In contrast with reverberation, which is based on the propagation and amplification of the sound waves emerging from an instrument, sustain signifies the autonomous continuation of sound after playing an instrument. Sustain is comparatively short in stringed acoustic instruments such as the ukulele. It has a tendency to become even shorter at the higher frets. Sustain is influenced by the nature and quality of the bridge, nut, frets, fretboard and body. Hard tonewood reflects the sound waves better and ensures a longer sustain.