Loud speech can mask its own high-frequency components via upward spread of masking

Evidence

More subtly, it is possible for speech that is produced at high levels to mask itself. That is, if the speech is too loud, then, notwithstanding the improvement in signal-to-noise ratio, the intelligibility is reduced, because the low-frequency components of the speech mask the high-frequency components due to the upward spread of masking. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2480

Categorization

• DEEPTHINKING: A genuine paradox — the intuitive fix (speak louder) degrades intelligibility via upward spread of masking within the speaker’s own voice.
• SEED: An essay on why louder public-address announcements so often become less intelligible.
• corroborated: 2 independent passages support this claim — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2480
• XREF: connects to: Masking spreads from a noise band to tones further away as masker level rises; A masker raises the audibility threshold of a test sound.; Masking acts within critical bandwidths, leaving distant-frequency sounds unaffected — Eric J. Heller, Why You Hear What You Hear- An…, 2009 (+9 more)