stopbands

Stopbands are frequency ranges in which energy is strongly attenuated or cannot propagate, depending on the context. They are a fundamental concept in filter design and wave propagation. In signal processing, a stopband is the portion of the spectrum where a filter suppresses signals, as opposed to a passband where signals are transmitted with minimal attenuation. The stopband is defined by a specified attenuation level, typically expressed in decibels (As). In a low-pass filter, the stopband lies above the cutoff frequency; in a high-pass filter, it lies below. For band-pass and band-stop filters, stopbands occur outside the desired passbands.

Attenuation in the stopband increases with filter order and design complexity. The transition band lies between the passband edge and the stopband edge, where attenuation rises from nominal passband values to the stopband specification. Real filters exhibit finite attenuation in the stopband and may show ripple in the passband; specifications include Ap for passband ripple and As for stopband attenuation. Digital and analog implementations share these concepts, with digital filters using normalized frequencies.

In physics and engineering beyond conventional filters, stop bands (band gaps) appear in periodic media such as photonic and phononic crystals, where waves cannot propagate within certain frequency ranges due to Bragg scattering or other periodic effects.

Applications of stopbands include suppression of adjacent-channel interference in communications, rejection of unwanted frequencies in audio and instrumentation, and the design of spectral filters to shape signals or guide wave propagation. Common filter families used to realize stopbands include Butterworth, Chebyshev, elliptical, and equiripple designs.