The newest machines on the market actually adjust their volume and frequencies automatically
. These machines have not only speakers going out but also microphones coming in, and they can read and respond to the sound that they're "hearing." This means that you can rely on your white noise machine to give out soft noises when there's not much going on around you, but when loud noises threaten to interrupt your sleep, the machine can gradually up its volume to cover the noise without waking you itself. This type of white noise machine can be great if you need a way to fall asleep without worrying about fiddling with volume controls.