Band Pass Butterworth Filter

Band-pass filters allow audio above a specified frequency range and bandwidth to pass through to an output. The center frequency is the starting point from where the frequency limit is set. Adjusting the bandwidth sets how far out above and below the center frequency the frequency band should be. Anything above that band should pass through.

import AudioKitPlaygrounds
import AudioKit

let file = try AKAudioFile(readFileName: playgroundAudioFiles[0],
                           baseDir: .resources)

let player = try AKAudioPlayer(file: file)
player.looping = true

Next, we'll connect the audio sources to a band pass filter

var filter = AKBandPassButterworthFilter(player)
filter.centerFrequency = 5_000 // Hz
filter.bandwidth = 600 // Cents
filter.rampTime = 1.0
AudioKit.output = filter

User Interface Set up

class PlaygroundView: AKPlaygroundView {

    override func setup() {
        addTitle("Band Pass Butterworth Filter")

            player: player,
            filenames: playgroundAudioFiles))

        addSubview(AKBypassButton(node: filter))

            property: "Center Frequency",
            format: "%0.1f Hz",
            value: filter.centerFrequency, minimum: 20, maximum: 22_050,
        ) { sliderValue in
            filter.centerFrequency = sliderValue

            property: "Bandwidth",
            format: "%0.1f Hz",
            value: filter.bandwidth, minimum: 100, maximum: 1_200,
        ) { sliderValue in
            filter.bandwidth = sliderValue

import PlaygroundSupport
PlaygroundPage.current.needsIndefiniteExecution = true
PlaygroundPage.current.liveView = PlaygroundView()