Click here to purchase the entire book in PDF format. Band-reject Filter Although bandpass filters are very useful at accentuating a small band of frequencies while attenuating others, sometimes we want to do the opposite. What if we want to attenuate a small band of frequencies while leaving the rest alone? This can be accomplished using a band-reject filter (also known as a bandstop filter) which, as its name implies, rejects (or usually just attenuates) a band of frequencies without affecting the surrounding material. The frequency response of this can be seen in Figure 6.4. Figure 6.4: The frequency response of a band-reject filter with a centre frequency of 1 kHz. The thing to be careful of when describing band-reject filters is the fact that cutoff frequencies are still defined as the points where we've dropped in level by 3 dB from the maximum output.