Alexander L. Butler
John L. Butler
Image Acoustics, Inc., 97 Elm St., Cohasset, MA 02025
It is common practice to make near-field pressure measurements on woofers as a means for avoiding low-frequency reflections from surroundings which can occur during far-field measurements. There is also a problem with making far-field measurements on ribbon tweeters. This is due to the long length of the ribbon compared to the wavelength which results in an extended near-field distance in the higher-frequency region and the requirement of a very distant far-field microphone. As a result of this measurement problem, a technique has been developed for near-field measurements of ribbon tweeters. This technique is based on the Helmholtz integral equation evaluated on the axis of the tweeter. In the case of a rigid baffle only the pressure gradient measurements are needed over the surface of the ribbon. This paper will present the results of measurements of the pressure gradient as well as the pressure distribution in the vicinity of the ribbon and present a simple technique for predicting the far-field from these near-field measurements.