Low-Frequency Loudspeaker Assessment by Nearfield Sound-Pressure Measurement

A loudspeaker test technique is described which depends on nearfield pressure mea-surements made in a nonanechoic environment. The technique allows extremely simple measurements to be made of frequency response, power response, distortion, and electroacoustical efficiency. GLOSSARY OF SYMBOLS r distance from pressure sample point to center of piston a radius of circular radiator R _ dc resistance of driver voice coil ao radius of diaphragm, = _/SD/_r E surface area ay radius of circular vent, = 5/Sv/_r SD effective projected surface area of driver diaphragm c velocity of sound in air, = 343 m/s Sv cross-sectional area of vent ein voltage applied to driver input SPL sound pressure level, in dB re 20/xN/m 2 [ frequency, in Hz Uo output volume velocity of acoustic radiator [ _ Helmholtz resonance frequency of vented box 3. wavelength of sound in air, = c//]s low-frequency cutoff (--3 dB) of speaker system, / nominal power transfer efficiency, = Pa/P_ Io acoustic intensity, in power per unit area, */o reference efficiency defined for radiation into a: pa/(2 po c) for a plane wave half-space free field k wave number, = 2_r/3. = to/c po density of air, = 1.21 kg/m s at 20 ° C