Funk bass has an "earthy, percussive kind of feel", in part due to the use of muted, rhythmic ghost notes (also called "dead notes"). 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) and imitate keyboard synthesizer bass tones (e.g., the Mutron envelope filter) and overdriven fuzz bass effects, which are used to create the "classic fuzz tone that sounds like old school Funk records". Other effects that are used include the flanger and bass chorus. 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".