Few sounds are intrinsically unpleasant. They become so if they intrude sufficiently on a listener's consciousness to distract him from his own pursuits. The listener must first be able to hear the intruding noise above the other ambient noises in his own environment. In addition, it must attract his reluctant attention. This requires that the sound have some special character, usually denoting a specific activity.
Speech sounds are particularly troublesome if they are intelligible or nearly so.
Similarly, a single identifiable source of noise is more troublesome than the same level produced by a random assortment of many noises from many sources. Sounds that are suitably random and meaningless sometimes serve the useful function of masking other sounds that would be distracting. The most acceptable form a masking sound can take is white noise, perhaps suitably tailored to mask a particular intruding sound. The widespread use of background music in restaurants and other public areas is an example of the use of meaningless noise to protect people from their neighbours. Unfortunately, some of the purveyors of background music misunderstand its function and use a collection of musical cliches that is itself annoying.