Time-varying noise control in motorcycle helmets
Acoustical Science and Technology
Introduction Motorcyclists are usually exposed to high noise levels that increase with speed. There have been a variety of medical studies that confirm that the prolonged exposure to excessively noisy environments may cause health problems, such as, noise-induced hearing loss (NIHL)     . Active noise control (ANC) has an extensive reference bibliography [5,6], but there have been very few ANC works on motorcycle helmets [1,7-9], probably due to the time-varying nature of their
... ature of their dynamics. Here, the active noise control problem in motorcycle helmets is studied, which has an intrinsic time-varying behavior. To achieve a reasonable noise attenuation, the noise source and its dynamic characteristics are computed through an experimental identification process. Also, in a robust control framework [10, 11] , the time-varying dynamics can be either considered as a dynamic uncertainty or treated as it is. Here a time-varying feedback (FB) controller is proposed and compared with a robust linear time-invariant (LTI) controller. The LTI controller optimizes against the lack of knowledge of the model dynamics using H 1 optimal control. The time-varying approach is designed to have an attenuation level that increases with the speed between the helmet and the air (since the noise level increases with speed) using a linear parameter-varying (LPV) controller. The main purpose here is to take advantage of this time-varying controller to tune the attenuation level in accordance with the noise environment. In this way, the attenuation is higher at speeds where the external noise is louder and lower at reduced velocities. Such tuning cannot be performed with LTI controllers, which provide a constant attenuation at all speeds. The result is illustrated by an experimental simulation, which uses the noise obtained from a freeway at different speeds.