A band-pass filter (BPF) passes only a certain band of frequencies and reduces the content below and above it. It works like a high-pass and a low-pass combined. You can use it to bring out part of a sound for checking, to create a telephone or radio texture, or to narrow the band feeding a distortion.
Deciding the range that passes
With a band-pass you decide the center frequency and how wide a range passes. The width is related to Q and bandwidth. A wide setting leaves the mids sounding natural, while a narrow setting makes a specific component stand out. The slope at both ends also affects the tone.
Using it for sound design
Narrowing a synth or a noise source to a thin band creates telephone, radio, or filter-sweep effects. It is also used to bring out part of the character of a guitar or drums. Strong settings are noticeable, so decide whether the sound is the star of the track or sits in the background.
Using it to find a band
Sweeping a temporarily narrow band-pass makes it easier to find where a harsh sound or the content you want is located. Do not leave it there, though; once you find the spot, go back to the EQ processing you actually need. Keep the settings for searching separate from the settings for the final sound.
The effect of Q and resonance
The narrower the width, the more it concentrates on a specific band, and a high resonance emphasizes the area around the cutoff. Extreme settings can seem to raise the level, so match output levels when you compare. Check the effect on phase and stereo image in both stereo and mono.
How it differs from other filters
A high-pass reduces the low end and a low-pass reduces the high end. The difference with a band-pass is that it reduces both sides and leaves only the middle. Choose the shape that fits your purpose. The basics of choosing filters in an EQ are explained in the EQ article.
