In distinguishing action from communication, Luhmann mentions social resonance:
Action occurs even when nobody is watching, when nobody else is there, when the agent does not expect that somebody else will react to her action–for instance, when somebody brushes her teeth while by herself. It is done merely because everybody knows that it ought to be done. True, one was told by somebody to do it and somebody put the toothbrush there for this purpose. However, in principle, action can be conceived of as a solitary, individual operation that has no social resonance. In the case of communication, this is not possible.(Introduction to Systems Theory, 2012, p. 54)
Whether a particular person does or does not brush one’s teeth has no effect on society; the choice to brush one’s teeth or go to bed without brushing one’s teeth does not resonate (or resister) with society; it does not challenge society or demand that society do something. But if I set my house (or my neighbor’s house) on fire, society notices.
In the same text, Luhmann discusses “system rationality.” He states that
a system is defined by means of its indifference in regard to the environment. In light of this definition, system rationality means that one takes this back or that one denies the indifference of the system (the fact that whatever happens in the environment does not happen to us) and instead strengthens the irritability, sensitivity, or resonance (or whatever term may be used) of the system.pp. 136-37
Luhmann equates resonance with a system’s irritability or sensitivity, and he suggests that
the differentiation of functional systems has the function of increasing chances for rationality, irritability, sensitivity, and resonance in the functional systems. It makes it possible for the ability to be disturbed to increase and at the same time provides counter-measures or procedural concepts, but only at the level of society in its totality.p. 138
This means that society stops being indifferent to certain environmental factors. For example, for thousands of years, society did not see child labor as a problem. Child labor did not register or resonate with society. It wasn’t “a thing.” Of course, society was aware of, or sensitive to, many problems (e.g., the plague, famine, religious heresy . . .), but child labor was not on the list of problems. In the English-speaking world, this changed in the 1880s. Here is Google Books ngram:
Judging by this chart, the social resonance of child labor peeked (in the English-speaking world) in 1937.