The 'fall off' shown in a
frequency analysis is somewhat irrelevant, as it only shows whether or not the audio has been through a lowpass filter. While undesirable, it has no meaningful use when determining if audio has been through lossy data compression. The only reliable way to be sure is to use a
spectrum analyzer to observe a 2-3 second span of the audio and examine for 'blocks' or the 'Lego effect' with your eyes. These 'blocks' are the result of audio chunks being deleted during the lossy data compression. With your ears, you can also listen for a 'glassy' or 'watery' sound in the high freq range... overly-shimmery, watery cymbals, etc.
in the case of your two AC/DC audio samples, all is well.. some very nice, open sounding audio with no evidence of lossy data compression being used.
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