These levels come from outside activities that penetrate the classroom structure, from teaching activities, and other noise generated inside, which can be exacerbated by room reverberation.
Subjects reported finding the predictable and unpredictable noise equally annoying, and all subjects performed at about the same level during the noise portion of the experiment.
When we are annoyed or made irritable by noise, we should consider these symptoms fair warning that other thing may be happening to us, some of which may be damaging to our health.