The outer ear
The pinna and the concha: an acoustic horn
Main role in sensing, and channeling exterior sounds towards the ear canal and ear drum.
Concha-pinna: Impedance matching (optimisation of the transfer to the ear canal).
Amplifying (resonances). Concha: resonance around 5000 Hz.
The ear canal : the resonator
To understand the resonator concept, see section 2.3 - part 2, chapter "propagation in a long tube", with examples.
The ear canal behaves like a tube closed on one side (by the ear drum), and open at the other.
This type of tube is called a wave guide, which exhibits resonances depending on its length.
This is like a organ pipe, with the note depending on the length of the tube.
The resonance frequency of the ear canal can be found at around 3400 Hz. This is a quarter wave resonance, thus the length of the tube and the frequency can be linked. If is the tube length: , with the speed of sound in the air.
We can therefore see that the ear is most sensitive to frequencies between 1 kHz and 4 kHz (This mostly being due to the outer ear).
The ear drum is the element that separates the middle and outer ear.
Total amplification due to the outer ear.
The blue curve represents the amplification due to the ear canal (maximal around 2500 Hz), the green curve represents the amplification due to the pinna and concha (maximal around 500 Hz), and the red curve represents the total amplification of the outer and middle ear.
We can see that the sensitivity depends mostly on the amplifications by the outer ear, as the red curve is very close to the sum of the green and blue curves. The remaining amplification is provided by phenomena in the middle and inner ear.