Spatial Sound in Virtual Reality

4 Elevation Cues

While the primary cues for azimuth are binaural, the primary cues for elevation are often said to be monaural. They stem from the fact that our outer ear or pinnae acts like an acoustic antenna. Its resonant cavities amplify some frequencies, and its geometry leads to interference effects that attenuate other frequencies. Moreover, its frequency response is directional dependent.

Figure 3: Pinna

The figure above shows measured frequency responses for two different directions of arrival. In each case we see that there are two paths from the source to the ear canal — a direct path and a longer path following a reflection from the pinna. At moderately low frequencies, the pinna essentially collects additional sound energy, and the signals from the two paths arrive in phase. However, at high frequencies, the delayed signal is out of phase with the direct signal, and destructive interference occurs. The greatest interference occurs when the difference in path length d is a half wavelength, i.e., when f = c / 2d. In the example shown, this produces a “pinna notch” around 10 kHz. With typical values for d, the notch frequency is usually in the 6-kHz to 16-kHz range.

Since the pinna is a more effective reflector for sounds coming from the front than for sounds from above, the resulting notch is much more pronounced for sources in front than for sources above. In addition, the path length difference changes with elevation angle, so the frequency of the notch moves with elevation. The pinna provides the primary cues for elevation.