The PPM's purpose is to ensure signal peaks are never so great that they over-modulate the transmitter which will result in the audio being clipped and cause transmitters to exceed their allotted bandwidth. Extremely fast transients are not displayed by these 'quasi-peak' PPMs. The USA tradition developed differently, with peak metering still needed for transmission management, but to allow the sound mixer to see what they hear they adopted the average reading Volume Unit (VU) meter. Despite UK engineers claiming it stood for 'virtually useless' the VU meter provides a helpful pointer towards something the PPM fails to show and that is how loud something may seem to be. Of course it is not a real loudness measurement.
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