The audio attenuator HI/LO switch is used to adjust the output volume for different audio sources. It has no effect on noise reduction.
Before using your headphones for audio, note the setting on the switch. The switch is at the headphone end of the audio cable, which is inserted into the headphone. If it is in place in the headphone, take hold of the audio cable as close to the left earcup as possible. Gently pull it out to access the switch, as shown below.
Select the appropriate setting for the sound source you want to use:
LO is typically used for airline audio or audio devices that use AC power, such as a home stereo system
HI is typically used for battery-powered audio devices
Then insert the switch into the opening on the left earcup and press it in so it's flush with the side of the earcup.
Connect the headphone plug to an audio source and make sure the audio source volume is turned down. You are now ready to put on the headphones and turn on the headphone power switch (located on the right earcup) to enjoy both audio and noise reduction. Use the Volume control on the selected audio source to make volume adjustments.
If the HI/LO switch is set to LO and you have to increase the volume control on the audio source above the halfway point, remove the headphones. Change the switch to the HI position. Turn the Volume control on the source down low. Put the headphones on and slowly increase the volume on the source.
