Pano gumagana ang Frequency Modulated IR Headphones..eh diba ang Ir led is Constant ang frequency/Wavelength???
The optical wavelenght of the IR device is fixed, so you are right in saying di ito pwedeng frequency modulate. But it can be pulsed/amplitude modulated.
Kaya sa mga IR headphones, the IR is actually amplitude modulated at ultrasonic-hypersonic(>60KHz) frequencies - used as the carrier signal
. The carrier signal is in turn frequency modulated by the audio signal. Kaya hayan, FM IR headphones na.