Some of my iPHone meditation apps feature "Binaural Beats". In theory, if you listen to a "Binaural Beat" with headphones, the sounds can change your state of mind. Depending on the frequency of the beat, you might relax deeply, feel sleepy, feel alert, or enter a meditative state.
I did some very cursory research into how these "Binaural Beats" are supposed to work. Here is a link to one of the more interesting articles: Stanford study of brainwave "entrainment". Basically, brainwaves--like any waves--occur at regular intervals or frequencies. These frequencies tend to be very slow for sleepy states, faster for relaxation, faster still for meditation, faster for normal "wide awake" states, and still faster for extreme focus. A recording with a "Binaural Beat" is designed to match, for example, the rhythm associated with meditation. When you listen to the recording, your brainwaves start to "dance" to the rhythm you are hearing. This is called "entrainment". Your brainwaves switch from an alert, or an even higher frequency stressed drumbeat, to a meditative drumbeat. Meditation then becomes easier.
Does it work? I haven't done enough research to claim any expertise. But it sounds reasonable, I know that brainwaves can be measured, and I've read that the shift in brainwaves to match the "Binaural Beat" has been measured repeatedly since they were first discovered in the 1970's.
My personal experience with "Binaural Beats" has just been in the last few months. One of my favorite meditation apps for iPhone and iPad is "SleepStream2". This app gives you high-definition recordings of sounds such as ocean waves, fire, and wind, and lets you choose a variety of "Binaural Beats" to play at the same time. I THINK it makes a difference for me. It seems to make it easier for me to go quickly from a hyper, task oriented state of mind to a meditative state of mind. This morning, I was even able to do something I can't normally do: meditate while walking the dogs. Normally, I'm too distracted by sights and sounds, negotiating traffic, keeping the dogs under control, and dealing with other dogs and pedestrians to ever feel as if I was able to achieve any kind of meditative state. But this morning, listening to the ocean sounds with a meditative "Binaural Beat" in the background, I think it worked. For the first time ever, I walked the dogs and meditated at the same time.