In general, the sounds produced by a digital piano are based on sampling, by which acoustic piano sound samples are stored in ROM. The samples stored in digital pianos are usually created using high-quality pianos, professional microphones, and high-quality preamps in a professional recording studio. Usually multiple samples are available for the same keystroke, attempting to reproduce the diversity of sounds observed on an acoustic piano. However, sample-based digital pianos do have limitations on the faithfulness with which they reproduce the sound of an acoustic piano. Because samples are taken for only a limited number of intensity levels, digital pianos usually lack the continuous timbral changes that characterize acoustic pianos. They may also lack the harmonic tones that result when certain combinations of notes are sounded and the natural reverberation that is heard when an acoustic piano is played percussively. They often lack the incidental acoustic noises associated with piano playing, such as the sounds of pedals being depressed and the associated machinery shifting within the piano, which some consider a benefit. These limitations apply to most acoustic instruments and their sampled counterparts, the difference often being described as "visceral".