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.