When a solid body vibrates, the alternating strain within it induces a correlated dissipation of energy resembling internal static or rubbing friction. No material is purely elastic. Elasticity must be regarded as having a phase angle which varies little, or not at all, with frequency, and in metals is a small fraction of 1°. This friction introduces losses in acoustic apparatus in amounts depending on design, and also on choice and heat treatment of materials. Taking metals as a class, this dissipation is generally least in light materials like aluminum and duralumin, and greatest in the heaviest substances like lead. They may be varied by a factor of as much as 10 by heat treatment.