Final answer:
The HRTF is frequency dependent because high and low frequencies interact differently with the listener's head, with high frequencies being particularly affected by head size. Sound's speed is nearly independent of frequency, and pitch perception is aided by hair cells in the inner ear which respond best to certain frequencies.
Step-by-step explanation:
The HRTF, or Head-Related Transfer Function, is indeed frequency dependent. The way sound interacts with the listener's head, notably diffraction, reflection, and attenuation, can differ based on sound wave frequency. For low frequencies, sound waves can bend around the head with marginal distortion, while higher frequencies cannot easily diffract around the head and are more likely to reflect and attenuate. This leads to head-related modifications of the sound that are complex and frequency-dependent, making HRTFs unique to an individual's ear shape and head size.
The speed of sound is nearly independent of frequency, which can be observed when listening to music from distant marching bands in a stadium; all frequencies travel at nearly the same speed despite their pitch. However, the perception of loudness and pitch will differ with frequency due not only to the physical changes in sound wave propagation but also due to the physiological sensitivities of the ear, particularly the basilar membrane within the cochlea.
Hair cells located near the base of the basilar membrane are more responsive to high-frequency sounds. These hair cells play a crucial role in differentiating among various pitches that we perceive, as they trigger nerve impulses to the brain where sound is ultimately interpreted.