Low-Frequency Loudspeaker Assessment by Nearfield Sound-Pressure Measurement*

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 p peak sound pressure co radian frequency variable, = 2 _-[. Pi* peak sound pressure in farfield of acoustic radiator Pz¢ peak sound pressure in nearfield of acoustic radiINTRODUCTION: The low-frequency evaluation of a ator loudspeaker system with respect to frequency response, P/grins root mean square sound pressure in nearfield of distortion, and power output has traditionally required the radiator, = pzdV2 use of a large and expensive anechoic chamber or a cumPR peak Sound pressure on axis of piston at distance r bersome and often equally costly open-field outdoor testPa acoustic output power lng Site. Recently, Small [1] pointed out that valid measureP_ nominal electrical input power ments could be made at very Iow frequencies in any reaQ ratio of reactance to resistance (series circuit) or sonable environment by sampling the pressure inside the resistance to reactanc e (parallel circuit) enclosure) QB Q or cabinet at/_ considering all system losses