Depending on the noise, this ratio can be linear, as in white noise or thermal noise, or can decrease linearly, as in pink noise. White noise is characterized by the fact that the noise density is always the same for the same bandwidth, regardless of frequency. For example, the noise density between 10.5 kHz and 11 kHz is the same as that between 50 kHz and 50.5 kHz.
The situation is different for pink noise, where the noise density is always the same in the same frequency ratios. Thus, the frequency range between 10 kHz and 20 kHz has the same noise density as that between 30 kHz and 60 kHz.