Gray & Black Noise

GRAY & BLACK NOISE

What is Gray Noise?

Gray Noise is a random white noise subjected to a psychoacoustic equal loudness curve (such as an inverted ‘A’ weighting curve) over a given range of frequency. Gray noise is also referred to as a Psychoacoustic White noise. The psychoacoustic curve gives the listener the perception that the sound is equally loud at all frequencies. In contrast, a white noise has equal power over the entire frequency range but it is not perceived as equally loud due to the non-linear frequency response of human ear. The psychoacoustic curve considers the non-linear frequency response of ear and hence gray noise is perceived as equally loud over the entire frequency range.

Fig1 Frequency response of a human ear

What is Black Noise?

The absence of sound or noise is formally termed as black noise. There exist certain informal definitions of black noise such as sound of silence, residue of noise after active noise cancellation, noise with a spectrum corresponding to the black body radiation and noise beyond the human range of hearing.

Power Spectral Density (PSD)

The power spectrum of a time series describes the distribution of power into frequency components composing that signal. The power spectral density (PSD) refers to the spectral energy distribution per unit time. PSD describes how the power of a signal or time series is distributed over frequency. The PSD is a graph of energy level of noise in decibel (dB) versus the frequency range in hertz (Hz).

The PSD of a white noise signal is flat which explains the energy is equal throughout the entire range of frequencies. The PSD of gray noise signal follows the psychoacoustic equal loudness curve and thereby is non-linear, which highlights the equally loud perception over the entire frequency range. Simply put White noise contains all the frequencies with equal energy whereas gray noise contains all frequencies with equal loudness.

Fig 2 PSD of white noise

Fig3 PSD of gray noise

Application

1. White noise is used for privacy shielding in open plan offices.

2. White noise is often used for inducing sleep.

3. Gray noise is useful in audiometric studies of hearing difficulties allowing researchers to assess how a particular person’s hearing differs from the average.

Audio file - Gray noise

https://soundcloud.com/user-65477107/gray-noise

Conclusion

There is no ONE gray noise because each human ear may have slightly different Equal loudness curve. Also, the EQC depends on the volume of noise playback. There however is ONE white noise which is as per the definition of flat power spectral density.