When searching for new sounds, it’s always a challenge to get the sounds that are in your head out of there and into your production. Especially being able to do it quickly and easily is something I really strive to do. Over time you get better at this, but there are also some cool techniques you can use. For me, one of the techniques I wanted to dive a little deeper into was convolution. I am always looking for ways to use the sounds around me, in real life, and use them in a production. Using convolution, you can record things like reverb, speakers, recording gear, and basically anything that alters sound.
Audio Ease, the manufacturer of Altiverb, recording an impulse response at Sound Kitchen Studios in Nashville
What is convolution? In essence, you’re taking one recorded sound and multiplying it with another. So, in a classic example, taking a dry instrument and multiplying it with a reverb that you recorded. In post production convolution is often used to mimic certain acoustics, such as the inside of cars or other ambient surroundings. But that’s the way they expect you to use it. I wanted to check out if I could use it as an effect to simulate a speaker from a phone or laptop. And these days it’s even used to simulate guitar amplifiers, with the dry signal just being a DI signal.
Early convolution reverbs were very expensive outboard pieces. I mean, check out the fake wood on the Sony DSE R777 below. That’s a cutting edge piece of technology right there. Fortunately over time, the technology got to the point where a normal modern computer can run it easily. With most DAW’s, it’s even included in a standard installation (Pro Tools has TL Space included since version 12, and Logic’s Space Designer is also included with Logic).
The Sony DRE-S777, the first commercially available convolution reverb
Recording an impulse response
So how do you get your sound into the software, into the convolution plugin of your choice? Well, first you have to capture the sound. To do this you have to use an excitation noise. Two kinds of excitation noises can be used:
A sine sweep. This is a sound that sweeps from a low frequency (~20 Hz) to a high frequency (~20 kHz or more, depending on the sample rate). You can hear an example below. This sound is played from a speaker and then captured with a microphone. If you’re going for realism, this is the way to go. It allows every frequency to be captured pretty well. Keep in mind that you do have to deconvolve the audio before you can use it in your DAW. The deconvolve process removes the sweep and converts it to an impulse. You can do this entire process easily in Apple’s Impulse Response Utility.
You can also use an impulse. An impulse is a very short sound that contains all frequencies. This is easier when recording on location than setting up a speaker to amplify a sine sweep. You can also pop a balloon, use a starter pistol, or clap your hands if you don’t have anything else. It’s less accurate than using a sine sweep, but much more easy. Personally I wasn’t going for realism with this experiment, so I popped a balloon. In the hallway of my apartment building it sounded like this:
Hallway Impulse Response
This file can be loaded into an Impulse Response plugin. I used the Waves IR-1, because of its built-in eq and editing capabilities. If you then combine it with a dry instrument, it sounds like this. First you’ll hear the dry signal, and then the reverb is slowly added.
Drums with hallway IR
The real reason I wanted to look deeper into convolution was because it can be used to simulate every sound-altering device. So in theory, everything can be sampled. That includes recording gear, pedals, and also things like laptop speakers and phones.
I really wanted to make it easier to use things like phone speakers for effects, so I made an impulse response for my own iPhone speaker. So, I started with a sweep (using only an impulse doesn’t really work that well, since there isn’t really a reverb tail). It sounded like this:
Playing it back through my iPhone speaker, and recording it with my Tascam DR-40 field recorder, it sounded like this:
iPhone Sine Sweep
Quick tip: record at high samplerates (96kHz or higher, if possible). This way you can increase the decay time later with less sampling artifacts.
After deconvolving, loading it into a plugin, and tweaking it a little bit it sounded like this. You’ll first hear the dry signal, then it will slowly morph to the signal convoluted with the iPhone impulse response. This example is pretty extreme, but you can hear how it transforms the sound well.
Drums with IR
Working with convolution
For me it’s a great tool to use, because it’s quick to load in and see if it’ll work in the music. It’s much quicker loading a convolution plugin than exporting, playing it back through a system, setting up mics for that, recording that, and syncing it with the tracks already in my session.
Of course there are also drawbacks to using convolution. The main one being that it’s a recorded sample and therefore fixed. This also means that unlike real reverberation, every decay and response is exactly the same. Is this noticeable? That will vary with each application, and for most instances, probably not. But something to keep in mind, nonetheless. Algorithmic reverbs, and physical reverbs such as springs and plates still have their place. If you want to read more about the realism of impulse responses, there’s a nice (technical) article on the blog Science of Sound.
So, what to sample next? My field recorder is always in my backpack, and a sine sweep can be played from my phone any time. Some nice guitar amplifiers would be cool. And a proper PA system in a medium to big concert hall, or making a phone call and recording the sine sweep through that would be great too. Recording an impulse response using some high- and lower end microphones would be interesting, to compare how it holds up to the real thing.
By the way, there are tons of Impulse Responses available online. You can check out a lot of different places where you can download them, in this article on Propellerhead’s website. Check out the impulse response of a club setting with a Funktion One speaker system. Or the impulse responses of the Bricasti M7 unit!