Occupational Soundscapes – Part 2: Mechanics of Sound and the Human Ear

     A rudimentary understanding of the physics of sound and the basic functions of the human ear is necessary to appreciate the significance of test results, exposure limits, and other elements of a hearing loss prevention program (HLPP).  Without this background, data gathered in support of hearing conservation have little meaning and effective protections cannot be developed and implemented.
     This installment of the “Occupational Soundscapes” series provides readers an introduction to the generation and propagation of sound and the structure and function of the human ear; it is not an exhaustive treatise on either subject.  Rather, it aims to provide a foundation of knowledge – a refresher, for many – on which future installments of the series build, without burdening readers with extraneous or potentially confusing detail.

Sound Generation and Propagation
     Sound can travel through solid, liquid, and gaseous media.  As our primary interest is in human hearing, this presentation focuses on sound propagation through air.  It should be noted, however, that vibrations in other media can be transferred to surrounding air and, therefore, ultimately perceptible by the human ear.  In fact, structure-borne noise is a prominent component of many occupational soundscapes.
     In air, sound is propagated via longitudinal pressure waves.  The movement of particles in a longitudinal wave is parallel to the wave’s direction of travel.  In contrast, particles move perpendicular to the travel direction of a transverse wave; a common example is a ripple in water.  A pressure wave consists of alternating regions of high and low pressure.  These are known as compressions and rarefactions, respectively, as the air pressure oscillates above and below ambient atmospheric pressure as portrayed in Exhibit 1.

     Sound waves are most-often referenced by their amplitude (A) and frequency (f).  A sound wave’s amplitude is the maximum pressure deviation from the ambient (μPa).  It is related to the perception of “loudness” of the sound.
     Instantaneous sound pressure is often a less useful measure than one that is time-based, such as an average.  However, the average of a sine wave’s amplitude is zero and, thus, unhelpful.  For a metric comparable across time and events, the root mean square (RMS) sound pressure (PRMS) is used.  To calculate PRMS:
(1) Consider the sound pressure waveform over a specified time period; the period of time considered is important for comparison of sound environments or events.
(2) Square (multiply by itself) the waveform (i.e. pressures) [P^2].
(3) Average the pressure-squared waveform (mean pressure squared) [P^2avg].
(4) Take the square root of the mean pressure squared (RMS pressure) [PRMS].
The steps of this process are represented graphically in Exhibit 2 and mathematically by the equation:

Squaring the pressures ensures that RMS values are always positive, simplifying use and comparison.
     A wave’s frequency is the number of complete wave cycles to pass a fixed point each second (Hz or cycles per second).  Frequency is related to the perception of a sound’s pitch.  A wave’s period (T), the time required for one complete wave cycle to pass a fixed point, is simply the inverse of its frequency:  T = 1/f (s).
     The wavelength (λ), the length of one complete wave cycle, is related to frequency and the speed of sound (c):  λ = c/f (m).  Though the speed of sound in air is influenced by temperature and density (i.e. elevation), an approximation of c = 343 m/s (1125 ft/s) is often used in lieu of calculating a more precise value.
 
     Speech is generated by forcing air through the larynx.  Movement of the vocal cords creates pressure fluctuations that manifest as complex sound waves.  These complex waves include carriers and modulation superimposed upon them.  Timing of modulation differentiates similar sounds, such as the letters “b” and “p;” therefore, resolution of these timing differences in the auditory system is integral to speech intelligibility.

     Maintaining speech communication abilities is paramount to a hearing loss prevention program (HLPP).  As such, understanding typical characteristics of voiced sounds is critical to its success.  Exhibit 3 shows the average (dark line) and range (shaded region) of sound pressures created by a small, homogeneous sample of subjects (seven adult males) reciting the same nonsensical sentence.  Though unrepresentative of the diversity of the broader population, the results are indicative of the variability that can be expected in a wider study.  A more-generalized data set is depicted in Exhibit 4, suggesting that the most-critical speech frequencies lie in the range of 170 – 4000 Hz (dB scales and the relation to speech communication will be explored in a future installment of the series).

     In addition to the voiced sounds of “normal” speech, humans generate unvoiced, or breath, sounds.  These occur when air is passed through the “vocal equipment” (i.e larynx, mouth) without activating the vocal cords.  Breath sounds, acting as low-energy carriers, make whispering possible.
 
     All sounds in an environment – wanted and unwanted – impinge on occupants indiscriminately.  It is up to the auditory system of each occupant to receive, process, resolve, differentiate, locate, and interpret these sounds collectively and/or individually as circumstances dictate.  Much of this work is performed by a sophisticated organ that often garners little attention:  the seemingly underappreciated ear.
 
Structure and Function of the Human Ear
     Exhibit 5 provides a pictorial representation of the ear’s complexity; a detailed discussion of each component is impractical in this introductory presentation.  Instead, a brief description of some critical components and their roles in the perception of sound is offered.  It won’t make “experts” of readers, but will provide the basic understanding needed to support hearing conservation efforts.

     Hearing – the perception of sound – takes place in three “stages” corresponding to the three regions of the ear:  outer, middle, and inner.  Common use of the word “ear” often refers only to the outer ear, the region highlighted in Exhibit 6.  Many times, it is intended to reference only the visible, cartilaginous portion called the pinna or auricle.  The pinna is most famous for adornment with jewelry and being the part of a misbehaving child pulled by a TV sitcom mom.

     Sound waves in the environment impinge upon the outer ear, where the pinna helps direct them into the external auditory canal, or simply ear canal.  The structure of the ear canal causes it to resonate, typically, in the range of 3 – 4 kHz, providing an amplification effect for sounds at or near its resonant frequency.
     The terminus of the outer ear is the tympanic membrane, commonly known as the eardrum.  The variable pressure of the impinging sound waves causes this diaphragm-like structure to flex inward and outward in response.
 
     In the middle ear, highlighted in Exhibit 7, the vibrational energy of the flexing eardrum is transmitted to another membrane in the inner ear via a linkage of three small bones, collectively called the ossicles or ossicular chain.  The malleus is attached to the eardrum and the stapes is attached to a membrane in the oval window of the cochlea.  Between these two lies the incus, the largest of the three bones.  The malleus, incus, and stapes are commonly known as the hammer, anvil, and stirrup, respectively.

     The configuration of the ossicular chain provides approximately a 3:1 mechanical advantage.  In conjunction with the relative sizes of the eardrum and oval window, the middle ear provides an overall mechanical advantage of approximately 15:1.  The ability to hear very soft sounds is attributed to the amplification effect produced in the middle ear.
     The Eustachian tube connects the middle ear to the nasal cavity, enabling pressure equalization with the surrounding atmosphere.  Blockage of the Eustachian tube, due to infection, for example, results in pressure deviations that can reduce hearing sensitivity, potentially to the extent of deafness.
     Two small muscles, the stapedius and the tensor tympanic, serve a protective function against very loud sounds.  These muscles act on the bones of the ossicular chain, changing its transmission characteristics to reduce the energy transmitted to the inner ear.  This protection mechanism is only available for sustained sounds, as the reflexive contraction of these muscles, known as the acoustic reflex, does not engage rapidly enough to attenuate sudden bursts of sound, such as explosions or gunshots.
 
     The inner ear, highlighted in Exhibit 8, is comprised primarily of the cochlea.  The cochlea is an extraordinary organ in its own right; its presentation here is, necessarily, an extreme simplification.  Many of its components and functional details will not be discussed, as a descriptive overview is of greater practical value with respect to hearing conservation.

     The motion of the stapes (stirrup) in the oval window induces pressure fluctuations in the fluid in the chambers of the cochlea.  The mechanical advantage provided by the middle ear serves to overcome the impedance mismatch between the air in the outer and middle ear and the liquid in the inner ear.  As mentioned previously, this maintains sensitivity to low-intensity sounds.
     The fluid movement, in turn, causes tiny hair cells in the cochlea to bend in relation to the sound energy transmitted.  These hairs are selectively sensitive to frequency; the extent of bending is proportional to the loudness of the sound.  It is this bending of hair cells that is translated into electrical signals that are sent to the brain for interpretation.  Damaging these sensitive hairs leads to reduced hearing sensitivity.  They are also nonregenerative; therefore, hearing loss caused by damaging these hairs is permanent and irrecoverable.  Though other mechanisms of damage exist, NIHL is a prominent and important one.
     The basilar membrane, separating the chambers of the cochlea, is also selectively sensitive to frequency, due to its varying mass and stiffness.  This results in the tonotopic organization of the cochlea, as depicted in Exhibit 9.  The highest frequencies (~ 20kHz) are detected near the basil end of the membrane (i.e. nearest the oval window); sensitivity shifts to progressively lower frequencies along the cochlear spiral.  Sensitivity to frequencies above ~ 2 kHz, including critical speech frequencies, is concentrated in the first 3/4 “coil” of the cochlea.  The range of human hearing and the critical speech frequencies will be discussed further in a future installment.

     The semicircular canals are highly recognizable, projecting from the cochlea’s distinct snail-like shape, but they play no significant role in hearing.  They are, however, integral to the critical function of maintaining balance which enables humans to walk upright.  Sharing a fluid supply with the cochlea, issues with hearing and balance can be correlated during a traumatic event.
     The introduction to Part 1 of the series called attention to several parallels between occupational soundscapes and thermal work environments.  The fluids contained in the cochlea may demonstrate a more-direct link.  There are two key fluids contained in chambers of the cochlea.  One, perilymph, is sodium-rich and the other, endolymph, is potassium-rich.  As discussed in the “Thermal Work Environments” series (Part 3), depletion of these electrolytes (salts) can be caused by profuse sweating and/or ingesting large quantities of water without balanced replenishment.  In addition to causing heat cramps and other afflictions, it seems heat stress could affect your hearing!

     There is a great deal more detail available from various sources to explain the mechanics of hearing, particularly the inner workings of the cochlea.  It is a fascinatingly complex organ; intrigued readers are encouraged to consult the references at the end of this post, as well as medical texts, to learn more.  Despite the requisite simplification of this presentation, sufficient information has been included to enable readers to continue on this journey of sound exploration in pursuit of the ultimate goal: effective hearing conservation practices.

     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] ”29 CFR 1910.95 - Occupational noise exposure.’  OSHA.
[Link] “Noise Control Design Guide.” Owens Corning; 2004.
[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] “How Hearing Works.”  Hearing Industries Association; 2023.
[Link] “OSHA Technical Manual (OTM) - Section III: Chapter 5 - Noise.”  Occupational Safety and Health Administration; July 6, 2022.
 
Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
jody@jaywink.com