The Stroh violin or Stroviol is a type of stringed musical instrument that is mechanically amplified by a metal resonator and horn attached to its body. The name Stroviol refers to a violin, but other instruments have been modified with the amplification device, including the viola, cello, double bass, ukulele, mandolin, and guitar. Johannes Matthias Augustus Stroh, an electrical engineer from Frankfurt, invented the instrument in London in 1899.