This Planet Waves O-Port is proven to enhance the sound of any acoustic guitar and kill feedback without the use of electronics. It"s has been tested under the scrutiny of world-renowned acoustic engineers and sound producers to improve tonal quality. The result is a fuller, richer sound with noticeably increased sustain and harmonic range. The tonal enhancement provided by the Planet Waves O-Port is particularly pronounced when the acoustic guitar is plugged in and amplified, resulting in a much fuller sound, a stronger signal to the internal pickup, and significant feedback control. The Planet Waves O-Port fits in most acoustic guitars with a round soundhole, regardless of make or model. The Planet Waves O-Port"s material is custom blended for flexibility to prevent damage to your guitar finish and inlays. The O-Port suppresses feedback and gives your acoustic or acoustic-electric guitar a fuller sound with more volume, better clarity, and enhanced projection. Just measure your acoustic guitar"s soundhole and choose the Planet Waves O-Port that fits best. Small: 3.375" - 3.5" Large: 3.875" - 4"Non-electrical acoustic sound enhancement guitar accessory Custom-blended, flexible material prevents damage to your guitar finish and inlays Fits in most acoustic guitars with a round soundhole Fuller sound Stronger signal to internal pickups Increased feedback control More volume Better sound clarity Enhanced sound projection Easy installation