The purpose of a low-pass filter is to filter out frequencies above a user-definable point, known as the cutoff, while allowing frequencies below the cutoff to pass through to the output. The simplest low-pass filter, known as a high-cut filter, has a single parameter for establishing the cutoff point in hertz. More sophisticated low-cut filters may include a resonance parameter, which is used to empahasize frequencies at the cutoff point. Other parameters for managing the filter’s effect are sometimes available.
In most cases, low-pass filters are used in audio production to remove unwanted higher-frequency noise from recordings of lower-pitched sounds. For example, recording a drum kit often involves multiple microphones, including a dedicated microphone for the kick drum. During mixdown, the audio engineer will often apply a low-pass filter to the kick drum track to filter out higher-frequency “bleed” from other kit elements, such as the snare.
Some home and car stereos include a basic low-pass filter, but its purpose and effect in this context is somewhat different from that seen in audio production. In car audio, a low-pass filter is used to designate what frequency range is sent to the subwoofer. All frequencies below the cutoff point will be passed to the subwoofer, thus directing the bass frequencies to the appropriate output. A low-pass filter on a home stereo functions in the same way, provided there is a subwoofer present.
Similar filters include high-pass and bandpass filters. High-pass filters function like low-pass filters, but allow frequencies above the cutoff to pass. Bandpass filters are used to define a frequency range, called a “band,” anywhere within the audible spectrum, and allow that band to pass. Equalizers, though not true filters, can perform a similar function by cutting specific frequency bands. Most graphic equalizers include basic low- and high-pass filtration.