The power spectral density of white noise is the same in all frequency bands. Because white light is a mixture of monochromatic lights with various frequencies (colors), this signal with flat power spectrum is called "white", while other noise signals without this property are called colored noise.
The ideal white noise has infinite bandwidth, so its energy is infinite, but this is impossible in the real world. In fact, flat signals with limited bandwidth are often considered as white noise in life, because it is more convenient in mathematical analysis. Generally speaking, as long as the spectral width of a noise process is much larger than that of the system it acts on, and its spectral density can be regarded as a constant within this bandwidth, it can be treated as white noise.
Application of white noise
One of the application fields of white noise is architectural acoustics. In order to weaken the distracting and unwanted noise in the internal space, continuous low-intensity noise is used as the background sound. White noise is also used in electronic communication. It is used directly or as the input signal of a filter to generate other types of noise signals, especially in signal synthesis, and is often used to reproduce signals with high noise components.
White noise is also used to generate impulse response. In order to ensure the balance effect of live concerts or other performances, instantaneous white noise or pink noise is emitted from PA system, and noise signals are monitored in different places, so that engineers can automatically amplify or reduce some frequencies of building acoustic effects, so that the overall balance effect can be adjusted to obtain balanced harmony.