Under the hood, this web app uses the Web Audio API — a set of audio-processing tools built directly into modern browsers, not a third-party plugin.
Because the filtering happens with native browser technology, there's no Flash, no Java applet, and no extension to install. It runs the same way in any modern browser that supports the standard, which today is effectively all of them.
Each band is powered by a BiquadFilterNode — a standard Web Audio building block for shelving and peaking filters. Stacking four of them in sequence is what gives the whole tool its real-time, four-band shaping ability without any external library.