1. A computer-implemented method of determining a configuration for an audio processing operation, wherein the audio processing operation comprises a predetermined set of one or more audio processing sub-operations, each audio processing sub-operation being configurable with one or more respective configuration parameters, the method comprising: specifying the predetermined set of one or more audio processing sub-operations; specifying a target frequency response; and performing a convergent optimization process to determine a configuration for the audio processing operation that reduces a difference between the frequency response of the audio processing operation and the target frequency response, wherein the configuration comprises a respective value for each configuration parameter of each audio processing sub-operation; wherein the convergent optimization process comprises a plurality of iterations for reducing a difference between the frequency response of the audio processing operation and the target frequency response, wherein performing the convergent optimization process comprises: for an audio processing sub-operation, at each iteration identifying, for one or more control settings related to that audio processing sub-operation, a corresponding value from a plurality of test values identified for that control setting for that iteration; and determining each configuration parameter of that audio processing sub-operation as a respective function of at least one of the one or more control settings related to that audio processing sub-operation; wherein, for the or each control setting, one or more of the following apply: (a) a change in the frequency response of the respective audio processing sub-operation caused by adjusting that control setting is monotonically related to the adjustment of that control setting; (b) adjusting that control setting causes a substantially localized change in the frequency response of the respective audio processing sub-operation; (c) the magnitude of a change in the frequency response of the respective audio processing sub-operation caused by adjusting that control setting is substantially proportional to the magnitude of the adjustment of that control setting.