But hearing assistance systems in this country more commonly use infrared or FM signals to transmit sound.

但美国的助听系统更广泛使用的还是红外信号和调频信号来传播声音。

Mark says they even have some listeners who switch over to AM when the FM signal starts fading.

马克说，他们甚至有一些听众在 FM 信号开始减弱时切换到 AM。