Uniform masking noise must drop 3 dB per octave above 500 Hz.

Evidence

The masking threshold plots have a constant value up to about 500 Hz, after which they increase with a slope of 3 dB/octave (10 dB/decade). If spectral masking with a constant effect at all frequencies is desired, a uniform masking noise must be used, where the spectral density decreases correspondingly by 3 dB/octave at frequencies above 500 Hz. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 993

Categorization

• XREF: Practical detail for engineering uniform masking noise in test and calibration rigs.
• corroborated: 2 independent passages support this claim — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 993