University of Wisconsin-Madison Physical Sciences Lab

Speaker Mover

PSL had served Prof. Fred Wightman’s research needs when he was a UW professor, & was contacted again to develop a speaker mover for his auditory research at Univ. of Louisville.

The human head and external ears modify incoming sound. The nature of this modification depends on the direction of the sound source relative to the listener. Because the human auditory system uses this information to decode directional information in the sound reaching the ears, it is important to be able to accurately measure the acoustical transfer functions that describe the sound transmission from a free-field source to the ear drum for many directions surrounding the listener. Such measurements are typically referred to as head-related transfer functions, or HRTFs, and are referenced relative to the same sound transmission in the absence of the head. The new robotic device will greatly facilitate the HRTF measurement process by allowing fast and accurate positioning of a reference loudspeaker used for the measurements. Such fast and accurate positioning is critical because a full set of HRTF measurements will typically involve over 500 measurement directions!

Engineers at PSL designed a mechanism & controls to move a speaker about the head of a blindfolded listener, at a radius of about 1.63 meters (64″), from 45 degrees below to directly overhead, at any angle relative to forward. It was constructed mostly at PSL, while some machining was subcontracted to a local firm. The mechanism incorporates extremely quiet drives, sound absorbing & isolating materials, and geometry to deflect any indirect-path sound propagation away from the experimental subject. It includes encoders, home reference, and limit switches for both rotating axes. An adapter plate and counterweights enable use with a selection of speakers. Construction, assembly and checkout were completed at PSL. It was shipped after development & testing of a graphical operator interface. The mechanism was installed in an anechoic chamber in Louisville, KY by PSL personnel and tested on-site.

This is a video of the speaker mover in action: (requires Quicktime)