Volume 127, Issue 1, January 2010
Index of content:
- ACOUSTICAL MEASUREMENTS AND INSTRUMENTATION 
127(2010); http://dx.doi.org/10.1121/1.3270392View Description Hide Description
This paper evaluates the potential of the field separation method (FSM) for performing subwoofermeasurements in a small test room with poor absorbing properties, as is commonly available. The FSM requires the knowledge of both acoustic pressure and velocity fields on a closed surface surrounding the tested source. Pressures and velocities,measured using a p-p probe on a half-sphere mesh, are collected under various conditions: in a room with variable reverberation time (6.4–0.6 s) and with four measurement half-sphere radii. The measured data are expanded on spherical harmonics, separating outward and inward propagation. The pressure field reflected by walls of the surrounding room is then subtracted from the measured field to estimate the pressure field that would have been radiated under free-field conditions. Theoretical frequency response of the subwoofer is computed using an analytical formulation derived from an extended Thiele and Small model of the membrane motion, coupled to a boundary element model for computing the radiated pressure while taking into account the actual subwoofer geometry. Measurement and simulation results show a good agreement. The effects of the measurement distance, the measurement point number, and the room reverberation time on the separation process are then discussed.
Full bandwidth calibration procedure for acoustic probes containing a pressure and particle velocity sensor127(2010); http://dx.doi.org/10.1121/1.3268608View Description Hide Description
Calibration of acoustic particle velocitysensors is still difficult due to the lack of standardized sensors to compare with. Recently it is shown by Jacobsen and Jaud [J. Acoust. Soc. Am.120, 830–837 (2006)] that it is possible to calibrate a sound pressure and particle velocitysensor in free field conditions at higher frequencies. This is done by using the known acoustic impedance at a certain distance of a spherical loudspeaker. When the sound pressure is measured with a calibrated reference microphone, the particle velocity can be calculated from the known impedance and the measuredpressure. At lower frequencies, this approach gives unreliable results. The method is now extended to lower frequencies by measuring the acoustic pressure inside the spherical source. At lower frequencies, the sound pressure inside the sphere is proportional to the movement of the loudspeaker membrane. If the movement is known, the particle velocity in front of the loudspeaker can be derived. This low frequency approach is combined with the high frequency approach giving a full bandwidth calibration procedure which can be used in free field conditions using a single calibration setup. The calibration results are compared with results obtained with a standing wave tube.