Spectral cues used in the localization of sound sources on the median plane

The spectral cues used for median-plane localization are described by three experiments. First, the frequency spectrum necessary for localization is measured by noting the accuracy of subjects localizing low- and high-pass-filtered white noise. Second, several high-pass, low-pass, bandpass, and bandstop filters are associated with the subjective impression of direction by observing what directions are most frequently perceived by subjects localizing white noise colored by each filter. Third, the frequency responses of several artificial ears are measured for different angles of median-plane sound incidence. Results show that sound spectra from 4 to 16 kHz are necessary for localization. Frontal cues are a 1-octave notch, with a lower-frequency cutoff between 4 and 10 kHz and increased energy above 13 kHz. The “above” cue is a 1/4-octave peak between 7 and 9 kHz, with a high-frequency cutoff at 10 kHz. The “behind” cue is a small peak from 10 to 12 kHz. Increases in frontal elevation are signaled by an increase in the lower cutoff frequency of the 1-octave notch. This notch appears to be generated by time-delayed reflections off the posterior concha wall interfering with sound directly entering the external auditory canal.