Basically a mobile audio amplifier, a car 'amp' is a term used to refer to a dedicated electronic amplifier separated from the other components of the system. Though most head units have an amplifier, some do not, or lack the desired power or additional features (e.g., equalization controls or crossover systems). External amplification is available and most often used when existing amplification is insufficient. External amplifiers can be mounted in a different part of the car than the "head unit"; in many cases, an additional amp is mounted in the trunk. This is usually the case when powering a subwoofer, where desired wattage may be several multiples more compared to other cabin speakers.