Much of what is currently written about brainwave entrainment, especially on the internet, is simply … incorrect.  It seems par for the course that when something of huge value is discovered as a potential ‘internet product’ that it is going to be marketed within an inch of it’s life. And often with less than stellar regard for actual science, research, or factual knowledge. 
I wouldn’t personally recommend listening to delta wave frequencies for depression, so I’m not sure who advised you to do that? People with depression usually have a higher ratio of theta and delta wave activity, so I would normally recommend listening to high alpha wave and low beta wave frequencies, to help balance things. I have some 10Hz alpha tracks for serotonin release, which you can try for free on my YouTube channel here: I also recommend you give these SMR (low beta wave) tracks a try: I hope that helps.
2. Dimensional sound insures that the brain perceives the entrainment as more realistic, which produces more natural and thorough engagement in each meditation. Not only does this create more pronounced effects and an intensified response, but it also avoids the fatigue generated by overuse of standard sound – as can occur with extended use of other audios.
You also may find that after pushing yourself really hard, now you have pushed yourself a little too much, and you’re getting into overwhelm. You may want to ride that overwhelm like a wave to where it’s pushing you just a little bit, but not so much that it will push you over the edge and have you take a dive. So, maybe on Saturday, after pushing hard all week, you might decide to put on the releasing tracks. In this way, you can still experience a very deep and blissful meditation, but without all the push from the carrier frequencies and the biofield amplitude.
Brainwave entrainment is a colloquialism for such 'neural entrainment', which is a term used to denote the way in which the aggregate frequency of oscillations produced by the synchronous electrical activity in ensembles of cortical neurons can adjust to synchronize with the periodic vibration of an external stimuli, such as a sustained acoustic frequency perceived as pitch, a regularly repeating pattern of intermittent sounds, perceived as rhythm, or of a regularly rhythmically intermittent flashing light.
All brainwave frequencies are useful and beneficial at certain times – there is no brainwave that is intrinsically better than another. However, by deliberately choosing to attain a particular brainwave state, a corresponding mental state can be brought about at the same time. For example, a working person who has been in an overly alert beta brainwave pattern for many hours can quickly shift their mind and body into a relaxed state by listening to a few minutes of brainwave entrainment music for inducing alpha or theta brainwaves.
Anyone who has sought out different methods for enhancing cognitive ability will probably have come across a technique known as Brainwave Entrainment. However, it is a fairly niche area of brain training, meaning that this form of stimulation is often overlooked in favour of more mainstream methods. The following outlines what Brainwave Entrainment actually is, how it is used, and some of the benefits attributed to it.
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.