This paper presents the apparent apex of a horn and how it relates to directivity response measurement. Several mathematical models are used to relate horn size, coverage angles, frequency, microphone distance, and measurement error. These show that directivity error is directly related to the length of a horn and inversely related to measurement distance. An empirically derived error-distance equation estimates the error. In general, far-field performance for many horns can be obtained at a measurement distance of approximately eight times the length of the horn.
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