[VIDEO]http://www.youtube.com/watch?v=5ze_Np09OZk]YouTube - Planet Waves O-Port Product Demo[/VIDEO]
Pretty interesting idea, and worth a shot for $27. Jump to 3:30 or so for the actual sound comparison.
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.