Yes, you can activate the noise reduction circuits alone, with no connection to a source device by Bluetooth or cable. The headphones will stay on as long as there is power in the battery, which is close to 30 hours for pure NC function. Please note that there is a slight residual noise from the NC system, which will be drowned out by music playing, or by surrounding noise such as airplane noise. If you use the NC in surroundings that are near quiet to begin with, you may not get an improvement in noise level.