the most significant were set up by erich von hornbostel, and this system generally divides instrumentso membranophones-- [rapid drumbeats] those are the skin, which are rubbed or struck with the mallet or the hand. aerophones, which set a column of air in vibration by splitting an air column over a sharp edge or by the movement of a reed. [jazzy saxophone music] there's also chordophones, which have stretched strings. those, of course, can be set into motion by bows or by plectra or picks. chordophones have all sorts of different timbres depending on what you bring to the instrument. another category is idiophones. those are those instruments that sound like themselves like a bell. its entire structural material is its acoustic material. [rattling] [dinging] we don't hear the steel in a piano so much as we hear the string and the felt hammer itself. idiophones sound like themselves, and there's some other categories too. of course, the new category is electrophones. [synthesized birds chirping and water trickling with new age music] (shapiro) any wave form that you can make in the air,