The terms meditation audio and meditation cd are a little broad. They usually mean one of the 2 following things:
Brain wave meditation audio:
A good example would be Deep Zen Meditation audio. This sort of meditation audio was first coined by a biophysicist: Dr.Gerald Oster M.D. The audio (in the form of music) is recorded using 2 different frequencies, each playing into a different ear. When the 2 audio frequencies are played at the same time, the brain perceives them on it's own 3rd frequency (the brain creates this 3rd frequency, after combining the 2 frequencies in the music). After listening to the recording for a short period of time your brain starts to "tune" into this frequency. This "tuningin" enables the brain to shift from one brain wave state into another. These recordings can help the listener to switch into the theta wave state. This state is similar to the one we sometimes achieve while in the shower or while driving on a very long and mundane stretch of high-way (you probably noticed how we get some unusual and good ideas when we're in that state). You can see how this audio can help you achieve a better meditation state.
This works on most of the people and there are many different places where you can get these types of recordings (they've become pretty main stream since coming out in 1977). The place I suggest is a web-site called Deep Zen meditation, which I think is the best meditation audio example, because it offers a free trial, to give you a good taste of how this technology works.
Guided meditation CD:
These meditation cds are known for speaker`s soft spoken voice, that paints elaborate visual descriptions. In essence it's a voice recording, that guides you through the meditation experience. A well recorded guided meditation cd will feature a pleasant voice that will give gentle orders such as "close your eye" , "relax" , "imagine [fill in the blank]". For example the relaxation meditation cd by Bodhipaksa does just that. It often helps people who have racing thoughts going through their heads and who can't concentrate on their own and need some external help.
Another important aspect of a good meditation CD is the timing. The narrator cannot be too quick (because you need enough time to analyze and react) but cannot be too slow either (leave you hanging). I think that Bodhipaksa's guided meditation cd hits just the right spot.
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