The degree to which two or more sound sources present an equal sound pressure level (equal loudness) when driven with the same material. Balance is generally referred to in stereo music systems but may apply to an entire home theater system. It essentially means that all the channels are emitting an equal signal in terms of loudness and intensity when presented with the same signal. Of course, the various channels in an audio system rarely output the exact same sounds, so a test tone is used to check and adjust balance. When all of the sound sources are balanced, no one source will sound louder than another and attract undue attention to itself or otherwise harm the sound field.