Vibroacoustic Investigations on Hand Prostheses and Sound Design

Modern hand prostheses are capable of mimicking finger or wrist movements using electrical motors, gear mechanisms and control elements. The drive systems and the transmission mechanisms emit machinery noises which can activate or enforce a foreign body sensation by users. Transfer-path-analysis is originally developed to investigate the individual vehicle noise paths. It is a useful tool for troubleshooting and optimization of the product sound quality. In this study, the vibroacoustical characteristics of a hand prosthesis are investigated using the transfer-path-analysis. First of all, the acoustical behaviour of the electrical motor is investigated. The electrical motor is coupled with supporting structures to cause mechanical vibrations of the housing, which, in some instances, radiates acoustical energy. Therefore sound and vibration measurements are carried out to determine the airborne and the structureborne transfer characteristics of the structural elements of the hand prosthesis. According to Jekosch the perceived quality of an entity results from the judgment of the perceived characteristics of an entity in comparison to its desired/expected characteristics [1]. Thus, an interview with prosthesis users is conducted to characterize the user expectations regarding prostheses sounds. A psychoacoustical experiment is carried out to evaluate the sounds regarding their reliability and pleasantness. The results of this study convey useful constructive design ideas for hand prostheses concerning the sound quality. MYOELECTRIC PROSTHESES AND THEIR SOUNDS The development of new sensor and actuator technologies in recent years is reflected in the progress of prostheses industry. Myoelectric prostheses can measure and interpret the residual muscular activities (action potentials) with the help of their sensors. Using this information, they generate control signals for electromechanical elements and conduct different movements, such as opening and closing of the hand, adaptive grasp, wrist rotation, elbow flexion, etc. Usual prosthetic hands are basically pinch type devices with two rigid fingers and a rigid thumb. These prostheses can achieve a two or three point contact with objects that are grasped. During grasping, the operation of electromechanical elements causes a noise. Such kind of noises can be troublesome or unpleasant for users, because of different reasons such as their machinelike character which reminds of robot movements, frequency spectrum or modulation features, etc. The expectations of the prosthesis users play an important role on the quality evaluation. The aims of this study are the investigation of the vibroacoustical behaviour of a myolectric hand prosthesis and the rules behind of its sound quality. In order to achieve these aims, a work plan was followed which includes different steps: the characterization of the user expectations regarding prostheses sounds, the investigation of the vibroacoustical behaviour of the hand prosthesis using transfer path analysis and the evaluation of the existent and designed prostheses sounds. USER EXPECTATIONS REGARDING PROSTHESIS SOUNDS The purpose of this part of the study was to prepare a catalog of user requirements regarding the features of the prosthesis sounds. Therefore an interview with prostheses users was conducted. Eighteen representative users, eleven men and seven women, aged between 12 and 80 years old, participated in this interview. They were paid on an hourly basis. The following criteria were evaluated in this interview: • The annoyance of the sound of prosthesis, which is daily used. • Description of the sound of existing prostheses. • Which feedback modality (visual, auditory, tactile) is normally used to control the operation of the prosthesis? • Does sound play any role as feedback during the prosthesis usage? • How should a prosthesis sound? Which noise behaviour is required (no sound, unobtrusive sound, no matter)? 23-27 August 2010, Sydney, Australia Proceedings of 20th International Congress on Acoustics, ICA 2010 2 ICA 2010 • Does sound deliver any information on the quality of prosthesis? The annoyance regarding the sounds of daily used prostheses was evaluated on a quasi-continous scale (1: not at all, 10: extremely). The results are shown in Figure 1.