★ FITS in soundholes from 3.82'' (97 mm) to 4.33'' (110mm) diameter.★ DOES not work with nylon guitar strings but only works with steel-string acoustic guitars. ★ DOES not work with 3/4-size guitars like Taylor Baby, Little Martin series. ★ HIGH impedance passive design, no battery required. ★ BUILT with Customized Rare Earth Neodymium Magnet and Single Coil with Handwound Oxygen-Free Copper. ★ SOLID Beech Wood Housing for better transmission of Acoustic Vibrations with Super String Balance and Pure Acoustic Clarity. ★ CAME with Customized Studio Grade Lossless Transmission Audio Cable with 1/4" Female Jack Assembly.