Interesting, as our university (biomed department actually) was doing research of influence of alpha brain frequencies on learning process in late 90s early 2000s. They were taking the EEG of a person to check for different bands and then they were stimulating person with the pure frequency or different percentage mix and checking how long the learning process took. As humans have problems with hearing infrasounds the audio stimulus was delivered via differential or modulated audio signals, the visual stimuli were delivered via precisely controlled LED light source (they were checking the shape of the wave controlling the LED - if I remember correctly a sinewave was giving the best results).

The difference with this new research was that they were using audio stimuli and the learning test was conducted during the stimulation, while in this new research the test was conducted immediately after the stimulus (and it was visual only). Audio stimulation is tricky as the frequencies are so low, so you have to use a carrier in the hearing range modulated with the frequency under test. IIRC they were testing a single ear, both ears with same signal and both ears with differential signal. And actually the differential signal was working (identical signal was working as well - it just had to be properly modulated - I don't remember if there was any difference in results between those two).

I am not saying that hemi-sync and other brands deliver what they promise, just that principle of stimulating brain with person's own frequencies was tested years ago and differential aural signal stimulus (known commonly as binaural beats for some reason) if matched with person's own frequencies can be used as a stimulus in this case.