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Encyclopedia > Bandpass
The frequency axis of this symbolic diagram would be logarithmically scaled.

A band_pass filter is (usually) an electronic circuit that lets through frequencies between two other given frequencies. For example, an ideal bandpass filter would let through all signals above 30 hertz but below 100 Hz. All of the signal outside this range is attenuated or damped. See RLC circuit for basic theory regarding the frequencies passed.

It can be created by a combination of a low-pass filter and a high-pass filter. In practice, no bandpass filters are ideal and do not attenuate frequencies just outside the desired frequency range completely. There is generally a smooth and quick decrease in transmitted frequency outside the band. This is known as the roll-off, and is usually expressed in dB per octave.

In the atmospheric sciences, for example, it is common to band-pass filter the data with a period range of, say 3 to 10 days, so that only cyclones remain as fluctuations in the data fields.

Between the lower cutoff frequency f1 and the upper cutoff frequency f2 of a frequency band there is the center frequency f0.
It is calculated as the geometric mean:

Often a mistake is made in calculating the arithmetic mean as the passed band:

If, for instance, we are looking for the center frequency of the telphone audio band from 300 Hz to 3300 Hz, we get (3300 + 300) / 2 = "1800 Hz" for the short arithmetic mean calculation, but the root of 300 x 3300 = "995" Hz with the correct geometric mean formula.

The bandwidth of the filter is simply the difference between f2 and f1.

• Calculations and comparisons between the geometric mean and the arithmetic mean (http://www.sengpielaudio.com/calculator-geommean.htm)

Results from FactBites:

 audio gear reviews - Enclosure Design - Bandpass Enclosures (1248 words) Bandpass enclosures differ from the rest of the enclosures in that all of the acoustical output is radiated from a vent or port. Sealed bandpass enclosures are sealed enclosures with the output filtered though a secondary tuned enclosure. Series bandpass enclosure differ from the two previous bandpass enclosures in that the low frequency limit of the woofer can be excursion limited by the tuned port that is vented to the outside of the enclosure.
 Bandpass Calibration (613 words) Bandpass calibration is necessary to correct for complex gain variations as a function of frequency. The shape of the bandpass is determined by the baseband filters as most other components have a flat response as a function of frequency. Sufficient duration implies that the signal-to-noise ratio reached on the bandpass calibrator in a channel for a particular baseline is at least as high as that on the target object.
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