The Alpha waves have for years been the most popular frequencies to use during Brainwave Entrainment. They have the ability to enhance light relaxation as well as positive thinking and they are often utilized for enhancing what is known as ‘super learning ability’. The Beta waves have a frequency of 13-25 Hz and they assist in enhancing a standard state of alertness. In addition, they are an effective remedy for both stress and anxiety.
Hi Marko, that isn’t one of my videos you referred to, so I can’t really answer you properly as I don’t know how their track was created. For the best answer, you should really contact the video creator. There isn’t any research that I’ve seen to suggest that you could harm your health by looping a delta track. During a typical sleep cycle, your brainwave activity will usually go up and down between the delta and theta range. It may be that you won’t experience the same quality of sleep if you spend most of your time producing mainly delta activity. With my 8-hour sleep track, I fluctuate the frequency range to try and emulate a typical sleep cycle http://www.mindamend.com/shop/sleeping-and-dreaming/deep-sleep-8-hour-sleep-cycle/.
According to both observation and research, after a series of brainwave entrainment practices, the brain learns how to state change. This enables you to achieve certain brainwave effects in the absence of entertainment. Additionally, research shows that children are usually in the Theta stage and this in itself could provide a detailed explanation for their ability to learn fast and remember what they have learned. We can take advantage of this effect and use Alpha and Theta waves to reduce learning time in adults.
Both brainwave entrainment and neurofeedback deal with brainwaves, but the similarity stops there. Entrainment pushes your whole brain into a pre-determined state, while neurofeedback teaches you how to move specific parts of your brain on your own. It is the differeence between forcing the brain into a given position, and skills building so you can move it there yourself. 
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