Introduction

　　Active audio filters may be used to compensate problems in frequency response of audio systems and loudspeakers. This paper deals with analog filters. These filters can produce a response that is approximately the inverse of a system response or a loudspeaker acoustic response so that when the two are summed the result is nearly flat. They may also be used to produce the difference of a target response minus the system or loudspeaker response， so that when the two are summed the result is nearly the target. In either case the final response is more accurate or more pleasing.

　　Cell phone and PDA speaker responses like the one shown below often have annoying peaks that reduce intelligibility. This is compensated with the response of a biquad filter， also shown. The sum， the compensated response， is much more pleasing and far more intelligible than the original.