Acoustic music is music that solely or primarily uses instruments that produce sound through acoustic means, as opposed to electric or electronic means. While all music was once acoustic, the retronym "acoustic music" appeared after the advent of electric instruments, such as the electric guitar, electric violin, electric organ and synthesizer. Acoustic string instrumentations had long been a subset of popular music, particularly in folk. It stood in contrast to various other types of music in various eras, including big band music in the pre-rock era, and electric music in the rock era. Music reviewer Craig Conley suggests, "When music is labeled acoustic, unplugged, or unwired, the assumption seems to be that other types of music are cluttered by technology and overproduction and therefore aren't as pure." Types of acoustic instruments Acoustic instruments can be split into six groups: string instruments, wind instruments, percussion, other instruments, ensemble instruments, and unclassified instruments. String instruments have a tightly stretched string that, when set in motion, creates energy at (almost) harmonically related frequencies. Wind instruments are in the shape of a pipe and energy is supplied as an air stream into the pipe. Percussion instruments make sound when they are struck, as with a hand or a stick. History The original acoustic instrument was the human voice, which produces sound by funneling air across the vocal cords. The first constructed acoustic instrument is believed to be the flute. The oldest surviving flute is as much as 43,000 years old. The flute is believed to have originated in Central Europe. By 1800, the most popular acoustic plucked-string instruments closely resembled the modern-day guitar, but with a smaller body. As the century continued, Spanish luthier Antonio de Torres Jurado took these smaller instruments and expanded the bodies to create guitars. Guitar use and popularity grew in Europe throughout the late 18th century and more acoustic instruments were crafted, such as the double bass. Its popularity later spread to cities and towns in the new United States. In the 19th century, the guitar became a recognized instrument played in grand galas and concerts. As electric instruments took hold during the 20th century, many stringed instruments were redefined as acoustic. Instruments that involve striking or vibrating the strings, such as the violin, viola and cello, fall under the acoustic category. The violin became popular during the 16th and 17th centuries, due to technological advancements in building them,