In the 1970s, experimenters noticed that bare speaker wires could deliver a jolt and began using recorded and live sound for electrostimulation. At that time, there were no professionally made attachments for such play, so people built their own out of copper plumbing parts and other metal pieces with attention to resistors placed in series with the human parts to control the current for safety. Although early e-stim units used only a simple, pulsed, sinusoidal wave, newer units use more complex wave forms and also allow for the use of ambient sound or prerecorded wave forms like music or specially designed computer files for specific types of stimulation. There are now sites dedicated to the creation of MP3 files specifically for erotic journeys or symphonies, which can include such routines as rewards, punishments, very strong, and pleasantly soft portions.