Why are impulse response files so important for acoustic instruments?

Acoustic instruments produce a natural sound amplified by resonance chambers specific to their design. However, this sonic authenticity becomes a challenge when it comes to sound reinforcement for these instruments in an environment requiring more powerful projection, such as a large concert stage. This requires the use of a pickup, which distorts the timbre of the musical instrument. Impulse response files are presented as the solution capable of reproducing the acoustic characteristics of the instrument. How do these files work? How can they enrich the musicians’ sound experience?

The importance of the acoustic properties of acoustic instruments

Often overlooked by musicians, Impulse Response (IR) files have a lot to offer to acoustic instrument players.

Natural tone of an acoustic musical instrument

The sound production of an acoustic instrument involves the natural amplification of sound through a resonance chamber. For instance, the sound of an acoustic violin is generated by the friction of a bow on the strings of the instrument. The vibration of the strings produces a sound that is transmitted from the bridge to the soundboard of the violin. The resonance chamber receives the sound signal and naturally amplifies it.

This natural amplification occurs without the need for an external system to the instrument. In contrast, an electric guitar requires an amplifier to amplify the signal it produces.

The difficulty of amplifying acoustic instruments

The downside of acoustic instruments is that they were designed to generate sound naturally, making them challenging to amplify. Take the example of a violin: if a violinist needs to perform on a large stage alongside amplified instruments (such as an electric guitar, bass, synthesizer, etc.), they must find a way to amplify their acoustic musical instrument to be heard. Without amplification, the sound they produce is inaudible to the audience due to being drowned out by the deafening noise of the other instruments.

In such cases, violinists often opt to install a piezoelectric pickup. This device transforms the vibrations into an electrical signal transmitted to an amplifier.
Unfortunately, this type of piezoelectric pickup is incapable of reproducing the instrument’s acoustics. This acoustic quality is achieved through the propagation of sound waves in the air until they are perceived by someone. A piezoelectric pickup captures the vibrations at the source without considering the propagation of sound waves in the air. The resulting sound is described as dry, direct, and lacking in resonance, often leading to disappointment for musicians. See below an example of a pickup installed on an acoustic violin. To know more, please read our article ‘How to amplify an acoustic violin?‘.

The use of IR files to reproduce the instrument acoustic properties

Acoustic instruments Impulse Response files address the absence of this acoustic quality. They simulate the acoustics of your instrument, generating a very close acoustic ambiance. This particular case applies to all acoustic instruments: acoustic guitars, violins, double basses, cellos, banjos, ukuleles, mandolins, etc.

How to use an impulse response file with an acoustic instrument?

You need to find a file corresponding to your instrument. If the sound spectrum of your instrument is similar to that of another instrument, impulse responses will work. For example, a viola and a violin have very similar sound spectra. The viola produces a deeper tone, being a fifth below the violin. However, their sound spectra remain very close. Therefore, a viola impulse response works with a violin, and vice versa.

Once you have acquired the impulse response file relative to your instrument, you need to integrate it into a system capable of processing the file to transform the sound. This system can be software, a plugin, a small effect pedal, or a multi-effects processor. The choice depends on your needs.

For live performances, you should opt for an effects processor or a small pedal (called an IR loader). This element will be positioned after the pickup of your instrument but before the sound is transmitted to the mixer.

For studio audio production, it is better to use the effects offered by your DAW or install a dedicated plugin. Look for convolution reverb plugins as well as a third party list of free VST impulse response loader plugins.

Example done with an acoustic guitar

The video below uses a source signal captured from an electric guitar equipped with its own pickup, and the same signal to which a Taylor Impulse Response file has been applied.

The source signal from the electric guitar is raw, without any effects or audio post-processing. It is directly captured via the guitar’s jack output. The final signal, that you can hear, is completely transformed thanks to the use of an impulse response. It was applied using a VST plugin integrated into a DAW. No other effects or equalization have been applied to this final signal. Only the processing resulting from the use of the IR file is responsible for the new sound of the electric guitar.

Of course, using a different impulse response file will alter the tone of the guitar to some extent. Each parameter during the recording influences the capture of the impulse response. The parameters are infinite: the musical instrument and its characteristics, the location of the capture, the microphone(s) used, the position of microphone(s), etc.

Are these impulse response files essential?

The answer to this question can only be yours. It depends on your desire to get closer to an acoustic sound. How important is this to you? How far do you want to push the sound quality offered by your instrument?

An impulse response file will never match the true acoustic of your instrument, a Stradivarius, or a Guarneri. No matter the quality of the file, you will never achieve a 100% equivalent result. IRs operate on acoustic simulation. They are designed to try to make real what is not. Therefore, they should be seen as a dynamic addition to the sound production of your instrument. It’s an enhancement to add to your knowledge! However, it’s essential not to neglect other effects. The use of impulse response files should not stand alone. Simply using the file is not enough to achieve an ideal sound. It will transform it, but it’s necessary to work with other effects, especially an equalizer. There’s a balance to be found between each element to achieve what you’re looking for.

The use of impulse response files represents a significant advancement for acoustic instrument musicians wishing to preserve the authenticity of their timbre despite amplification. This is certainly not the natural timbre of their instrument, but a sound reproduction of another instrument. These files do not achieve the perfection of natural sound projection, nor the authenticity of the instrument being played. Nevertheless, they offer a nuanced reproduction of the acoustic properties of the musical instrument. Judiciously integrated into the produced audio signal, impulse response files are the key to addressing the problem of dry and direct timbre produced by pickups.