A system for modifying an audio signal comprises a computer including a first digital signal processor (DSP). The first DSP includes a first module and a second module. The first module is coupled to the interface to provide a musical instrument with a set of testing input signals and obtain a set of testing output signals from the musical instrument via the interface. The second module is configured to perform functions including: analyzing the set of testing output signals to obtain a set of parameters, constructing an acoustic transducer to model characteristics of the musical instrument based on the set of parameters, and receiving the audio signal to modify the audio signal using the acoustic transducer, wherein the first DSP module obtains the set of parameters at least by identifying occurrences when an output level state determined at least based on the set of testing output signals significantly changes.