Sallen-Key filters

Hey all.. I'm referring to the Sallen and Key type second order active filters using op-amps... The frequency response of low pass and high pass filters, under certain values of 'Q' has a peak (a sort of feline ear shape), just after or just before the fall/rise of 40 dB/dec (respectively).. Could anyone enlighten me on what the practical application of such a peaked response would be?

Download FilterPro from the TI web site. Install and play with it. As you select Butterworth vs Chebycheff vs Bessel vs Linear Phase filters it will show you the amplitude and phase response of the different filter types. For example you can trade sharpness of knee vs ripple by using Butterworth vs Chebycheff

The peaked response generally is not desirable. But in some cases you may tolerate a small peak since it usually means a sharper rolloff near the corner frequency.

The Butterworth filter is used in audio because it has a flat bandpass (no peak) then a sharp rolloff.