Every year television stations receive hundreds of complaints about the loudness of advertisements. However, federal rules forbid the practice of ads louder than the programming. In addition, television stations always operate at the highest sound level allowed for reasons of efficiency. According to one NBC executive, no difference exists in the peak sound level of ads programming. Given this information, why do commercials sound so loud The sensation of sound involves a variety of factors in addition to its speak level. Advertisers are skillful at creating the impression of loudness through their expert use of such factors. One major contributor to the perceived loudness of commercials is that much less variation in sound level occurs during a commercial. In regular programming the intensity of sound varies over a large range. However, sound levels in commercials tend to stay at or near peak levels. Other "tricks of the trade" are also used. Because low-frequency sounds can mask higher frequency sounds, advertisers filter out any noises that any drown out the primary message. In addition, the human voice has more auditory (听觉的) impact in the middle frequency ranges. Advertisers electronically vary voice sounds so that they stay within such a frequency band. Another approach is to write the script so that lots of consonants (辅音) are used, because people are more aware of consonants than vowel (元音) sounds. Finally , advertisers try to begin commercials with sounds that are highly different from those of the programming within which the commercial is buried. Because people become adapted to the type of sounds coming from programming, a dramatic change in sound quality draws viewer attention. For example, notice how many commercials begin with a cheerful song of some type. The attention-getting property of commercial can be seen by observing one-to-two-year-old children who happen to be playing around a television set. They may totally ignore the programming. However, when a commercial comes on, their attention is immediately drawn to it because of its dramatic sound quality. One of the reasons why commercials are able to attract viewer attention is that______.