Communications equalization?
In the '70ies I worked with a very large very high quality business intercom system. I designed, built and installed a background music system to it. The little intercom speakers did not produce enough low frequencies so I added an equalizer circuit to boost the low music frequencies then it sounded great. My government wanted a very high quality high fidelity intercom system for airports so I added high frequency equalization to the one I worked with and it was the only intercom system that met their hifi spec's.
In 1980, when my government allowed competition with BELL, the company I worked for began to sell, install and repair business telephone systems. I knew that the capacitance between wires in a landline cut high audio frequencies on long distance calls so the carrier company equalizes the frequency response by boosting the high audio frequencies. One day I called out on one telephone line and received on another telephone line then I measured the frequency response. It was horrible, 3kHz was down -12dB so I asked the carrier company why it was so bad. They said it was normal, their spec's limit is -15dB within a city where they do not equalize the audio.
Then I worked for a company that conferenced many telephone lines for banks, investment companies and other large companies. Users complained about the badly muffled sound. So I designed, built and demo'd an audio equalizer that boosted the high audio frequencies and it sounded so good that every demo I made sold one for each member in their conference.