The auditory canal represents an indispensable biological conduit that bridges the acoustic environment with the delicate biomechanical apparatus of the middle and inner ear. Far more than an inert passage, this sophisticated anatomical structure modulates incoming sound frequencies, preserves tympanic homeostasis, and protects deeper auditory systems through dynamic physiological defenses. Comprehending its intricate morphology, acoustic resonance, and clinical vulnerabilities is essential for audiology, otolaryngology, and sensory neuroscience.
Auditory Canal
1. Concise Definition
The auditory canal, systematically designated in medical nomenclature as the external acoustic meatus (meatus acusticus externus), is an osseocartilaginous tubular passage of the outer ear that extends from the concha of the auricle to the tympanic membrane. Functioning as an open acoustic waveguide, it funnels airborne sound waves toward the middle ear while conferring physiological protection and mechanical dampening against environmental hazards.
Morphologically, the adult canal spans approximately 2.5 centimeters (1 inch) in length and averages 7 to 9 millimeters in diameter, traversing an S-shaped trajectory through the temporal bone. It is anatomically divided into an outer fibrocartilaginous third and an inner rigid bony two-thirds. The canal is lined with stratified squamous epithelium that exhibits a unique self-renewing, laterally migratory property, continually cleansing the conduit of debris and physiological secretions.
Acoustically and physiologically, the auditory canal acts as a quarter-wave acoustic resonator, systematically amplifying frequencies vital for human speech intelligibility, predominantly between 2,000 and 4,000 Hertz. Concurrently, it maintains a microclimate characterized by specific lipidic secretions and antimicrobial enzymes that shield the delicate tympanic membrane from desiccation, mechanical trauma, and microbial colonization.
2. Etymology and Linguistic Origin
The designation "auditory canal" derives from classical Latin roots that reflect both sensory function and structural architecture. The adjective "auditory" stems from the Latin auditorius, meaning "pertaining to hearing," which itself originates from the verb audire ("to hear"), tracing further back to the Proto-Indo-European root *h₂ows-, signifying "ear." The substantive noun "canal" originates from the Latin canalis, which historically denoted a channel, conduit, pipe, or watercourse, emerging from canna ("reed" or "cane").
In formal anatomical taxonomy, governed by the Federative International Programme on Anatomical Terminologies (FIPAT), the structure is designated meatus acusticus externus. The Latin noun meatus signifies a "passage," "pathway," or "motion," derived from meare ("to go" or "to pass"). The modifier acusticus is a Latinization of the Ancient Greek ἀκουστικός (akoustikos), meaning "pertaining to the sense of hearing," rooted in the Greek verb ἀκούειν (akouein, "to hear"). Modern vernacular English adopted "auditory canal" during the rise of empirical anatomical treatises in the seventeenth and eighteenth centuries to describe this gateway into the auditory apparatus.
3. Pronunciation and Grammatical Form
The term is pronounced phonetically in Received Pronunciation and General American as follows:
- International Phonetic Alphabet (IPA): /ˈɔː.dɪ.tər.i kəˈnæl/ (British English) or /ˈɑː.də.tɔːr.i kəˈnæl/ (American English).
- Syllabification: au·di·to·ry ca·nal.
- Part of Speech: Compound nominal phrase (noun).
- Plural Form: Auditory canals.
- Adjectival and Derivative Forms: "Canalicular" (referring to diminutive channels), "aural" (pertaining to the ear as a whole), and "endoaural" (situated within or administered through the auditory canal).
In clinical discourse, the term frequently appears as an attributive modifier within complex medical descriptions, such as "auditory canal stenosis," "external auditory canal cholesteatoma," or "auditory canal resonance."
4. Detailed Conceptual Explanation
The auditory canal is not a uniform, static cylinder; rather, it is a complex, dynamic biomechanical conduit that performs critical sensory, protective, and physiological functions. Its average course exhibits an inferior, anterior, and medial curvature from the auricle inward. To achieve direct visualization of the tympanic membrane during diagnostic otoscopy, the examiner must pull the adult auricle posterosuperiorly, thereby aligning the cartilaginous and bony segments into a relatively straight optical path.
Anatomically, the outer third of the canal (approximately 8 millimeters) is cartilaginous, continuous with the elastic cartilage of the auricle. This segment contains deep fissures in its anterior and inferior wall known as the fissures of Santorini (incisurae santorini). These cartilaginous dehiscences afford mechanical flexibility to the ear canal during mastication and speech, but they also represent potential conduits through which severe infections or neoplasms may traverse into the parotid gland, infratemporal fossa, or mastoid process. The medial two-thirds (approximately 16 millimeters) is entirely osseous, carved within the tympanic and squamosal portions of the temporal bone. The junction between these two zones is marked by a structural narrowing termed the isthmus, an area particularly prone to the entrapment of foreign bodies and accumulated secretions.
The epithelial covering reflects a specialized continuation of facial skin. Over the cartilaginous segment, the dermis is relatively thick and populated by pilosebaceous units, robust hair follicles (tragi), and specialized apocrine-derived glands known as ceruminous glands. These glands, in coordination with sebaceous glands, produce cerumen, a protective waxy substance composed of sloughed keratinocytes, branched-chain fatty acids, squalene, and cholesterol. Within the osseous segment, by contrast, the cutaneous layer becomes exceptionally thin (often less than 0.1 millimeters thick), completely lacking hair follicles and glandular structures, adhering intimately and directly to the underlying periosteum.
A remarkable physiological feature of the auditory canal is centrifugal epithelial migration. Unlike cutaneous surfaces throughout the rest of the human body, where desquamated stratum corneum sheds perpendicularly to the skin surface, the epithelial lining of the auditory canal migrates laterally from the center of the tympanic membrane (the umbo) outward toward the meatal orifice. Operating at a speed of approximately 0.05 to 0.07 millimeters per day—comparable to the growth rate of fingernails—this physiological conveyer belt actively transports fallen squames, particulate dirt, and cerumen out of the canal, sustaining an unobstructed path for acoustic wave propagation without manual intervention.
5. Historical Development
The anatomical understanding of the auditory canal evolved concurrently with the broader trajectory of human anatomical dissection and otological science. In antiquity, the canal was recognized by Hippocratic physicians as an aperture capable of receiving airborne sound and discharging pathogenic humors. Galen of Pergamon (129–c. 216 CE) recognized its non-linear, tortuous trajectory, postulating that its curved orientation was designed by nature to attenuate harsh environmental winds and prevent traumatic foreign matter from damaging the sensory soul within.
During the Renaissance, anatomical dissection reached unprecedented rigor. Andreas Vesalius, in his landmark 1543 work De humani corporis fabrica, produced the first anatomically accurate depictions of the temporal bone, outlining the distinct boundaries between the cartilaginous meatus, the tympanic ring, and the internal auditory structures. Subsequent anatomists, including Gabriele Falloppio (1523–1562) and Bartolomeo Eustachi (c. 1500–1574), further defined the relationship between the external canal, the facial nerve canal, and the auditory tube connecting the pharynx to the middle ear.
The nineteenth century marked the birth of modern otology as a distinct scientific and surgical discipline. The Austrian physician Adam Politzer (1835–1920) and British otologist Joseph Toynbee (1815–1866) established scientific otoscopy, utilizing concave reflective mirrors and artificial illumination to inspect the living auditory canal and tympanic membrane in clinical practice. Concurrently, the German physicist and polymath Hermann von Helmholtz (1821–1894) integrated acoustics with anatomy in his 1863 work Die Lehre von den Tonempfindungen als physiologische Grundlage für die Theorie der Musik, identifying the outer auditory canal as a biological resonator that mathematically shapes the frequency spectrum of speech sounds.
Throughout the twentieth and early twenty-first centuries, the development of binocular operating microscopes by Carl Olof Nylén and Horst Wullstein revolutionized surgical access through the canal (transcanal and endomeatal approaches). The invention of high-resolution computed tomography (HRCT) and miniature probe-tube microphones during the late twentieth century permitted in vivo real-ear measurements, transforming both surgical reconstructive procedures and the acoustic customization of contemporary digital hearing aids.
6. Theoretical Foundations
The functional mechanics of the auditory canal rest upon classical physical acoustics, specifically the principles governing wave mechanics in acoustic tubes. Acoustically, the external ear canal functions as an idealized pipe open at one end (the external meatus) and closed at the other (the tympanic membrane). According to quarter-wave acoustic resonator theory, a cylindrical pipe open at one end and closed at the other exhibits a fundamental resonant frequency ($f_0$) governed by the formula:
$$f_0 = rac{v}{4L}$$
where $v$ represents the velocity of sound in warm, humid biological air (approximately 343 to 350 meters per second) and $L$ represents the acoustic length of the canal. Taking into account an average physical canal length of 0.025 meters and an end-correction factor attributable to acoustic radiation impedance at the meatal boundary, the effective acoustic length approximates 0.027 to 0.029 meters. This yields a primary fundamental resonance peak situated between 2,700 and 3,200 Hertz.
This quarter-wave resonance provides an energetic passive pressure gain of 10 to 15 decibels (dB) in sound pressure level at the tympanic membrane relative to the free sound field outside the head. When combined with the broader anatomical resonance of the pinna and the head-related diffraction effects, this physiological amplification forms the foundation of the head-related transfer function (HRTF). This specific frequency augmentation aligns with the higher-frequency consonant sounds of human language (such as /s/, /f/, /t/, and /θ/), which are essential for speech comprehension and spatial localization along the horizontal and vertical acoustic planes.
In parallel, the neurobiological and somatosensory foundations of the canal reflect an intricate convergence of cranial nerves. The anterior and superior walls receive sensory innervation from the auriculotemporal branch of the mandibular nerve ($V_3$), a division of the trigeminal nerve. The posterior and inferior walls are innervated by the auricular branch of the vagus nerve (cranial nerve X, often termed Arnold’s nerve), with additional collateral contributions from the facial nerve (cranial nerve VII) and glossopharyngeal nerve (cranial nerve IX). This sensory architecture explains why mechanical manipulation of the canal can provoke profound autonomic reflexes, such as cough (Arnold’s reflex), bradycardia, syncope, or pharyngeal sensation.
7. Key Components, Types, and Dimensions
The human auditory canal can be broken down into discrete anatomical zones, tissue layers, and functional dimensions:
- Cartilaginous Meatus (Outer Third): Comprises approximately the lateral 8 millimeters of the canal. It is flexible, surrounded by fibrocartilage continuous with the pinna, and contains hair follicles and cerumen-producing glands. It features the fissures of Santorini, which provide mechanical elasticity during mandibular motion.
- Osseous Meatus (Inner Two-Thirds): Comprises the medial 16 millimeters of the canal. Formed predominantly by the tympanic ring and the squamosal temporal bone, it is covered by a thin, fragile, aglandular cutaneous layer adhering directly to bone.
- The Isthmus: The narrowest constriction of the auditory canal, situated just medial to the osseocartilaginous junction. It represents a protective boundary that prevents deep mechanical penetration while presenting a common site for foreign object impaction.
- Anterior Tympanic Recess (Sulcus): An acute angle formed between the anteroinferior canal wall and the obliquely oriented tympanic membrane, creating an anatomical recess that can harbor debris, desquamated skin, or fungal spores.
- Epithelial and Glandular Complex: Stratified squamous keratinized epithelium embedded with apocrine ceruminous glands and holocrine sebaceous glands, which together synthesize cerumen.
- Innervation Network: A multimodal sensory matrix supplied by the auriculotemporal nerve ($V_3$), Arnold’s nerve (vagus nerve, CN X), and communicating fibers from cranial nerves VII and IX.
8. Examples and Illustrative Cases
The structural and physiological characteristics of the auditory canal are clearly demonstrated in everyday clinical otolaryngology through diverse pathological presentations:
- Acute Diffuse Otitis Externa ("Swimmer’s Ear"): A 24-year-old competitive swimmer presents with severe otalgia, erythema, and profound soft-tissue edema of the external canal that occludes the lumen. The pathophysiology involves excessive water exposure disrupting the acidic, lipidic cerumen barrier, allowing opportunistic bacteria (such as Pseudomonas aeruginosa or Staphylococcus aureus) to invade the epithelial layer. Treatment requires gentle aural toilet and topical antibiotic-corticosteroid drops rather than systemic agents, highlighting the canal’s function as a specialized cutaneous pocket.
- Ceruminosis and Conductive Hearing Loss: A 72-year-old patient experiences sudden unilateral hearing impairment and aural fullness following attempts to clean the ears with cotton swabs. Physical examination reveals an impacted, dense plug of dry cerumen forced deep into the bony isthmus against the tympanic membrane. The swab disrupted the natural outward centrifugal epithelial migration, physically packing the secretion against the drum and abolishing the canal’s acoustic resonance, yielding a 25-decibel conductive hearing deficit reversed immediately by microsurgical removal.
- Congenital Aural Atresia: A newborn presents with unilateral microtia and an absent external auditory meatus. High-resolution temporal bone imaging demonstrates complete failure of the first branchial groove (pharyngeal cleft) to canalize during embryogenesis, resulting in an osseous plate sealing the middle ear space. This developmental anomaly illustrates how the canal forms via invagination of surface ectoderm during the early embryonic period.
- Arnold’s Nerve Reflex Syncope: During an ear canal impression procedure for custom high-end in-ear audio monitors, an audiologist introduces an otoblock into the deep osseous canal of a healthy 30-year-old client. The patient develops paroxysmal coughing, followed rapidly by lightheadedness and transient vasovagal bradycardia. This clinical presentation highlights the vagal innervation (Arnold’s nerve) of the inferior-posterior canal wall, illustrating the reflex pathways linking ear stimulation to central autonomic centers.
9. Measurement and Assessment
Assessing the auditory canal requires a multimodal approach encompassing direct visual examination, electroacoustic measurement, and advanced radiological imaging:
- Otoscopy and Video-Otoscopy: The primary clinical examination method. Utilizing a lighted instrument with a speculum, the clinician evaluates meatal patency, epithelial health, vascularity, and the integrity of the tympanic membrane. Pneumatic otoscopy introduces controlled positive and negative air pressure to assess tympanic membrane mobility through the sealed canal volume.
- Tympanometry and Equivalent Ear Canal Volume ($V_{ec}$): An acoustic impedance evaluation where a probe tip is hermetically sealed within the meatus, introducing a 226-Hz probe tone. The diagnostic instrumentation calculates the physical volume of air between the probe tip and the tympanic membrane. Normal adult values range from 0.6 to 1.5 cubic centimeters ($cm^3$); values exceeding 2.0 to 2.5 $cm^3$ indicate a perforated tympanic membrane or a patent tympanostomy tube, whereas values below 0.4 $cm^3$ suggest canal obstruction or probe misdirection against the meatal wall.
- Real-Ear Measurement (Probe-Tube Microphone Acoustics): Performed by audiologists using an ultra-thin probe tube positioned within 5 millimeters of the tympanic membrane. This measures the Real-Ear Unaided Response (REUR), capturing the individual’s unique biological resonance curve, and the Real-Ear to Coupler Difference (RECD), ensuring that prescriptive hearing amplification matches the unique acoustic profile of that patient’s canal.
- High-Resolution Computed Tomography (HRCT): The gold-standard imaging modality for evaluating the bony auditory canal. Thin-slice (0.5 mm) bone-window scans delineate congenital stenosis, bony exostoses, temporal bone fractures traversing the meatus, and the bone-eroding margins of necrotizing external otitis or cholesteatoma.
10. Applications and Practical Significance
The auditory canal is central to multiple medical, audiological, industrial, and consumer applications:
In audiologic rehabilitation, the auditory canal provides the physical foundation for personal amplification devices. From custom in-the-ear (ITE) hearing aids to deep completely-in-the-canal (CIC) and invisible-in-canal (IIC) designs, the device must couple securely within the cartilaginous and bony segments. An improper acoustic seal results in acoustic feedback (high-frequency whistling), whereas an unvented, occluded canal creates the "occlusion effect"—an uncomfortable, boomy amplification of the patient’s own bone-conducted voice caused by sound vibrations trapped within the cartilaginous canal walls.
In otologic and neurotologic surgery, the canal serves as the primary corridor for transcanal endoscopic ear surgery (TEES) and classic microscopic tympanoplasty. Surgeons utilize the canal’s natural lumen to repair tympanic perforations, reconstruct the ossicular chain, and eradicate middle ear pathology without external facial incisions. When pathology constricts the lumen, surgical procedures such as canalplasty or meatoplasty widen the bony or cartilaginous framework to restore natural ventilation and clear chronic disease.
In industrial hygiene and occupational safety, the canal is the direct recipient of personal hearing protection devices (HPDs). Foam, silicone, and custom-molded earplugs attenuate acoustic energy by forming an airtight mechanical seal across the canal lumen. Proper sizing and insertion depth are essential; poor insertion compromises attenuation, leaving workers exposed to noise-induced hearing loss. In consumer technology, the engineering of high-fidelity in-ear monitors (IEMs) and active noise-canceling earbuds relies on precise computational modeling of auditory canal acoustics and impedance to ensure accurate frequency response curves and comfortable wear.
11. Research and Empirical Evidence
Modern physiological acoustics was substantially shaped by the landmark empirical investigations of Canadian physicist E.A.G. Shaw. In his foundational 1974 treatise, Transformation of Sound Pressure Level from the Free Field to the Eardrum in the Horizontal Plane, Shaw mapped the individual acoustic components of the human outer ear. His empirical data confirmed that while the head and torso contribute low-frequency gains and the concha generates a secondary resonance around 5,000 Hz, the external auditory canal is the primary source of the critical 10 to 15 dB acoustic boost observed between 2,500 and 4,000 Hz across diverse human populations.
In epithelial biology, research by Alberti (1964) and Litton (1968) systematically demonstrated the lateral migratory phenomenon of the human canal lining using microscopic ink spots and photographic tracking over months. Their work proved that the auditory canal possesses a unique self-cleaning mechanism absent elsewhere in the integumentary system. Disruption of this migratory mechanism was subsequently shown by contemporary cellular biologists to play a key role in the pathogenesis of keratosis obturans and canal cholesteatoma.
Extensive biochemical investigations have clarified the antimicrobial mechanics of cerumen. Studies by Stone and Fulghum, as well as subsequent analyses by Megarry and colleagues, isolated lysozyme, immunoglobulin A (IgA), transferrin, and antimicrobial peptides (including human beta-defensin-1 and beta-defensin-2) within native cerumen. These empirical findings demonstrated that the auditory canal maintains a hostile biochemical barrier against pathogens, functioning as an active immunological barrier rather than a passive collector of physiological waste.
12. Cultural and Cross-Cultural Considerations
Human variation across the auditory canal exhibits notable geographic and genetic differences. One of the clearest examples of human polymorphism centers on cerumen consistency, controlled by a single nucleotide polymorphism (SNP) on the ABCC11 gene located on chromosome 16. Populations of European and African ancestry predominantly carry the dominant G allele (coding for wet, sticky, yellowish-brown cerumen rich in lipids), whereas East Asian and Native American populations show high frequencies of the recessive A allele (coding for dry, flaky, white cerumen with lower lipid content).
These biological differences have fostered distinct cultural practices and tools across different societies. In many East Asian cultures, particularly Japan (where the practice is known as mimikaki), China, and Korea, the prevalence of dry, flaky cerumen led to the traditional use of specialized ear picks made of bamboo, metal, or ivory. Ear cleaning in these societies is often performed as an intimate, relaxing familial ritual or offered as a specialized service in traditional barbershops and wellness salons. Conversely, in Western societies, the prevalence of wet cerumen frequently prompts individuals to use manufactured cotton-tipped swabs, despite widespread public health warnings against this practice.
Furthermore, access to specialized otologic care and diagnostic equipment varies substantially around the world. In low- and middle-income countries, untreated otitis externa and unrecognized canal stenosis or atresia are more likely to lead to long-term conductive hearing impairment due to shortages of micro-otoscopic equipment and trained otolaryngologists. This disparity underscores the importance of public health initiatives designed to protect ear canal health and prevent hearing loss worldwide.
13. Criticisms, Debates, and Limitations
A longstanding debate in otology and public health centers on ear hygiene and mechanical instrumentation. A strong consensus across otolaryngologic societies—including the American Academy of Otolaryngology–Head and Neck Surgery (AAO-HNS)—warns against inserting cotton-tipped applicators, hairpins, or household implements into the auditory canal. Clinical evidence indicates that these tools disrupt natural epithelial migration, cause cutaneous micro-abrasions, strip protective cerumen lipids, and impact debris against the tympanic membrane. Despite consistent clinical guidelines, personal ear-cleaning practices remain widespread, driven by consumer perceptions that cerumen is unhygienic rather than protective.
Another controversial practice is "ear candling" (or auricular thermal therapy), an alternative medicine technique promoted for extracting cerumen, clearing sinus congestion, and purifying the body. Critical scientific evaluations, including controlled laboratory testing by Seely and colleagues, have shown that ear candling creates neither a partial vacuum nor an extraction force. The residual debris found within the burned candle consists entirely of candle wax and fabric soot rather than canal cerumen. The procedure carries documented risks of thermal burns to the canal wall, tympanic membrane perforation, and secondary conductive hearing loss, leading regulatory bodies such as the U.S. Food and Drug Administration (FDA) and Health Canada to issue public advisories against its use.
Within the audiological and audioprosthetic engineering fields, debates persist regarding standardized versus individualized canal acoustic modeling. Traditional prescriptive fitting algorithms for hearing aids have frequently relied on standardized acoustic models, such as the 2-cubic-centimeter acoustic coupler or the KEMAR (Knowles Electronics Manikin for Acoustic Research) acoustic simulator. However, clinical researchers argue that substantial interpersonal variations in canal length, curvature, and cross-sectional volume make generalized models inaccurate. This variation can lead to over- or under-amplification of acoustic energy, underscoring the growing requirement for individualized, real-ear probe-tube measurements in modern audiological fittings.
14. Related Terms and Distinctions
To prevent clinical and anatomical confusion, several closely related terms must be distinguished from the auditory canal:
- Internal Acoustic Meatus (Meatus acusticus internus): A completely separate bony channel within the petrous temporal bone that transmits the facial nerve (CN VII), vestibulocochlear nerve (CN VIII), and labyrinthine artery from the posterior cranial fossa to the inner ear, lacking any direct communication with the external atmosphere.
- Eustachian Tube (Pharyngotympanic Tube): An osseocartilaginous channel that connects the anterior wall of the middle ear cavity (tympanic cavity) to the nasopharynx. Its primary functions are equalizing middle ear pressure with ambient atmospheric pressure and clearing middle ear secretions, in contrast to the external canal’s role as a receiver of airborne sound waves.
- Pinna (Auricle): The visible, outer cartilaginous structure projecting from the side of the head that collects sound waves and directs them into the auditory canal, serving as the first stage of outer-ear acoustic filtration.
- Tympanic Cavity: The air-filled middle ear space situated medial to the tympanic membrane, housing the auditory ossicles (malleus, incus, and stapes) that convert acoustic sound pressure waves into mechanical vibrations.
- Isthmus of the Auditory Canal: The physiological and anatomical narrowing situated at the boundary between the cartilaginous and bony segments of the external canal, rather than a separate independent organ.
15. Summary and Key Takeaways
The auditory canal is a complex, dynamic osseocartilaginous passage that plays a crucial role in human hearing and ear health. Its morphology, combining a flexible outer cartilaginous third with an inner bony two-thirds, curves gently to protect the tympanic membrane from mechanical trauma while funneled acoustic energy enters the ear. Operating as a natural quarter-wave acoustic resonator, the canal amplifies sounds between 2,000 and 4,000 Hz by 10 to 15 dB, boosting speech perception and aiding spatial sound localization.
Beyond its acoustic properties, the canal maintains an active physiological defense system. Its epithelial lining exhibits a lateral, centrifugal migratory property that cleanses the channel, working alongside ceruminous and sebaceous secretions that provide acidic, water-repellent, and antimicrobial barriers against microbial infection. When these protective mechanisms are compromised by mechanical trauma, moisture, or pathology, conditions such as otitis externa, cerumen impaction, and conductive hearing deficits can develop.
Understanding the auditory canal requires synthesizing perspectives from physical acoustics, temporal bone anatomy, epithelial cell biology, and clinical otology. Whether evaluating hearing aid acoustics, designing personal hearing protection, diagnosing ear pathology, or performing delicate microsurgery, modern hearing health depends on appreciating this anatomical gateway that connects our sensory consciousness with the acoustic world around us.
References
- Alberti, P. W. (1964). Epithelial migration on the tympanic membrane. The Journal of Laryngology & Otology, 78(9), 808–830. https://doi.org/10.1017/s0022215100062799
- Helmholtz, H. von. (1885). On the sensations of tone as a physiological basis for the theory of music (A. J. Ellis, Trans.; 2nd English ed.). Longmans, Green, and Co. https://archive.org/details/onsensationsofto00helmrich
- Politzer, A. (1909). A text-book of the diseases of the ear (5th ed., M. J. Ballin & C. L. Heller, Trans.). Lea & Febiger. https://archive.org/details/textbookofdiseas00polirich
- Schwartz, S. R., Magit, A. E., Rosenfeld, R. M., Ballachanda, B. B., Hackell, J. M., Krouse, H. J., Lewis, C. M., Lin, K., Parham, K., Stutz, D. R., Walsh, S., Woodson, E. A., Yanagisawa, K., & Cunningham, E. R. (2017). Clinical practice guideline (update): Earwax (cerumen impaction). Otolaryngology–Head and Neck Surgery, 156(1_suppl), S1–S29. https://doi.org/10.1177/0194599816671491
- Shaw, E. A. G. (1974). Transformation of sound pressure level from the free field to the eardrum in the horizontal plane. The Journal of the Acoustical Society of America, 56(6), 1848–1861. https://doi.org/10.1121/1.1903522