Speakers use a vibrating membrane or diaphragm to produce sound in response to an electric, digital, or wireless signal. They include an integral or external amplifier, drivers, and a crossover circuit that divides an audio signal into specific bands. There are three basic types of speaker drivers: woofers, tweeters, and midrange devices. Woofers, the largest drivers, are designed to produce low frequency sounds. Tweeters are much smaller than woofers and are designed to produce higher frequencies. Speakers are often housed in a case, chassis or enclosure made from wood, metal, plastic, or composite materials. Sealed speaker enclosures or acoustic suspension enclosures prevent air from escaping so that the internal air pressure is constantly changing. Other enclosure designs redirect the inward pressure outward, using it to supplement the forward sound wave. In these bass reflex speakers, the backward motion of the diaphragm pushes sound waves out of the port in order to boost the overall sound level.