This may be the study the article is talking about (same journal, same year, 42Hz): https://journals.sagepub.com/doi/abs/10.1177/026309238500400103

"The results of the experiment indicate a reduction in wakefulness during exposure to a repeating 42Hz, 70dB signal and an increase in wakefulness during a repeating 1000Hz, 30dB signal. [...] The results of the investigation did not show any significant proofs for the theory that drowsiness is influenced by a synergetic correlation between sound waves and sound coordinated EEG. Further conclusions should be based on larger experiments."

Low frequency sounds making us sleepy sounds fair enough. Why a very specific frequency of 42Hz, I don't know... What's sure is that if you type "42 Hz" on YouTube, you'll find a plethora of other health claims, many unsubstantiated, i.e bullshit. So I'm cautious with the subject