Masking by bandlimited noise depends only on masker energy within the critical band.

Evidence

Below the cut-off frequency of the low-pass noise, the masked thresholds are the same as found using white noise as masker. The same holds true for frequencies of the test tone above the cut-off frequency of the high-pass noise. There, the masked threshold increases with the test-tone frequency by about 10 dB per decade. This means that masking on the slopes produced by bandlimited noises can be approximated by the sound pressure levels of the masker falling within the critical band at the cut-off. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 426

Using Eq. 3–2 as a guide, we can see that increases in the LPC of the noise will increase its masking effectiveness. However, we also see that increasing the noise bandwidth beyond the critical band will not lead to increases in masker effectiveness. The noise bandwidth is not a component of Eq. 3–2. However, note that this equation only applies for maskers that have bandwidths greater than the size of the critical band. — Jennifer Lentz, Psychoacoustics- Perception of Normal…, 592

Categorization

• XREF: This is the psychoacoustic expression of spectral integration: once noise fills the ear’s filter at the probe frequency, extra bandwidth adds nothing - parallel to how energy summation works within a single auditory filter.
• corroborated: 2 independent passages support this claim — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 426; Jennifer Lentz, Psychoacoustics- Perception of Normal…, 592