That is a general assumption people make but there isn’t really much science to back it up. What we do know though is that isochronic tones are ineffective below 4hz, and possibly a bit higher. So for theta and delta entrainment you are best off with binaural beats. In my experience binaural beats will be very effective all the way up to around 40hz.

I would like to share with you my experience after using EquiSync for three weeks. First of all, I am so amazed how fast I can put myself in a meditation state. During meditation, I feel some kind of a sensation I could not explain. I feel the rain dropping on my face and sometimes my legs. I feel the breeze of a tropical weather as if I was in the forest. I always sleep for a very short time at night and I have a very high sense of smell all my life. Using equisync I still sleep less, but am more relaxed when I wake up. During meditation, I can also smell the forest as if I am in the forest.

Binaural beats were the first method discovered for brainwave entrainment and works by delivering tones of different audible frequencies to the two ears with the difference in frequency between the two tones being the frequency of entrainment. The difference in frequency between the two tones must be less than 30 hertz, and this resulting frequency is called a beat or the target frequency, and it is processed in a brain region called the olivary body. When listening to such tones with stereo headphones, the two hemispheres of the brain become synchronized at the target frequency.
Neural oscillations are rhythmic or repetitive electrochemical activity in the brain and central nervous system. Such oscillations can be characterized by their frequency, amplitude and phase. Neural tissue can generate oscillatory activity driven by mechanisms within individual neurons, as well as by interactions between them. They may also adjust frequency to synchronize with the periodic vibration of external acoustic or visual stimuli.[3]