From a brainwave entrainment effectiveness perspective, it’s my understanding that the response from isochronic tones stimulation starts to diminish over 30Hz and that 40Hz is about the limit for using them. So from what I’ve read on the topic a 100Hz beat wouldn’t work, probably because it’s too fast for the brain to process and synchronise with it.
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.
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. 
In addition to the exercise prescription, other characteristics of the exercise session (e.g., group vs. individual) and the individual also are important considerations. Because of health consequences associated with stress, high-stress clients are likely to be at increased risk for cardiovascular disease and cardiovascular events during exercise. Therefore, using the preexercise screening procedures outlined and endorsed by the American College of Sports Medicine is essential (1). Monitoring exercise intensity for those looking to “blow off steam” to reduce stress might be a concern when the client has high blood pressure or other cardiovascular risk factors that further the risk increase for cardiovascular events.
This session uses a proven technique for inducing deep relaxation, playing one frequency for two minutes, then alternating to a second after a 30-second ramp, then reversing the process. The first half of the session switches between a serotonin-boosting 10Hz and the grounding and rejuvenating 7.83Hz Schumann resonance. The second half of the session goes even deeper, alternating between the relaxing 7.83Hz and 6.3Hz, a frequency associated with releasing anger and irritability. The session ends on a chilled out 10Hz.
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