Occupational Soundscapes – Part 16: A Rainbow of Noise

     There are several sounds, with defined spectral compositions, that have been assigned colors for identification.  These colors invoke the rainbow of the visible light spectrum to facilitate comparative references to the spectral nature of various sounds.  The analogy is not completely accurate, however.
     Each named color in the visible light spectrum consists of a single frequency of radiation, the equivalent of a pure tone sound.  A named color of sound, in contrast, refers to a specific combination of frequencies.  It should also be noted that the characteristics and effects associated with certain colors by traditional color theory are not applicable to sound.  There are some interesting parallels, however, between color theory and colored noise.

     This installment differs significantly from others in the “Occupational Soundscapes” series.  The series is loaded with information derived from research and refined by experience.  Recommendations provided were developed with scientific and engineering rigor.  The information related in this installment, in contrast, lacks the empirical data necessary to gain universal acceptance.
     Though research has been conducted on the effects of colored noise exposure, it simply is not thorough enough to claim “clinically proven” benefits.  Anecdotal evidence suggests that some people may benefit from colored noise “therapy;” therefore, this sidebar installment is deemed worthy of inclusion in the series.
     A note on usage of the terms sound and noise in this installment should be made.  Throughout the series, noise has been loosely defined as “unwanted sound.”  The difference between sound and noise remains a matter of perspective when discussing colored noise.  In this context, the term noise is applied to indicate a lack of recognizable, or coherent, sounds, such as speech or music, though it is intentionally added (i.e. “wanted”) to the soundscape.

Hear the Rainbow
     A brief summary of several named sound/noise colors is provided below.  A description of the spectral composition, source examples, and potential benefits of employing each are presented.  Visual representations of some colored noise spectra are shown in Exhibit 1. 
White Noise
     Readers are likely familiar with the term “white noise,” though it is often misused; many times, any random collection of sound frequencies is called white noise.  Its precise definition is a broadband sound consisting of all frequencies in the audible range at equal intensity.  If this were the only named color of sound, the analogy to light would be accurate.
     White noise sources include fans, vacuum cleaners, and older TVs and radios when tuned to an unused channel.  Claimed benefits include improved sleep and memory, mitigation of tinnitus (see Part 8), and improved cognitive performance in children with attention-deficit hyperactivity disorder (ADHD).  Some relief from tinnitus is attained by way of masking (see Part 9) the internal noise with an external white noise.  Cognitive performance can be improved by adding white noise to increase the S/N (see Part 9) of the desired message; this phenomenon is called stochastic resonance.
Pink Noise
     The intensity of pink noise constituent frequencies decreases at a rate of 3 dB/octave. This bias toward lower frequencies results in a deeper sound than white noise.  Sometimes called “the sound of nature,” pink noise sources include rainfall, rustling leaves, and crashing waves.
     Claimed benefits include increased attention span, improved focus, memory, sleep, and productivity, as well as stress reduction.  It is believed that pink noise triggers the hypothalamus, increasing signal speed to and from the brain.
Red Noise
     Red noise has a stronger bias toward low frequencies, with intensity decreasing at a rate of 6 dB/octave.  This results in a very deep, “bassy” sound.  It is also called brown noise, named for Brownian motion of particles.  Red is used here, as it is better aligned with the visible light analogy, as imperfect as it may be.  Sources of red noise include waterfalls, heavy rain and thunder, and some heavy equipment.
     Claimed benefits include improvement in memory and organization skills, increased attention span, and reduced reaction times.  Its bass-laden content may also be used to mask low-frequency annoyances.
Orange Noise
     Orange noise consists of a flat spectrum (i.e. white noise) with the frequencies corresponding to notes on a musical scale removed (i.e. zero intensity).  The removal of “in-tune” frequencies causes a discordant sound that is often equated to an out-of-tune orchestra.  It may also be called yellow noise.
     Sources of orange noise include the aforementioned musical instruments in need of tuning and some forms of electronic feedback.  Specific benefits are not often claimed, but orange noise can be used in studies of auditory perception and in experimental music.
Green Noise
     Green noise has been described as “bounded Brownian noise” and “vocal spectrum noise” with an energy peak at its center frequency, 500 Hz.  A more-precise definition is elusive.
     Allegedly, green noise is the sound of nature sans human influence; it is associated with gentle flows of water and wind.  It seems that internet gurus have more to say about green noise than reputable researchers, claiming that it calms anxiety, promotes relaxation, and improves sleep quality.
Blue Noise
     Blue noise is the opposite of pink noise; the intensity of its constituents increases at a rate of 3 dB/octave.  Its high-frequency dominance results in a hissing sound.
     Blue noise is typically associated with electronic equipment, as both source and application.  It is used in audio engineering to improve the quality of digital audio.  Benefits to individuals are not usually claimed; blue noise hiss tends to be unpleasant.
Violet Noise
     Also called purple noise, violet noise is the opposite of red noise; intensity increases at a rate of 6 dB/octave.  Though it does not match the profile exactly, a free-flowing water faucet approximates a violet noise source.
     Violet noise may be used to mask tinnitus.  Some claim that it can also improve attention and memory.  For many, attention will be focused on finding and silencing the source of this terribly shrill hissing noise!
Grey Noise
     Grey noise consists of all frequencies in the audible range, with intensities adjusted to achieve the perception of equal intensity at all frequencies.  This is like subjecting white noise to an inverted A-weighting scale; high and low frequencies are enhanced to compensate for the ear’s sensitivity to middle frequencies.
There are no naturally-occurring sources of grey noise; it is generated for customized tinnitus treatment, auditory testing, and similar applications.  Many substitute grey noise for white noise, finding its “softer,” balanced sound more soothing.
Black Noise
     Black noise is not a widely-used term, nor does it have a universally-accepted definition.  It is included here to bookend the comparison with visible light.  For this purpose, black noise refers to an absence of sound (i.e. silence) in the same way that “black” is an absence of visible light (i.e. darkness).
     To experience black noise, an exceptionally efficient anechoic chamber or a trip to deep space may be needed.  The experience could also be quite disconcerting.  In addition to free-floating in outer space (Where is my spaceship!?!), sounds from within the human body would be experienced in a whole new way.  Every heartbeat and vascular peristalsis, respiratory and digestive function would reveal the auditory components masked by normal life on earth.  If not for the unmitigated tinnitus, you probably could hear yourself think!

Caveats for Use of Colored Noise
     While studies have been conducted to test the effects of various noises on individuals in various situations, there is no scientific consensus.  Given the highly-variable nature of individuals’ lives and highly-subjective nature of sound exposure, a definitive guide for use cannot be provided.  In fact, the claims of benefits cited here are given to be thorough, but with no confidence in their veracity.
     One conclusion that appears to be universal is that there is no lasting detriment caused by experimenting with colored noise.  Therefore, a trial-and-error approach is a completely acceptable method of determining if one can reap benefits similar to those reported by others.  If not, the entire sound spectrum is available for experimentation; a cacophony to one is a symphony to another.  Colored noise generators are widely available as standalone devices and smartphone apps.  Extended-length files are also available online to facilitate overnight or workday trials of chosen spectra.
     Individual preferences require that great care be taken when adding colored noise to a shared space.  Whether at home, to improve sleep, or at work, to improve concentration and productivity, everyone in a shared space must approve of the added noise.  If there is no consensus, personal listening devices must be used to deliver the colored noise only to those who benefit from it.  In all cases, other auditory requirements (e.g. hearing alarms, speech, etc.) must not be compromised.
     Colored noise is a form of “acoustic perfume” used to transform a soundscape from an annoyance to an asset.  It is preferred to music in many situations, because it has less potential to create a distraction of its own.  Music played in early elevators was intended to distract passengers.  Its entertainment value made the trip seem shorter, while masking mechanical sounds of the lift equipment that could be unnerving to the uninitiated.  In an occupational setting, however, entertainment can be counterproductive.

     Part 17 is the capstone of this series, bringing together all of the information presented throughout previous installments to outline a comprehensive soundscape management program.  This includes the ways in which colored noise may – or may not – be an important component of an individual’s sound exposure.

     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Occupational Soundscapes” volumes on “The Third Degree,” see Part 1: An Introduction to Noise-Induced Hearing Loss (26Jul2023).

References
[Link] “Low intensity white noise improves performance in auditory working memory task: An fMRI study.”  Elza Othman, et al.  Heliyon; September 2019.
[Link] “Spectral Content (colour) of Noise Exposure Affects Work Efficiency.”  Shih-Yi Lu, Yuan-Hao Huang, and Kuei-Yi Lin.  Noise and Health; January – March 2020.
[Link] “All the Colors of Noise, Explained.”  Jacob Thrall.  August 2023.
[Link] “Music and Noise.”  The Physics Hypertextbook.
[Link] “Noise Types Uncovered Discover the Difference Between Various Sounds.”  Alecia Steen.  Prime Sound; March 4, 2024.
[Link] “The Ultimate Guide to Colored Noise.”  Abby McCoy.  Sleepopolis; November 6, 2023.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
[email protected]