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Measurement procedure

The measurements were made in MIT's anechoic chamber. The KEMAR was mounted upright on a motorized turntable which could be rotated accurately to any azimuth under computer control. The speaker was mounted on a boom stand which enabled accurate positioning of the speaker to any elevation with respect to the KEMAR. Thus, the measurements were made one elevation at a time, by setting the speaker to the proper elevation and then rotating the KEMAR to each azimuth. With the KEMAR facing forward toward the speaker (0 degrees azimuth), the speaker was positioned such that a normal ray projected from the center of the face of the speaker bisected the interaural axis of the KEMAR at a distance of 1.4 meters. This was accomplished using a tape measure, plumb line, calculator, a 1.4 meter rod, and a fair amount of eyeballing. We believe the speaker was always within 0.5 inch of the desired position, which corresponds to an angular error of +/- 0.5 degrees.

The spherical space around the KEMAR was sampled at elevations from -40 degrees (40 degrees below the horizontal plane) to +90 degrees (directly overhead). At each elevation, a full 360 degrees of azimuth was sampled in equal sized increments. The increment sizes were chosen to maintain approximately 5 degree great-circle increments. The table below shows the number of samples and azimuth increment at each elevation (all angles in degrees). A total of 710 locations were sampled.

Table 1: Number of measurements and azimuth increment at each elevation

If the KEMAR was perfectly symmetrical and its ear microphones were identical, we would only need to sample either the left or right hemisphere around the KEMAR. However, our KEMAR had two different pinnae, and consequently the responses were not identical. This was actually a bonus, because by sampling the entire sphere we obtained two complete sets of symmetrical HRTFs.



Next: Speaker and headphone Up: HRTF Measurements of a Previous: Measurement technique


kdm@media.mit.edu