The reciprocity principle defines a general relationship between transmission and reception in an acoustical system. By applying this principle, the transformation of sound pressure level from a diffuse sound field to the eardrum can be expressed in terms of the frequency. the radiation impedanceZradof the external ear looking outward from the eardrum position, and the load impedanceZdpresented by the eardrum. The formula, which contains no assignable constants, has been used to calculate the diffuse field response curve for a physical model of the external ear combined with an eardrum impedance simulator. An impedance tube was used to measureZradandZd. The magnitude of the response and the frequencies of the resonance peaks are in excellent agreement with other measurements. The diffuse field pressure gain at the primary resonance frequency (2.7 kHz) is approximately 18 dB.