The Butterworth filter is a medium-Q filter that is used in designs the require the amplitude response of the filter to be as flat as possible. The Butterworth response is the flattest pass-band response available and contains no ripple. The typical response of such a filter might look like that of Fig. 3-8.
Since the Butterworth response is only a medium-Q filter, its initial attenuation steepness is not as good as some filter but it is better than others. This characteristic often cause the Butterworth response to be called a middle-of-the-road design.