By A Mystery Man Writer
File:PH Scale.svg - Wikipedia
File:Avalanche considerable danger level.svg - Wikipedia
File:ETCS Level 1.svg - Wikipedia
File:Line.svg - Wikipedia
File:NV-energy-levels.svg - Wikipedia
File:Pge-overview.svg - Wikipedia
What we mean by 'audible', 'inaudible' and 'ultrasonic', by James Nesfield, Chirp
File:Valence atomic energy levels for At and Uus horizontal.svg - Wikipedia
File:Sound levels.svg - Wikipedia
File:Electronic levels.svg - Wikipedia