YouTube Speech Audio Filter (TamperMonkey UserScript)

userscript.js

// ==UserScript==
// @name         YouTube Speech Audio Filter
// @namespace    NONE
// @version      1.0
// @description  I wrote this because of background noise in Rachel Thomas's Computational Linear Algebra series, but it probably could be used elsewhere.
// @author       August Karlstedt
// @match        https://www.youtube.com/watch?v=*
// @grant        none
// @run-at       document-end
// ==/UserScript==

(function() {
    'use strict';

    var audioCtx = new (window.AudioContext || window.webkitAudioContext)();
    var source = audioCtx.createMediaElementSource(document.querySelector('.video-stream'));

    var highpassFilter = audioCtx.createBiquadFilter();
    highpassFilter.type = "highpass";
    highpassFilter.frequency.value = 440;

    var lowpassFilter = audioCtx.createBiquadFilter();
    lowpassFilter.type = "lowpass";
    lowpassFilter.frequency.value = 4400;

    source.connect(highpassFilter);
    highpassFilter.connect(lowpassFilter);
    lowpassFilter.connect(audioCtx.destination);

})();