Characterisation of nonlinearities of electroacoustic devices using volterra kernels

Classical methods of characterization such as Total Harmonic Distortion, SMPTE Intermodulation or Difference Frequency Intermodulation, are widely used for electroacoustic device characterization. However, these methods only put in evidence some of the nonlinear effects for arbitrary excitations, resulting in non-ideal transfer functions, or interaction between two frequencies. More complex effects are not taken into account and simply ignored. We propose the use of Volterra kernels as a mean to obtain further characteristics of an electroacoustic device. By applying an appropriate multitone excitation, we can trigger the apparition of distortion products that did not show up with a single or two tones excitation. Frequency repartition in a multitone signal must be chosen carefully to avoid the risk of frequency recovering. A careful choice of the number and magnitude and phase of the excitation components allows an easy and effective extraction of Volterra kernels at their different orders. When expressed in the frequency domain, these latter constitute a generalization of transfer functions at orders greater than one. The rules for a valid composition of excitations will be exposed here. Under the assumption that the electroacoustic device is weakly nonlinear and by using the described multitone signal, an efficient, and above all, systematic characterization can be achieved. The experimental method of analysis of the device response to a multitone excitation, based on Volterra series, will be explained. This allows us to obtain the Volterra kernels, containing thorough information about distortion products at different orders. In this article we will present results containing the simulation of the nonlinear system response and compare experimental and simulated results.