Isochronic tones work just the same in delta as they do in alpha, theta and beta and they are widely used in the brainwave entrainment community to help people sleep. Like you, I’ve also seen some websites saying they don’t work in delta, but it’s a bit like the game of Chinese Whispers, where someone makes a comment and then after it gets passed around and shared a lot the message gets distorted and appears to be a fact. I don’t know of any scientific reason why they wouldn’t work in delta. I remember some people talking about this on a brainwave entrainment forum many years ago. They were saying they found isochronic tones a bit too abrupt for using to help them sleep and they preferred binaural beats, as they thought they were a more soothing sound. That was just a personal preference shared by a couple of prominent forum members at the time and some people then took that as a fact for everyone. That’s where I think that belief originated from.
So, I tried those frequencies. And after listening to the 1st one (High Beta to Gamma, Isochronic tones: 18HZ ramped up to 40HZ within 20 minutes) – I immediately stood up and started organizing! (Something that I kept forgetting to do!) Overall, I’ve found that isochronic tones & monaural beats are more immediately effective than binaural beats, though they are still good.
The Delta waves range from 1-3 Hz; these frequencies can assist in enhancing deep sleep. They also play a big role in boosting the immune system to facilitate natural body healing. The Theta waves have a frequency that ranges from 4-7 Hz. In turn, this brain state can enhance deep relaxation and is known to assist individuals who have poor memory. They are the best frequencies to use if you want to focus or meditate.
Brainwave entrainment is clearly NOT an effective standalone treatment for mental illness, but it may be an effective adjunct treatment in certain individuals. It should also be mentioned that when using brainwave entrainment, you may want to have a specific purpose in mind. If your goal is to relax, yet you’re using a protocol that is stimulating, you may falsely conclude that it doesn’t work even though you’re using the wrong protocol.
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.