When the directional response pattern is highly symmetric about the reference axis, which is commonly the case for directional microphones isolated in a free field, Q can be determined from data obtained only in the horizontal plane at [alpha] = 0[degrees] between the azimuth angles of 0[degrees] and 180[degrees] (11), (12).
The standard uncertainties [u.sub.6] arise from imperfections in setting the elevation angle in combination with imprecise vertical aiming of the directional response pattern. Limits used to calculate [u.sub.6] were estimated from the results of calculations reported for a second-order directional system using an elevation angle misalignment of no more than [+ or -]2[degrees] for any of the elevation angles .
For these measurement conditions, the directional response patterns of the hearing aids tested are considerably asymmetric about the reference axis.
To show examples of the directional response patterns programmed in the aids used in the interlaboratory comparison and intrinsic to the unaided manikin, polar response patterns measured at NIST in the horizontal plane at [alpha] = 0[degrees] for three of the programmed memories and the unaided manikin are displayed in Fig.
was calculated for each of the 119 values of [Q.sub.N] measured (seven directional response patterns as a function of seventeen frequency bands).