A car subwoofer is a specialized type of speaker designed exclusively to reproduce very low audio frequencies, commonly known as bass. Conventional door or dashboard speakers are designed primarily to play mid and high tones, such as the human voice or the higher frequency sound of musical instruments. Due to their smaller size and design, deep bass puts a physical strain on them and cannot be played fully and cleanly. A subwoofer therefore focuses on the part of the sound spectrum that is represented in music by, for example, bass guitar, kick drum or thumping electronic sounds. To function properly, it requires a rigid and sealed enclosure called a soundboard or "box" that is designed for a specific type of speaker. Since reproducing low frequencies requires a significant amount of power, a separate amplifier is almost always needed to drive the subwoofer. This supplies the necessary power that a conventional car radio would not be able to provide. There are also active subwoofers that have the amplifier already built right into the sound box, making installation easier. Its job is to add the lowest tones to the sound system and give a sense of depth and fullness to the listening experience.