. Your blend is usually expressed as a percentage: 0% means all dry, 100% means all wet. A great starting place for IR blending is 50/50. You can tweak from there to your taste.

One example of an affordable, compact speaker cab IR pedal that will work to load instrument IRs is the Mooer Radar IR pedal. The only downside is that you will not be able to blend your dry signal into the IR sound. Blending dry/wet sound is very helpful for injecting a little life and personality back into what can be a somewhat generic sounding instrument IR. If you have your own custom blended IR, you may not need blend, in which case the Mooer Radar would work well.

Placement in signal chain

Instrument IR: Whether you're using a physical pedal or a software channel strip to load your instrument IR, you'll need to place it somewhere in your chain, but where? IRs are best used at (or very near) the beginning of your signal chain, since they a) work best when applied to your dry pickup signal and b) can improve tone on all subsequently applied effects.
Speaker Cab IR: Applied later on in your signal chain, towards the very end, just before Reverbs/delays.

IR Pedals

These pedals all feature the ability to load impulse responses (IR files). Each is designed to a different purpose, so look closely at the benefits and limitations of each...

Prosody is a top-notch instrument IR pedal that comes with a set of high-end instrument IRs, software for loading them to the pedals, and several useful EQ and effects functions. It does not handle speaker cab IRs, however.
Line 6 Helix and HX multi-effects processors feature an IR loader that allows you to load an instrument IR, a cab IR, or both together. Helix comes with a plethora of speaker cab IRs, but you will need to source instrument IR files elsewhere.
Mooer Radar is a cost-effective pedal capable of loading either an instrument or a speaker cab IR, but it does not have a wet/dry blend function.
LR Baggs Voiceprint DI allows you to create a custom IR of your own acoustic instrument, but it cannot load 3rd party IR files.
BOSS ME90, can only load speaker cab IRs, but not instrument IRs.