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How to Mix with Impulse Responses

When mixing with impulse responses, you’ll need the impulse response file, and a convolution reverb or space designer plugin. The most popular way to mix with impulse responses is to reverberate a signal, but they can also be used for equalization, emulation, and occasionally delay.

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What is an Impulse Response?

An impulse response is a recording that captures a room or piece of hardware or software’s temporal and frequency response. It’s created by taking a 1 sample full-frequency waveform, running it through some hardware, or software, or amplifying it into a room, then recording the result.

The recorded result can then be loaded into a convolution plugin, which often comes free with a DAW. From there it can be further processed.

They’re typically used for reverb, equalization, and gear emulation, so let’s start by taking a listen to an impulse response from a Lexicon 480L reverb unit, and notice how it impressively mimics its sound.

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Using IRs for Reverb

As we covered last chapter, impulse responses work well at mimicking reverb - what’s great about using reverb impulse responses loaded into a convolution plugin is that the CPU usage is often lower allowing us to use more instances of the reverb, and we can further process the reverb response.

So if I wanted a brighter sound, I could use this free Space Designer in Logic’s daw and amplify the high frequencies. Or if I wanted a reverse reverb, I could just click reverse.

We’ll cover some convolution plugins in more detail in a future chapter, but for now, let’s listen to Logic’s stock reverb impulse response, and change the EQ.

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Equalizing with Impulse Responses

Although a less popular way to use Impulse responses, they can also emulate the frequency response of EQ units. For example, I have some impulse responses from a real Pultec unit - if we measure the frequency response of the output, we’ll notice how the response is changed.

Let’s take a listen to one of these Pultec IRs being used.

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Vintage Reverb with IR

One of my favorite ways to mix with impulse responses is to first use a classic reverb or effects plugin - in this case I’ll use this SP2016 reverb, and then run it into an IR of tape. For the tape, I’ll use an Impulse response of a Studer 2-track.

This way, the reverb sounds as if it was tracked to tape. We’ll make this effect more realistic in a future chapter, but for now, let’s take listen to the effect.

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Introduce Delay with Impulse Response

Introducing delay with impulse responses is a little more finicky - the reason being, the taps are set in stone so to speak, so if the delay taps are 250ms and 500ms for the left and right channels, it’s difficult to realign or change those to fit the tempo of a song.

That said, we could shift the length and change some other parameters to make it more useful, but in short, delay isn’t typically the best use for impulse responses.

Let’s take a listen.

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Shaping Responses with Convolution Reverb

As I was saying in chapter 2, we can use convolution reverb plugins that come with our DAW and take advantage of their added processing. For example, Logic’s Space Designer lets me change the ADSR, the EQ, the predelay, reverb size and length, and the wet/dry.

If I used Ableton’s Hybrid Reverb, I can alter the pre-delay, feedback, EQ, if the bass is mono, and more.

And If I used Fruity Loops I could normalize the peak, run it into more processing, stretch the response, equalize it, and more.

Lastly, I could use this free MConvolutionEZ plugin by Melda Audio, to change the stereo width, normalize the impulse response, change the routing, and introduce some filters.

Let’s listen using the Space Designer, and alter functions like pre-delay, reverse, and volume envelope.

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Using Impulse Response Plugins

Some plugin developers are using impulse responses as the primary processing in their plugins. For example, Acustica Audio plugin use impulse responses to create equalizers, and many other effects - Liquid sonics uses impulse responses for their reverbs, then combines that with additional processing, similar to some free convolution plugins.

Let’s take a listen to an Acustica Audio EQ and keep in mind that all changes are being made with a series of impulse responses.

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Creating Realistic Emulations with IRs

So far we’ve covered that Impulse Responses affect the timing and frequency response - that means they don’t affect modulation, harmonic distortion, or compression in a complex way. So let’s say I want realistic tape emulation - first, I could add in some odd harmonics.

Then, I’ll use this pitch time shift plugin to mimic small timing and frequency variations caused by the tape.

Then, I could introduce some subtle soft-knee compression, followed by my impulse response of a Studer Tape machine.

Let’s listen and notice how these additional forms of processing create a very realistic emulation.

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Creating Realistic Tape Delay with IRs

Following the ideas of last chapter, let’s add in one more effect - with a regular delay plugin I’ll introduce 167ms of delay - the same amount of delay introduced by tape machines when both read heads are active at the same time - then I’ll introduce the impulse response.

As a result, we achieve a very realistic tape delay, more so than most plugins.

Let’s take a listen.

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Transient IR Reverb Tip

For this last chapter, let’s get creative with our reverb, and add the SplitEQ before our impulse response reverb - again, since impulse responses take up so little CPU, we can stack effects like this without an issue. Then, I’ll lower the tonal band of the EQ.

This way, only my signal’s transients reverberate. This is a cool effect for drums and other percussive signals.

Let’s take a listen.

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