Ultrasonic cleaning is a process that uses ultrasound (usually from 20–400 kHz) and an appropriate cleaning solvent (sometimes ordinary tap water) to clean items. The ultrasound can be used with just water, but use of a solvent appropriate for the item to be cleaned and the type of soiling present enhances the effect. Cleaning normally lasts between 20 and 30 minutes, but can also exceed 90 minutes, depending on the object to be cleaned.