Some people believe that using this stuff can be a replacement for meditation. While sitting passively and listening to something that can give you the same effect frequency-wise as meditation, there is still far more benefit to meditating than just entrainment. Meditation and mindfulness allow you to really understand yourself on a level far deeper than technology will bring you.
The pitch frequency measures and describes what a beat sounds like, i.e. does the isochronic beat have a deeper bass sound, or is it high pitched and sharper sounding? The beat waveform frequency describes how many times the beat is repeating per second, i.e. how fast it is beating. You don’t really need to concern yourself about the pitch frequency, as that doesn’t have a direct influence on brainwave entrainment and doesn’t play a part on the measurement on an EEG. I only mentioned it because you were referring to humans not being able to hear below 20Hz. I change the pitch frequency just to suit the mood of the track. For tracks that are to be relaxing, for meditation or sleep, I tend to use a lower pitch frequency so it sounds deeper and more relaxing and I never have that lower than 100Hz. For an energizing, high focus track I might use a more higher pitched 200Hz isochronic tone, because that is sharper sounding and less likely to make you feel sleepy. That’s all you really need to know about the pitch frequency.
You don't have to buy one, though. It's not too hard to make your own binaural beat, and free software is widely available to do just that. The one that I used to make that little sample is an open-source program called Gnaural, available on the Sourceforge website. It's pretty easy to use, though it takes some practice before you can generate some of the really cool, more professional sounding beats. A binaural beat consists of two simple tones, and most people add that background pink noise. Nothing special.
In physics, entrainment is the process of two oscillating systems coming to assume the same periodic rhythm, such as is observed when two clocks slowly synchronize their ticking and tick together in harmony after some time. Pendulums also achieve this same synchronicity when swinging in close proximity to one another, a phenomenon first observed and written about in 1665 by Christiaan Huygens, a Dutch scientist.
When you listen to binaural beats/tones, you are actually listening to two slightly different pitched frequencies. Each frequency goes to each of your ears independently. When this happens your brain responds by creating a third tone, making up the difference between the two frequencies. This shift in frequency then shifts your brain’s consciousness and changes your state.
This is probably the most popular choice of frequency. The theta state is a state of tremendous stress relief. The benefits associated with theta level relaxation include improved concentration, reduced hyperactivity and improved memory. While in a state of theta relaxation, one’s blood pressure, breathing and heart rate all slow to a much more restful and healthy level that promotes natural healing. During theta relaxation one may also experience vivid flashes of mental imagery as the mind opens to memories or subconscious information that is not available to the conscious mind during the normal waking state. The theta state is a very deep state of relaxation that is excellent for deep hypnosis and mental programming.
In addition, Western culture reinforces thought and communication styles associated with the left brain hemisphere, which controls logic, language, and linear thinking in a majority of people, as opposed to the right hemisphere which controls brain centers for emotional, intuitive, creative, and non-linear thought processes. Note that a significant minority of people has the opposite hemispheric dominance than the majority of people.

Consider the following analogy. Imagine a ballroom full of people dancing together. When the music changes to a faster tempo, the dancers move faster in response to this. When a slower piece of music is played, the dancers’ rhythm slows down as well. In a similar way, the frequency of your brain will change in response to the frequency of the binaural beat that it is exposed to. For example, a person who is in a state of very deep meditation may have a dominant brainwave frequency of 5 hertz, so by listening to a binaural beat with a frequency of 5 hertz you can entrain your own brainwaves to a similar state.
Eric Bartel is the creator of the Free Binaural Beats website and the sole creator of the audios found here. He specializes in creating binaural beats and isochronic tones along with relaxing ambient meditation music. From his home studio in the Black Hills of South Dakota, Eric is committed to bringing the healing power of binaural beats to anyone who wants to live a more peaceful, relaxed and fulfilled life.
Although there is a general stress response pattern, there can be variations in the response according to the characteristics of the stressor (10). Individuals tend to respond differently based on the familiarity of the stressor. For example, the perceived level of stress and physiological response when giving a presentation to a group of work colleagues will likely be less than when presenting to an unfamiliar group. The stress response also varies depending on the level of perceived control one has over the stressor (10). If there is a way for one to actively cope with the stressor that is reasonable, then the individual usually perceives more control over the situation. Consider an individual who has to take a certification examination for work and has 6 months to prepare. He can adjust his schedule to accommodate study time. However, waiting for medical test results that show whether one has a serious illness does not allow a sense of control over the stressor, and the individual passively endures the stressor or may try to avoid the stressor. With this uncontrollable type of stressor, there is a more negative reaction with greater productions of cortisol, which can have damaging health effects because of the suppression of immune function (10).
Because the brain filters and interprets reality in a split-brained way, we tend to see things as separate and opposed, rather than as connected and part of the oneness spoken of by the great spiritual teachers (and, in the last few decades, by quantum mechanical physicists). Thus, at a deep level, the dual structure of our brain, in conjunction with brain lateralization, predisposes us to see and experience ourselves as separate from, and often in opposition to, the rest of the world—instead of experiencing the elegant interconnectedness between us and everything else. Our childhood associations and programming build on this inborn tendency by training us to seek this and avoid that, to move toward pleasure and away from pain, to do good and not bad, and so on. The greater the lateralization in the brain, the greater the feelings of separation—and the greater the feelings of separation, the greater the fear, stress, anxiety, and isolation.
According to Dr. Mark Hyman, eating whole, real foods restores balance and reduces the effects of stress on your body. Replacing harmful substances such as caffeine, alcohol, and refined sugars, with clean proteins, fruits, vegetables, and healthy fats helps regulate your hormone levels, including stress hormones. Food As Medicine Education Director Kathie Swift, MS, RDN, LDN, FAND, EBQ cites the connection between the gut and brain in relieving stress. The gut and brain are constantly sending signals to each other, so by keeping your microbiota (the bacteria in your gut) healthy, your brain feels less stressed.
A 2008 study at Hofstra University played two different binaural beats and a control sound (a babbling brook) to patients with high blood pressure. There was no difference between the groups. In one small study from Japan that was published in the Journal of Neurophysiology in 2006, they played various binaural beats to nine subjects, and observed the resulting EEGs. They found great variability in the results. Their conclusion was that listening to binaural beats can produce activity on the human cerebral cortex, however the cause was more likely a conscious auditory reaction and was not correlated to the frequency of the binaural beat. However, a 2005 study published in Clinical Neurophysiology found that they were able to induce a desired frequency in the EEG matching the phantom beat frequency encoded in a binaural beat, however this was with a single subject and was neither blinded nor controlled.
The use of brainwave entrainment techniques offers many benefits for overall health and well-being, including improved emotional stability, increased cognitive function, and a deepening of creative insight. Much of this benefit derives from the hemispheric synchronization occurring as a result of entrained brainwaves. This effect happens when the electrical impulses in both hemispheres synchronized to the same frequency being delivered through the entrainment source.
Binaural beats are auditory brainstem responses which originate in the superior olivary nucleus of each hemisphere. They result from the interaction of two different auditory impulses, originating in opposite ears, below 1000 Hz and which differ in frequency between one and 30 Hz (Oster, 1973).For example, if a pure tone of 400 Hz is presented to the right ear and a pure tone of 410 Hz is presented simultaneously to the left ear, an amplitude modulated standing wave of 10 Hz, the difference between the two tones, is experienced as the two wave forms mesh in and out of phase within the superior olivary nuclei. This binaural beat is not heard in the ordinary sense of the word (the human range of hearing is from 20-20,000 Hz). It is perceived as an auditory beat and theoretically can be used to entrain specific neural rhythms through the frequency-following response (FFR)--the tendency for cortical potentials to entrain to or resonate at the frequency of an external stimulus. Thus, it is theoretically possible to utilize a specific binaural-beat frequency as a consciousness management technique to entrain a specific cortical rhythm.  
Brain Wave Entrainment is any procedure that causes one's brainwave frequencies to synchronize with a periodic stimulus (sound, vibration or light) having a frequency corresponding to the intended brain-state (for example, to induce a trance, dreams, sleep or relaxation.) It is also called the Flicker-response because of how staring at a campfire or the flickering of a burning candle can lull you into a state of calmness and serenity. There was an extensive article on this phenomenon by Gerard Oster in Scientific American in 1973. It may sound novel, but in many ways, this is old tech.
If brainwave entrainment leaves you with unwanted side-effects (see below) or discomfort, you’re probably encouraging a range of brainwaves that are already excessive in some area of your brain. The way around this is to get a brain map to see what your brain’s strengths and weaknesses are, and see what (if any) brainwaves could use some encouragement.