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Impulse response definition
How to use an
Impulse Response?
You may have arrived here without really knowing what an impulse response is? What it offers? How to use it with a musical instrument? There are a plethora of other questions that we are asked daily. We will try to decipher the vocabulary and techniques that lie behind this term.
What is an Impulse Response ?
An impulse response defines the characteristics of a system when it is subjected to an impulse.
Impulse responses of an environment
As part of the analysis of an environment, we send a sound pulse through the same space to determine the projection and reflection behavior of sound waves.
Let’s take a concrete example. We want to generate the impulse response of a theater. We will send a sound pulse into the theater space. It will be projected through the spaces and propagate through a complex system of reverberations. Then, we will capture the response of the space after sending the pulse. This capture is then the subject of mathematical functions in order to extract and store in a digital file the reaction capacity of the space. The result is the impulse response of the place.
When we talk about an impulse response file, we are referring to the digital file that contains the impulse response.
Impulse responses of acoustic instruments
Without going into vast and rather complicated details, in the case of an acoustic musical instrument, the impulse response has the acoustic characteristics produced by the instrument’s sound box.
Other types of impulse responses
What you need to understand is that impulse responses can be generated for any system that can react to an impulse.
Thus, you can find impulse responses of famous places (ancient theater, cathedral, chapel, etc.), musical instrument bodies (guitars, cellos, violins, mandolins, etc.), electrical devices (amplifiers, speakers, microphones, etc.), objects (bottles, glasses, etc.) and many others on the Internet.
Why use impulse responses?
Each of the previous impulse responses are aimed at different audiences and uses. They are also edited by using different techniques. The process to create these IR files is different from one to the other.
What is the impulse response of a place used for?
For example, impulse responses of a place are widely used in the film industry. They allow you to simulate the acoustics of a space. If a film scene takes place in a large church, but the filming took place in a studio, the sound recorded during filming cannot have the reverberation offered by a church. We will therefore simulate it in post-production using an impulse response file. The latter will function as a filter by adding the reverberation of a place to the original signal.
These IR files are captured with ambisonic mics. They are used for capturing 360-degree sound.
What is the impulse response of an amp used for?
Another example, electric guitarists are fervent fans of amplifier impulse response files. Some amps no longer exist on the market or are prohibitively expensive. They can simulate the tone offered by Jimmy Hendrix’s amp by simulating it using an impulse response.
These IR files can be done using different methods (white noise, impulse, test tone, etc.) with a software like REW (Room EQ Wizard), or a DAW and some plugins.
What is the purpose of the impulse response of the body of a musical instrument?
Finally, and these are the ones that interest us, the impulse responses of the bodies of acoustic musical instruments allow us to simulate the tone of the captured instrument by adding it to a source audio signal. For example, a violinist who plays an electric violin cannot benefit from the natural tone of an acoustic violin. An electric violin does not have a sound box, so it is unable to produce the very specific tone of the acoustic violin. However, the violinist can simulate an ‘acoustic tone’ to the audio signal of his electric violin by using an impulse response file of an acoustic violin. Let’s see below the principle of these impulse responses and their interest.
Let’s take the example of a folk guitar. There are two ways to amplify this guitar:
- using the built-in pickup and its jack
- using a microphone
The guitar’s built-in pickup detects the vibrations of the strings to produce an electrical signal. It only captures vibrations traveling through the body and bridge of the instrument. It is unable to capture sound waves traveling through the sound box and then through the surrounding space.
The only way to capture sound waves traveling through the air is to use a microphone. The difference lies in the method of capture. The sound produced cannot be the same if the signal is produced using a pickup or a microphone. The tone is therefore different. The audio signal from the pickup is direct and dry, while that of the microphone is more ample, warm and natural.
However, if we transform the signal produced by the pickup using an impulse response file, we can simulate the acoustic properties of a guitar.
- We capture the system comprising the guitar and the microphone to generate an impulse response file.
- We apply this IR file to the signal produced by the pickup.
- It will then benefit from the acoustic characteristics of the first system. It is explained in this diagram.
These IR files are done by recording two separate captures of the source signal. Then, the signals are correlated to generate an impulse response. The process uses the convolution/deconvolution functions.
How to use an impulse response file?
Application of the impulse response file
The impulse response file is a digital file. It is therefore necessary to have a computer or any device with a processor capable of performing the calculations necessary for its application. It is therefore possible to apply an impulse response file with a multitude of software and devices. They each have their advantages and disadvantages. The choice of a device or software must be made according to your setup, your playing conditions (studio recording vs. live performances), your musical instrument, your budget, etc.
List of compatible devices
| Name | Type | Upload of 3rd IR files | Blending |
|---|---|---|---|
| Line 6 Helix | Effects processor | Yes | Yes |
| Headrush MX5 | Effects processor | Yes | Yes |
| TC Electronic Impulse IR Loader | IR Loader | Yes | No |
| Two Notes Torpedo Captor X | IR Loader / DI box | Yes | Yes |
| Fog Convolver 2 | Plugin VST | Yes | No |
| Nugen Audio Paragon ST | Plugin VST | Yes | – |
| Lancaster Audio Pulse | Plugin VST (free) | Yes | Yes |
In the table, the ‘Type’ column specifies the type of devices. The ‘Upload of 3rd IR files’ column is important. This is an option offered by some devices to load 3rd party impulse response files. Without this possibility, you cannot add your own files and are obliged to use the files integrated by the manufacturer. Finally, the ‘Blending’ column characterizes the possibility of mixing impulse response files together. If the device allows it, it is possible to mix the impulse response of a Martin D-28 guitar with that of a Martin D-18 guitar. This allows to generate new tone. However, the result is sometimes imperfect because of ‘synchronization’ problems between the Impulse Response files. Any user has to do some tests.
The list of devices is not exhaustive, because there are dozens of other possibilities offered by many companies (Nux, BOSS, Zoom, Mooer, Altiverb and many others).
Sources: Zylia 3D Solutions Audio, mh acoustics, ResearchGate.