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March 1987

Acoustic Impedance Transformation-Reply

Author Affiliations

Stanford, Calif

Arch Otolaryngol Head Neck Surg. 1987;113(3):329. doi:10.1001/archotol.1987.01860030105022

In Reply.—Dr Kimberley is incorrect when he states that "a mechanical/acoustical system such as the external ear is quite incapable of impedance transformation, at any frequency." One way to model the acoustic function of the external ear is as a small horn whose mouth contacts the low specific impedance air of the external environment (41.5 acoustic Ω, cm/g/s), while the throat ends at the higher impedance tympanic membrane (TM), typically around 450 acoustic Ω, cm/ g/s. The knowledge that horns act as impedance-matching devices in this situation was first described in 19191 and is well covered in acoustics texts.2,3

I prefer to think of the external ear functioning as a quarter wavelength pipe resonator closed at one end by the TM and connected to a cavity resonator at the other. The pressure gain at the TM in the resonant frequency range (2500 to 6000 Hz) almost completely offsets

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