Now this module will examine both specifications in more detail. Output Noise Spectral Density is the output noise energy with respect to frequency. Noise spectral density is made up of two noise sources: 1/f Noise and Thermal Noise. 1/f noise is also known as Flicker Noise or Pink Noise and arises due to the nature of the component and the impurities of the manufacturing process. There is no general equation to define this noise.