I have 2 fans from eqdizzle here on TDH. Mine are powered by 4 AA batteries in a switched box all mounted in the helmet, which balances the weight of the rangefinder servo relay/radio receiver and batteries:
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I experimented with different placement of the fans while running to lessen their affect on my hearing. I settled on the back panel location with the fans outflow aimed up the back. In this way the airflow passes over my head and down the front to defog the visor. Also the intake is at the nape of my neck as add additional cooling sensation, all with minimal impact on my hearing. Only needed in short bursts in my experience, since the helmet is so open around the bottom. Helps to exhale out your nose instead of your mouth, especially in cold weather.