practice amps and basic combo amps may only have a single indicator light: an LED to indicate when the amp's power is on. More expensive amps may also have LEDs to indicate when the preamp has a signal present from the instrument; when a limiter or similar speaker protection feature is activated; when clipping is occurring; or when the amp is in standby mode. Amplifying the double bass Almost all bass amplifiers are designed for use with an electric bass, which has magnetic pickups. The signal from a double bass usually comes from a piezoelectric pickup mounted on the bridge or beneath the feet of the bridge. These pickups require a preamplifier or preamp-equipped DI box before the signal is sent to the bass amp. The preamplifier helps to ensure that the impedance of the pickup signal matches the impedance of the amplifier, which improves the tone. Some preamplifiers also have equalizers which can be used to modify the tone. Double bass players performing in genres where the bass is slapped, either by pulling the string until it snaps back onto the fingerboard or striking the strings, such as traditional blues, rockabilly, psychobilly jazz, folk, and bluegrass often blend the sounds picked up by a piezoelectric transducer with the sounds picked up by a small condenser microphone mounted on the bridge. The microphone picks up the resonance coming from the body and the sounds of the strings being plucked, bowed, or slapped. The two sound signals are blended using a simple mixer and then routed to the amplifier. While many upright bass players use combo amplifiers, bassists in genres that use high stage volume, such as the punk-rockabilly genre of psychobilly use "bass stacks". Some jazz bassists and other bass players who play in small venues use specialized, expensive upright bass amps, like the Acoustic Image combo amplifier. Double bass players playing in genres where a louder amplified tone (emphasizing the fundamental frequencies) is desired may encounter audio feedback. Feedback for double bass generally manifests itself as a sharp, sudden high-volume "howling" sound that can damage loudspeakers. When acoustic instruments with resonant bodies are amplified with microphones and piezoelectric transducer pickups, they are prone to have feedback problems. For acoustic bass guitars, soft plastic discs are available to block the instrument's sound holes, thus reducing feedback. Upright bass players sometimes use homemade foam inserts to fill in the "f" holes.