An ambulance siren is very important for operating ambulance safely, and the level of siren is regulated by the law in Japan so that it is not allow to make siren soft. In the cabin of an ordinary ambulance operating in Japan, sound level of siren reaches 80 to 85 dB SPL at the ear position of driver. This level obviously make not only crew members but also patient tired to hear the sound of siren continuously. In order to control the sound of siren, which consists of two harmonics 770 Hz and 960 Hz altered every 0.65s, ordinary ANC system used for cabin noise is not adequate. There are mainly two reasons; higher frequency components to be controlled and requirements to make sound other than siren unchanged to allow the crew members to communicate with each other, with patient or over radio. In this paper, two types of ANC systems are discussed for controlling ambulance siren within a cabin. One is ANC headset for crew members and the other is designed as a pillow for patient. The proposed system is operated by Delayed-X Harmonic Synthesizer algorithm and selectively attenuates sound of siren. Experimental results for ANC headset show more than 14 dB attenuation by ANC headset. And computer simulation shows that ANC pillow can reduce the noise even if patient moves her/his head in some extend.
展开▼
