The Function of the Outer Ear
As mentioned above, the pinna has certain sound-gathering functions. Cupping the hand behind the ear augments this sound gathering and allows us to hear fainter sounds. In Fig. 2, the solid line shows the increase in sound pressure at the eardrum above that of the free, undisturbed sound field. The sound amplification, however, is the result of more than the sound-gathering effect of the pinna. The auditory canal acts something like an organ pipe closed at one end. When the length of such a pipe is one-fourth of the wavelength of the sound impinging on the open end, a significant rise in sound pressure results at the closed end. When the sound falling on the open end of the auditory canal is of such a frequency that the length of the canal is one-fourth wavelength, this increase in sound pressure occurs at the eardrum. From the solid curve of Fig. 2, we see that such a sound amplification approaches 20 dB in the region of 2000 Hz.
Figure 2. The sound pressure at the eardrum. Solid line: normal. Broken line: with hand cupped behind the pinna, which amounts to an enlargement of the pinna (after Weiner and Ross)
An interesting question is, “How much does it help to cup the hand behind the hear?” The broken curve in Fig. 2 shows this condition. The difference between the two curves, the contribution of the cupped hand, varies from just a few dB at most frequencies to about 20 dB at 1500 Hz. This amount of gain obtained by cupping the hand behind the ear can mean the difference between understanding and not understanding speech for someone with impaired hearing.
We see, then, that the pinna, the auditory canal, and the eardrum constitute a complex acoustical cavity that increases the sound pressure at the eardrum. Diffraction of sound by the head also adds a bit to this amplification of sound pressure. The amount of the overall amplification varies with frequency, but its maximum occurs in the speech frequencies and amounts to about 20 dB (tenfold) at the peak.