History

The stapedius contraction in response to loud sound was described as a curious anatomical observation in the 19th century by Hensen (1878) — visible through a perforated drum in cadaver experiments. Lüscher (1929) showed that it occurred in live human ears via stroboscopic observation. Clinical use came with Otto Metz's 1946 monograph that founded clinical immittance audiometry. Metz observed that hearing-impaired patients with recruitment showed reflex thresholds at much smaller sensation levels than their audiometric thresholds would predict; he proposed the *Metz test* as a recruitment indicator in 1952. The pattern still bears his name. The four-condition decision matrix is owed to James Jerger (1972). Jerger systematised the use of ipsi/contra threshold patterns to localise lesions — and crucially, he showed that the contralateral reflex selectively fails in brainstem lesions, a finding that has remained one of the strongest acoustic-reflex predictions across fifty years of imaging-confirmed retrospective studies. The role of reflex testing has shrunk somewhat in the imaging era — MRI now diagnoses what reflex decay used to screen for — but reflex testing remains the *fastest, cheapest, most universally available* localising tool in audiology. A reflex panel takes about three minutes, is non-invasive, and produces a pattern interpretable across the entire central auditory pathway. As Jerger put it in a much-cited 1980 review: "the reflex pattern is the audiologist's neurological exam."

The first electronic hearing aid was the carbon-microphone "Acousticon" of 1902, marketed by Miller Reese Hutchison. It was the size of a small radio and the user wore the microphone at chest level. The carbon microphone's signal-to-noise ratio was poor and the device produced only mild amplification. Vacuum tubes (1920s) allowed dramatically more gain but required large batteries; a "wearable" hearing aid of the 1940s was a 1-pound body-worn device with a wire to an earphone. The first transistor hearing aid (1953, Sonotone 1010) reduced the body to wristwatch size; the body-worn era persisted into the 1970s. The first behind-the-ear hearing aid (Otarion Listener Model L8, 1956) put the entire device behind the pinna. The 1980s saw mass-market analog ITE devices. The first commercial digital hearing aid was the Nicolet "Phoenix" of 1987 — a single-channel device that arrived in offices on a wheeled trolley because the DSP chip wouldn't fit in the device itself, but the principle (digitise the signal, compute, output) was settled. The shift to fully on-device digital processing came with the Widex Senso (1995), the first commercial fully-DSP hearing aid that fit entirely behind the ear. Multichannel compression (Resound DigiFocus, 1996), adaptive feedback cancellation (Phonak Claro, 1999), and adaptive directional microphones (Oticon Adapto, 2001) followed in rapid succession. The RIC form arrived in 2005–2008 (Phonak Audéo, Resound Live) and quickly became the dominant form. Bluetooth audio streaming through MFi (Made for iPhone, 2014) and LE Audio (2022) closed the connectivity gap. By 2026, most "premium" hearing aids contain 12–24 compression channels, two omnidirectional microphones, adaptive feedback cancellation, statistical noise reduction, Bluetooth LE Audio with Auracast, rechargeable lithium batteries with a few days of life, and either explicit AI/DNN-based environmental classification or noise-reduction algorithms (Widex Moment Sheer, Starkey Genesis AI, Phonak Sphere). The pace of change has not slowed.

The Central Institute for the Deaf (CID) in St. Louis, founded in 1914, became the primary American research centre for clinical hearing measurement under its director Edmund Prince Fowler and then Hallowell Davis. During and after WWII, with thousands of veterans needing aural rehabilitation, CID developed the standardised word lists that became the US clinical baseline. The **CID W-1** spondee list (36 two-syllable words) was published by Hudgins, Hawkins, Karlin, and Stevens in 1947 and remains in clinical use today, though digital recordings have replaced the original 78-rpm phonograph records. The **CID W-22** mono-syllabic word list (50 phonetically-balanced words across four 50-word sub-lists) was developed by Hirsh, Davis, Silverman, Reynolds, Eldert, and Benson in 1952 as the open-set word-recognition standard. Raymond Carhart, at Northwestern, championed using both — SRT plus a separate WRS at a comfortable level — as the speech-audiometry "fingerprint" of a hearing loss. Carhart's clinical protocols, codified in his 1971 *Modern Developments in Audiology* chapter on speech audiometry, are essentially the protocols US audiologists still follow. The biggest modern evolution is the move from quiet to *noise*: speech-in-noise testing (HINT, QuickSIN, BKB-SIN, AzBio) developed from the 1990s onward to address the well-documented fact that quiet WRS poorly predicts real-world function. We cover those in [3.3](/tools-of-audiology/speech-audiometry/speech-in-noise).

Pure-tone audiometry as we now practise it descends from the Western Electric 1A audiometer (1922), the first electronic instrument capable of generating calibrated tones at controlled levels. Through the 1930s, audiometers were calibrated in dB *SPL* — the physical acoustic pressure — but this made audiograms hard to interpret: a 30 dB SPL threshold at 250 Hz means something completely different from a 30 dB SPL threshold at 4 kHz, because human hearing sensitivity is wildly frequency-dependent. Raymond Carhart, working at Northwestern after WWII, championed the use of *hearing level* (HL) — a frequency-dependent reference that subtracts the normal-hearing threshold at each frequency, so that "0 dB HL" means "at threshold for an average normal-hearing young adult at this frequency." This makes the audiogram horizontal for a normal-hearing patient and renders the shape of any hearing loss directly visible. The conversion table — the Reference Equivalent Threshold Sound Pressure Level (RETSPL) — was standardised first by ASA (the American Standards Association, 1951) and revised by ASA/ANSI (American National Standards Institute, 1969), then internationally by ISO 389 (1964 onwards). The current US standard is ANSI S3.6-2018, with frequency-specific RETSPL values for several earphone types (TDH-39 / TDH-49 supra-aural, insert phones, circumaural). Different earphones have different RETSPLs; the calibration converts ear-canal pressure to dB HL via the earphone's known acoustic response. The 5-dB step size on the audiometer dial is also Carhart's: he advocated for it as the resolution that balances test-retest reliability (about ±5 dB even for well-trained patients) against test time. Audiometers with 1- or 2-dB steps exist but are mostly used for research and threshold-microstructure studies, not routine clinical work.

Clinical audiology as a distinct profession emerged from the *aural rehabilitation* programmes the US military set up during and after World War II. Thousands of veterans returned with noise-induced hearing loss from gunfire, explosions, and aircraft engines. Existing otolaryngology services could diagnose, but the scale of the rehabilitation problem — fitting hearing aids, training speech-reading, counselling — exceeded what medicine alone could handle. Raymond Carhart, a speech-language pathologist with a PhD from Northwestern University, was tapped to direct the aural-rehabilitation programme at Deshon General Hospital in Pennsylvania (1944–1946). He developed the audiometric techniques that the US Veterans Administration then propagated nationally — including the speech-reception threshold (SRT), the *speech detection threshold*, and quantitative methods for hearing-aid candidacy. After the war Carhart returned to Northwestern and built the country's first university programme in audiology (the term "audiology" was itself coined around this time, from the Latin *audire* "to hear" plus the Greek *-logia* "study of"). He is widely regarded as the father of the field. The American Speech-Language-Hearing Association (ASHA) issued its first audiologist certification in 1959. The clinical doctorate (AuD) was introduced in the 1990s as the entry-level credential. Today audiology in the US is a doctorate-required clinical profession with about 15,000 active practitioners; analogous credentials exist worldwide. Most of the standardised tests this book covers — pure-tone audiometry calibrated in dB HL, the Carhart-notch interpretation, the SRT, the basic tympanogram types — descend more or less directly from the Northwestern programme and its WWII-era origins. The instruments have grown immensely more sophisticated; the conceptual structure that organises their interpretation is largely Carhart's.

The idea of measuring the middle ear's mechanical response by a probe in the canal goes back to the 1920s — Schuster (1934) and Metz (1946) used bridge circuits to measure mechanical impedance acoustically. Otto Metz's 1946 monograph *The Acoustic Impedance Measured on Normal and Pathological Ears* established that conductive, mixed, and sensorineural losses gave distinct middle-ear impedance patterns, even before any clinical instrument existed to capture them practically. The breakthrough was technological: in the late 1950s and 1960s, transistorised electronics let the bridge circuit be packed into a hand-held probe, and the differential measurement scheme became practical with electret microphones. The Madsen Z0-61 (1961) was the first clinical impedance bridge; the Madsen 70 (1969) was the first practical clinical tympanometer. James Jerger's 1970 paper *Clinical experience with impedance audiometry* defined the **five classical tympanogram types** (A, As, Ad, B, C) that bear his name and remain the standard clinical taxonomy fifty years later. Jerger's contribution was clinical rather than physical — the underlying admittance measurements had existed for decades — but his five-letter shorthand gave audiologists a reproducible language for tympanograms that made the test universally interpretable. Like Carhart's audiogram conventions (Ch 2), Jerger's typology persists not because it is optimal but because it is *agreed*: it works as a clinical shorthand precisely because every audiologist in the world uses the same letters. The shift from impedance to admittance as the displayed quantity was driven by Margolis and Shanks in the 1980s; admittance's parallel-additivity makes the canal-correction trivial. The 226-Hz probe tone became standard in ANSI S3.39 (1987); the high-frequency probes for infants came in with universal newborn screening in the late 1990s.

The first cochlear implant in a human was performed by William House in Los Angeles in 1957, on a 36-year-old patient with bilateral acquired deafness. House's device was a single-electrode implant — one platinum wire delivering whole-nerve stimulation. The patient could detect sound presence and crude rhythm but could not understand speech. Single-electrode implants persisted into the 1980s in some clinical settings; they remained a sensation aid (alerting the user to environmental sounds) rather than a speech-perception device. Multichannel implants were pursued from the 1970s by groups led by Graeme Clark in Melbourne, Blake Wilson at Research Triangle Institute, Ingeborg Hochmair-Desoyer in Vienna, and Robert Schindler in San Francisco. The Cochlear (Nucleus 22) device launched commercially in 1982 was the first 22-channel implant, the first to provide consistent open-set speech understanding in adult post-lingual deaf patients. Insertion depth was a battle of the 1990s. Conservative early practice placed electrodes only in the basal 15 mm of cochlea; deeper insertion was thought to be too traumatic. Animal studies (Eshraghi, Adunka) and intraoperative imaging (Verbist's CBCT studies) gradually established that careful insertion to 22+ mm was achievable with minimal trauma and produced measurably better outcomes. Modern surgical practice favours the deepest possible insertion consistent with structure preservation, with intraoperative electrode mapping (electrocochleography) increasingly used to monitor cochlear health during insertion. The 2010s and 2020s have seen progressive miniaturisation (lateral-wall arrays under 0.4 mm), residual-hearing preservation as a routine goal even in conventional CI candidates (atraumatic surgical technique, steroid washouts, slow insertion protocols), and increasing pediatric implantation under 12 months — driven by the sensitive-period evidence we will encounter in Lesson 9.3.

The ABR was discovered by Donald Jewett and John Williston at UCSF, published in their 1971 *Brain* paper *Auditory-evoked far fields averaged from the scalp of humans*. Earlier work by Sohmer and Feinmesser (1967) had shown that brief sound-evoked potentials could be recorded from the human scalp with averaging, but Jewett and Williston's contribution was to systematically characterise the waveform — naming the five peaks I through V in the order of their appearance — and to relate them to brainstem anatomy by analogy to cat physiology that Buchwald and colleagues had developed in parallel. The five-wave naming convention is universal and has the unusual property of being correct: each peak does correspond to a discrete brainstem generator, with reasonable specificity (wave V being the most reliable, generated by the lateral lemniscus rather than the inferior colliculus per se, but close enough that the textbook attribution holds). Wave V latency is the single most useful number in the ABR battery and has remained so for 55 years. Clinical ABR moved quickly into audiology. Hyde and Riko (1994) and others established the ABR threshold-behavioural threshold correlations that underpin its diagnostic use. Stapells (2000) developed the tone-burst ABR protocol that is now standard for frequency-specific objective threshold estimation in pediatrics. Selters and Brackmann (1977) and Eggermont, Don, and Brackmann (1980) established the latency criteria for vestibular schwannoma screening that dominated retrocochlear diagnostic practice from 1980 until MRI scaling in the 1990s. The ABR has remained essentially unchanged in its clinical implementation for forty years — the same five peaks, the same threshold criterion, the same Cz-mastoid montage. The technique has been so stable because the underlying neurophysiology hasn't changed: it is, like Carhart's audiogram, a *settled* clinical instrument.

Pertti-Erkki Tjellström, working in Göteborg in the 1970s, was a pioneer of dental-implant osseointegration — the discovery that pure titanium implants can be made to bond directly to bone tissue without an intervening fibrous capsule. The clinical opportunity was straightforward: dental implants were already in routine use. Tjellström's insight was that the same principle could be applied to *acoustic* coupling — a titanium implant osseointegrated into the temporal bone could provide a low-loss vibration pathway to the cochlea. The first BAHA implantation was performed in 1977 in Sweden. The early devices were body-worn (the processor was too large to fit behind the ear). Miniaturisation through the 1980s and 1990s produced behind-the-ear BAHA processors; CE marking in 1995 and FDA clearance in 1996 opened the device to wider clinical use. The 2000s and 2010s saw progressive improvements: smaller fixtures (4 mm to 3 mm), reduced surgical trauma (single-stage punch technique), improved abutments (longer abutments accommodate thicker scalps without requiring tissue reduction), and transcutaneous variants (Sophono 2009, BAHA Attract 2013, Osia 2019). The shift in the 2020s has been from "BAHA + alternatives" to "Osia / Bonebridge + percutaneous fallback." Active transcutaneous devices have become the first-choice option in most contemporary centres because they preserve the acoustic advantages of percutaneous coupling without the through-skin hardware. The percutaneous BAHA remains the first choice when maximum output is needed (severe mixed losses, large air-bone gaps) — but the indications are narrowing.

David Kemp, then a graduate student at the Institute of Laryngology and Otology in London, was studying ear-canal acoustics in 1977. He noticed that when he presented a click stimulus, his microphone recorded — buried in averaging — an echo arriving about 10 ms after the click. The echo's amplitude depended nonlinearly on the click level (it saturated at high stimulus levels but was clearly present at low ones), and its spectral content showed the click had been broken down by frequency: high components arriving first, low components arriving later. The signal was unmistakably from inside the ear, not a microphone artefact, but no passive structure could account for the energy. Kemp's 1978 paper (*Stimulated acoustic emissions from within the human auditory system*, JASA 64: 1386–1391) proposed that the cochlea contains an *active* mechanical element that does work on the traveling wave. The proposal was controversial — Békésy's passive cochlea was the dominant picture, and adding active elements seemed to conflict with the steady-state energy budget — but the evidence rapidly accumulated. Mountain (1980) showed efferent activation of the medial olivocochlear bundle could suppress OAEs; Brownell *et al.* (1985) discovered that isolated OHCs in tissue culture changed length in response to current injection; the prestin gene was cloned by Dallos's group in 2000. The first clinical OAE system (the Otodynamics ILO88) was launched by Kemp's own company in 1988. By the late 1990s, multiple US states had universal-newborn-screening mandates that effectively required OAE technology in every birthing hospital. Kemp's discovery moved from curiosity to standard-of-care in 25 years — a faster trajectory than most basic-science findings achieve.