The acoustic impedance Z of a surface or medium is the ratio of the amplitude of the SOUND PRESSURE p and the amplitude of the PARTICLE VELOCITY v of an acoustic WAVE that impinges on the surface or medium. By analogy to Ohm's law for electrical impedance,
Z = p / v
For air, Z = r . c where r is the density and c the SPEED OF SOUND. The numerical value of Z for air is 43 g/cm2 -sec. If a sound wave changes media, the ratio of the acoustic impedances of the media determines the efficiency of the energy transfer. The acoustic impedance of the eardrum, for instance, corresponds well with that of the auditory canal, guaranteeing maximum efficiency of energy transfer, but it does not correspond well with air. The PINNA may be described as an impedance matching device between the air and the auditory canal. Likewise, the flared end of a trumpet results in less energy being lost by being reflected back down the tube of the instrument.
Compare: AMPLIFICATION, RESONATOR.
Surfaces may be thought of as falling into two groups: those that are impervious to the passage of air, and those that are not. The first group includes most structural materials (brick, concrete, etc.), the second group includes materials such as foams, acoustic tiles, etc. High impedance materials are found in both groups. For example, both concrete and densely packed glass fibre are high impedance materials relative to air, whereas grass and some foams are low impedance surfaces.
See: SOUND PROPAGATION. Compare: ABSORPTION COEFFICIENT, NOISE ISOLATION CLASS, NOISE REDUCTION COEFFICIENT, SOUND INSULATION, SOUND TRANSMISSION CLASS.