Pinnae shape the sound spectrum to encode elevation and front-back position

Evidence

Although the previous review has discussed how ITDs and ILDs provide information about sound localization in the horizontal plane, humans also have an ability to localize sounds in the median plane (e.g., elevation). These abilities are primarily influenced by the pinnae, which alter the characteristics of sound in a way that depends on the angle of incidence, thereby providing information about the elevation of a sound source and whether a sound is in the front or back. The pinnae contain ridges and cavities that modify incoming sound and ultimately alter the spectrum of the sound received by the ear. Depending on the angle between the sound source and the pinnae, the amplitude of specific frequencies is altered, producing spectral peaks and valleys that are unique to different angles of incidence in the median plane. In this way, the pinnae provide cues to a sound’s location in 3D space, with the largest effects occurring for frequencies above 4 kHz (due to the small size of the cavities). Because it is the pattern of peaks and valleys across frequency that provide these cues, experimental evaluation of the role of the pinnae in sound localization is only possible using complex sounds, and specifically complex sounds that contain high frequencies. — Jennifer Lentz, Psychoacoustics- Perception of Normal…, 1505

Categorization

• DEFINE: Explains the pinnae-based spectral mechanism for median-plane localization, a core concept for understanding 3D hearing acoustics.
• QUESTION: How does the binaural auditory system segregate monaural spectral pinna cues from genuine interaural differences? The cue-conflict question is worth pursuing.