Occupational Soundscapes – Part 7: Perceptions

     The previous installment (Part 6) of the series dealt with objective measures of sound exposure.  Objective measurements, however, do not fully describe one’s experience of the surrounding soundscape.  The subjectivity of human perceptions of sound plays a vital role in effective noise control and communication system design.
     This installment of the “Occupational Soundscapes” series explores aspects of the human experience of sound that SPLs and TWAs alone do not explain.  These include the concepts of “loudness” and “noisiness” – terms that reflect the subjective nature of sound.

Loudness/Level
     “Loudness” is the term used to describe the subjective perception of a sound’s intensity or pressure.  Sometimes called a “loudness index,” loudness is quantified in sones, a linear scale defined as follows:

• One sone is the loudness of a 1000-Hz pure tone at a sound pressure level (SPL) of 40 dB (the “reference tone”).
• A sound of any frequency and SPL perceived to be of equal loudness to the reference tone is 1 sone.
• A sound “twice as loud” as the reference tone is 2 sones, a sound three time as loud is 3 sones, and so on.
• A sound that is “half as loud” as the reference tone is 0.5 sone, a sound one-quarter as loud is 0.25 sone, and so on.
• The threshold of hearing at 1000 Hz is 0 sones.

     The loudness of a sound has a corresponding “loudness level.”  The loudness level is analogous to the power, intensity, and pressure levels introduced in Part 3, though it is not derived in the same way.  Loudness level is also defined for a reference tone in units of phons.  A sound’s loudness level, in phons, is numerically equal to the SPL, in decibels (dB) of a 1000-Hz tone of equal loudness.  The reference tone (1000 Hz, 40 dB) has a loudness level of 40 phons by definition.  The loudness level is conceived as a perceived or “equivalent” SPL, as it ascribes an equivalent experience to a subject sound and the reference tone.
     Contours of equal loudness and loudness level are shown in Exhibit 1 for pure tones; Exhibit 2 presents curves for equal loudness levels of octave bands.  Alternative representations, using 1/3 octave band curves, are shown in Exhibit 3 and Exhibit 4.  These may facilitate estimation of perceived loudness in some applications, depending on the data available.  Equations have been developed to convert between perceived loudness/level and SPL.  Such calculations will not be presented here, however, as their value is dubious; the precision of a calculated value can be misleading.  For example, citing an SPL can obscure the fact that is was “converted” from subjective assessments, implying that knowledge of the sound is more “concrete” than it really is.

     To further illustrate the potential for confusion, consider the inverse square law example presented in Part 3.  There, it was shown that doubling the distance from a sound source reduced the sound’s intensity by a factor of 4 and its intensity level by 6 dB.  This equates to a reduction in sound pressure by a factor of 2, achieving an SPL reduction of 6 dB.  However, this is not the same as the change in loudness level.  A change of approximately 10 dB is needed to perceive a change in loudness by a factor of 2, as shown by the relationship of sones to phons (i.e. an increase of 10 phons doubles the sones) depicted in Exhibits 1 – 4.
     The curves represent the average responses of research subjects.  Human subjectivity, as always, renders the curves approximations or estimates of individuals’ actual experience.  Nonetheless, these estimates are useful references for communication system and noise control designers.
 
Noisiness/Annoyance
     The terms perceived noisiness and annoyance are often used interchangeably to describe the extent to which a sound is unwanted, unacceptable, or bothersome.  Analogous to the sone for loudness, noisiness has been assigned the unit of noy, where a 2-noy sound is twice as noisy as a 1-noy sound, a 3-noy sound is three times as noisy, and so on.  The perceived noise level (PNL) is “translated” into units of PNdB according to the following:  PNdB = 40 + 10 log2 (noy).  There is also a conversion scale on the right side of Exhibit 5, which provides equal-noisiness contours.  These curves were originally developed by assessing the sound of aircraft flyovers; as such, they may not be broadly applicable to occupational settings.  The caveats offered in regards to loudness are also applicable to noisiness.

     The concept of noisiness and techniques for its assessment have been developed to an extent far beyond the scope of this series.  In lieu of mastering the nuances of “synthetic” measurement units, it may be of greater practical value to bear in mind the characteristics of sound that contribute to annoyance.  Five parameters have been identified as significant factors in noisiness assessments:

• Spectral composition and levels – this relates to frequency weighting scales (e.g. dBA, dBC) and equal-noisiness curves.
• Spectral complexity – this refers to concentrations of energy within a broadband sound spectrum.
• Duration of exposure.
• Time of increasing levels prior to reaching maximum (continuous sounds).
• Level increase within a 0.5-second interval (impulsive sounds).

Awareness of these factors facilitates effective soundscape management, even if perceived noisiness (PNdB or other unit) numbers are unknown.
 
Frequency/Pitch
     The relationship between frequency and the perception of pitch, first mentioned in Part 2, has been formulated in a fashion similar to that between SPL and loudness.  Again, a reference tone of 1000 Hz at 40 dB SPL is used; it is assigned a value of 1000 mels.  A sound perceived to be twice the pitch of the reference tone is 2000 mels, and one-half the pitch of the reference tone is 500 mels.
     The sensitivity of human perception to changes in sound also varies with its frequency.  Curves showing the variation in sensitivity to changes in frequency are shown in Exhibit 6 and to changes in SPL in Exhibit 7.  High sensitivity to pressure and frequency changes in the 2 – 5 kHz range contribute to speech intelligibility.

     The Doppler Effect is a frequency-related phenomenon.  It is experienced when a sound source and listener are in relative motion (either or both can be moving).  As the distance between source and listener increases, the pitch of the sound decreases, and vice versa.  In this case, the frequency of the emitted sound does not change; it is the relative motion that causes a change in perception.
     Other frequency-related phenomena include perceived frequency shifts due to extended exposure or high intensity, consonance and dissonance, and beating.  Frequencies deemed “pleasant” in combination are said to be consonant, while those found objectionable are called dissonant.  This concept is applicable to annoyance and musical preferences, for example.
     Beating occurs when two sounds with similar frequencies are coincident, causing periodic reinforcement and cancellation.  The occurrence of beats can be used to identify mismatched operating speeds of equipment.  When two identical fans, for example, are synchronized, beating ceases.

Localization/Directionality
     A “binaural effect” allows humans to locate the source of a sound, or its directionality; this is called localization.  A sound may reach each ear at different times (“phase difference”) or at different intensity.  These differences are caused by the human body itself; this effect is known as the Head-Related Transfer Function (HRTF) among other names.  By analyzing the type and magnitude of perceived differences, the brain can determine the direction in which the source lies.

Other Perceptual Phenomena
     A sudden unexpected sound, particularly one of high intensity, can cause a startle reaction.  The startle itself may not be dangerous, but induced stress could cause uncontrolled movements or distraction that jeopardizes safety or task performance.  A sudden change in sound can produce a similar effect.
     High-intensity sounds (> 130 dB SPL), whether continuous, intermittent, or transient, can exceed the threshold of pain.  In addition to extreme discomfort and distraction in the short term, permanent hearing loss is likely to also occur.
     Another phenomenon of sound involves a lack of perception.  Ultrasonic and infrasonic “sounds” are those with frequencies above and below, respectively, the audible range.  Exposure to significant energy at these frequencies can cause discomfort or illness.  The inability to recognize the exposure often leaves victims baffled, though, in some cases, vibrational sensations may aid diagnosis of the discomfort.
     The concepts of masking and audibility vs. intelligibility are also perceptual in nature.  However, their relevance to communication warrants deferring detailed discussion to a future installment focused on the design of effective communication signals and systems.

In Conclusion
     The preceding discussion of auditory perception phenomena is merely a cursory introduction to the topic.  Expectations for direct application of the information presented are much lower than for other installments of the series.  The subjectivity of loudness, pitch, etc. requires substantial research to tailor a soundscape to a specific group of people.  Doing so proactively, while ideal, is beyond the capability of most commercial operations.
     Nonetheless, familiarity with perceptual variations is valuable to anyone that designs, maintains, or uses a communication system in an imperfect environment.  Even shallow knowledge of these concepts can aid in troubleshooting existing or potential performance issues.  With this knowledge, a focused project can be undertaken, requiring a manageable number of sound measurements and experiments to be conducted.

     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] Engineering Noise Control – Theory and Practice, 4ed.  David A. Bies and Colin H. Hansen.  Taylor & Francis; 2009.
[Link] The Noise Manual, 6ed.  D.K. Meinke, E.H. Berger, R.L. Neitzel, D.P. Driscoll, and K. Bright, eds.  The American Industrial Hygiene Association (AIHA); 2022.
[Link] “Hearing Protection.”  Laborers-AGC Education and Training Fund; July 2000.
[Link] Noise Control in Industry – A Practical Guide.  Nicholas P. Cheremisinoff.  Noyes Publications, 1996.
[Link] “Noise – Measurement And Its Effects.”  Student Manual, Occupational Hygiene Training Association; January 2009.
[Link] An Introduction to Acoustics.  Robert H. Randall.  Addison-Wesley; 1951.
[Link] The Effects of Noise on Man.  Karl D. Kryter.  Academic Press; 1970.
[Link] “Handbook for Acoustic Ecology.”  Barry Truax, Ed.  Cambridge Street Publishing, 1999.
[Link] “Protection and Enhancement of Hearing in Noise.”  John G. Casali and Samir N. Y. Gerges.  Reviews of Human Factors and Ergonomics; April 2006.

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