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.
