Funk bass has an "earthy, percussive kind of feel", in part due to the use of muted, rhythmic ghost notes[31] (also called "dead notes").[30] Some funk bass players use electronic effects units to alter the tone of their instrument, such as "envelope filters" (an auto-wah effect that creates a "gooey, slurpy, quacky, and syrupy" sound)[32] and imitate keyboard synthesizer bass tones[33] (e.g., the Mutron envelope filter)[27] and overdriven fuzz bass effects, which are used to create the "classic fuzz tone that sounds like old school Funk records".[34] Other effects that are used include the flanger and bass chorus.[27] Collins also used a Mu-Tron Octave Divider, an octave pedal that, like the Octavia pedal popularized by Hendrix, can double a note an octave above and below to create a "futuristic and fat low-end sound".[35]