There is nothing which is unable to serve as a medium for sound, although tone manifests more clearly through a sonorous body than through a solid one, the former being open to vibrations, while the latter is dosed. All things which give a clear sound show life, while solid bodies choked up with substance seem dead. Resonance is the reserving of tone; in other words it is the rebound of tone which produces an echo. On this principle all instruments are made, the difference lying in the quality and quantity of the tone, which depend upon the construction of the instrument. The instruments of percussion, such as the tabla or the drum, are suitable for practical music, and stringed instruments like the sitar, violin or harp are meant for artistic music. The vina is especially constructed to concentrate the vibrations; as it gives a faint sound, audible to the player only, it is used in meditation.