The concept of “colored noise” refers to a family of sounds used in acoustics and sound therapy, categorized by their specific distribution of energy across the frequency spectrum. While many people are familiar with white and pink noise, grey noise is a distinct category engineered with a unique focus. Grey noise differs because its frequency distribution is specifically tailored to how the human ear perceives sound, rather than relying on a mathematically flat distribution.
The Unique Sound Profile of Grey Noise
The defining characteristic of grey noise is that it is designed to sound equally loud across all frequencies audible to the human ear. This is complex because the human auditory system does not perceive all frequencies at the same physical intensity level as having the same loudness. Our ears are most sensitive to mid-range frequencies (1,000 to 5,000 Hertz) and less sensitive to very low and very high frequencies.
To achieve this perceived neutrality, the physical energy of grey noise is intentionally manipulated to counteract the natural biases of human hearing. Scientists use the equal-loudness contour, sometimes referred to as the Fletcher-Munson curves, to model this perception. Grey noise is created by filtering a random signal, like white noise, with a curve that is the inverse of a standard loudness contour, such as the A-weighting curve. This process ensures that sound intensity is boosted in the low and high frequencies where our hearing is weaker. The goal is a neutral sound experience where no single frequency band stands out as louder or softer than any other.
Grey Noise Versus White and Pink Noise
Grey noise is distinguished from two more common types: white noise and pink noise. White noise contains equal energy at every frequency across the audible spectrum. Because the human ear is highly sensitive to high-frequency sounds, white noise is often perceived as a harsh, hissing sound, similar to static.
Pink noise is an adjustment to white noise, where the energy decreases by approximately three decibels for every doubling of frequency (an octave). This decrease favors the lower frequencies, making pink noise sound flatter and deeper, often compared to heavy rainfall. While pink noise is perceptually more balanced than white noise, it still follows a mathematical rule of energy distribution per octave.
Grey noise, in contrast, abandons the mathematical rules of energy distribution used by white and pink noise in favor of psychoacoustics, the study of sound perception. Instead of equal energy across all frequencies (white) or equal energy per octave (pink), grey noise is the only one designed for equal perceived loudness across the entire spectrum. This tailoring makes grey noise the most acoustically neutral option, feeling balanced and seamless to the listener.
Why Grey Noise is Used for Hearing Health and Relaxation
The psychoacoustic neutrality of grey noise makes it a valuable tool in both clinical and personal sound environments. In hearing health, its balanced nature makes it an ideal background signal for audiologists performing threshold tests. When used during an audiogram, the perceptually flat sound provides an unbiased acoustic backdrop, ensuring that a person’s hearing sensitivity is tested without the influence of a frequency band sounding louder than another.
Grey noise is also applied in managing conditions like tinnitus, the perception of ringing or buzzing in the ears. Because tinnitus often involves a high-frequency component, a perceptually balanced sound like grey noise can be more effective for sound masking than white noise. The smooth, balanced sound helps to distract the auditory system and provide relief from intrusive ringing.
For relaxation and concentration, the unique sound profile of grey noise helps it blend into the background more effectively than other noises. Its equal perceived loudness prevents any one frequency from drawing the listener’s attention, promoting focus or calm. This lack of auditory peaks allows the sound to become a non-distracting acoustic blanket, helping to drown out sudden environmental noises that might interrupt sleep or concentration.