Acoustic signals arriving at the eardrum in free-space carry directionally dependent temporal and spectral information resulting from the acoustical effects of the body, head, and external ear as well as from differences in the length of the sound path to each ear. Through analysis of the responses of single auditory neurons, the acoustical and neural mechanisms by which sounds in free-space are localized are being studied. The approach involves simulation of free-field signals at the two eardrums of a cat via earphones and a study of the neuronal responses to such a virtual acoustic space. This approach makes it possible to manipulate different stimulus parameters independently in order to examine their role in determining the spatial characteristics of neuronal response. This report describes an insert earphone system designed for the delivery of such simulated signals which are broadband transients having complex spectra that mimic the acoustic transfer function of the external ear for frequency components up to 30 kHz or more.