I am not sure how cats process sound waves but I can vouch for a fact that they capture them better and with a higher intensity than us humans. Thats why cats flee at any sudden burst of sound. Thats why we should avoid high decibel sounds in the vicinity which would make pets uncomfortable including dogs. I have reason to believe that cats enjoy a constant pitch and tone of sound like that of a small radio. You can test this by giving them some snack or milk to check whether they would take it. This is a testimony that they are taking the sounds positively. Gradual increases i volume tend to make little difference since view have acclimatized. Cats tend to cope better with environmental stressors like dogs and tend to process both danger and disturbance for a brief period of time before discarding the same. This is insured by their agility unlike dogs who are less agile and tend to rely on their senses and are on guard most of the times and possess a good memory of the goings on on a daily basis.