Devices that accept a signal but allow only certain frequencies within those signals to pass through, blocking all others, are called band-pass filters. Some of these filters are electronic, and can be used in audio equipment, radio equipment, or other types of circuits. Others are optical and allow only certain wavelengths, or frequencies, of light to pass through.
A band-pass filter is made up of two filters put together: a high-pass filter, which lets frequencies above a certain threshold pass through, and a low-pass filter, which allows frequencies below a certain threshold through. An audio filter that allows only frequencies between 20 and 20,000 hertz through, as those represent the typical range of human hearing, is a good example of a band-pass filter. This filter would combine a 20,000 hertz low-pass filter with a 20 hertz high pass filter to accomplish this.
Electronic band-pass filters can be put into an integrated circuit (IC) chip, or can be made out of resistors and either capacitors or inductors, although capacitors are generally preferred. By changing the resistance values of the resistors and the capacitance rating of the capacitors, the effective area of the filter can be changed. Many filters use variable resistors to allow their effective area to be easily changed by the user, eliminating the need to rebuild them for minor tweaks.
One of the key challenges in filter design is that most band-pass filters have a gradual effect. Taking the above example, the filter would not eliminate all frequencies below 20 hertz and above 20,000 hertz but would, instead, gradually reduce their size. The gradual effect of the filter can be modified by a more complicated structure, using additional parts.
Band-pass filters are also used in the world of optics, although instead of only letting certain frequencies of signal through, they allow only certain wavelengths of light through. An example is a pair of anaglyph red-blue 3D glasses. The red lens is a band-pass filter which allows the red band of light to pass, and the blue lens only allows the blue band of light to pass. Astronomers also use band-pass filters, for instance, to look only at ultraviolet or infrared light emitted from distant stars and galaxies to better understand the cosmos.