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What’s an Impulse Response? What does it do?
Sunday 19 January 2020
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An impulse response is the response of an object when hit. For example, a glass will ring at its resonance frequency when hit. This because when you briefly hit an object you excite all the frequencies. If one frequency is resonant, it will "pop out".
A guitar has more than one resonant frequency as illustrated by this youtube video featuring the Chladni patterns of a guitar top. In this video, instead of hitting/tapping the top as some luthier do, a loudspeaker is used to scan all the frequencies. Some powder is put on the guitar top to reveal each resonance pattern.
To further illustrate that, here is a simulation of a guitar top movement when the strings vibrate.
The movement of the top is due its multiple resonances.
Because an undersaddle pickup only sens the vibrations at the saddle, it is impossible to represent such a complexity. On the picture, below the signal chain of an acoustic-electric guitar is represented when amplified by a piezo UST pickup or a microphone. The fingers pluck the strings that vibrates. That vibration is coupled to the top and to the body of the guitar. The top of the guitar then acts as a loudspeaker membrane and pushes the air into motion. That’s where the acoustic tone comes from.
A piezo UST pickup mostly sens the string motion and probably a bit of the guitar bridge motion. Whereas a microphone will sens the air motion giving a much more realistic "picture" of the acoustic tone.
The idea of Impulse response convolution is to recreate that acoustic tone by capturing the response of the guitar top through a microphone. That "impulse response" (IR) can then be loaded into an IR loader pedal that will be added between the pickup and the amplifier/PA system.
Link to read what Larry Fishman has to say about IR.
