Immittance/Impedance Measurement
The history of immittance measurement is said by some to go back to early 19th century observations on the effects of air pressure on hearing published by William Hyde Wollaston and Sir Charles Wheatstone in England – others refer to research done on the principles governing transoceanic transmission lines and on early telephone transducers (Oliver Heaviside, 1886, Webster AG, 1919, West W, 1928).
Olliver Heaviside (1850-1925)
Certainly, the underlying principles are very complex and usually better understood by physicists and acoustic engineers than those working with diseases of the ear.
Otto
Metz comes to Denmark
Although the concept
of acoustic impedance was described by Webster as early as 1919, our story starts in the 1930s when
the German-born doctor Otto Metz (1905 – 1993) escaped persecution in Nazi Germany and found employment
at the University Hospital of Copenhagen (Rigshospitalet) in 1938.
Dr. Otto Metz (1905 – 1993)
When Metz started in otolaryngology in the 1930s, he found the methods for distinguishing between conductive and perceptive hearing loss inadequate and thus began to look for objective measures of the eardrum and middle ear. In 1938, he came across a recent work by Waetzmann entitled “Absorptionsmessungen am Trommelfell mit der Schusterschen Brücke”.
Acoustic impedance is a significant parameter in the adjustment of acoustic components, e.g. telephones, to the ear. For this purpose, Waetzmann modified a mechanical bridge developed by K. Schuster in 1934 for the measurement of sound absorption in construction materials – in effect an acoustic version of the Wheatstone bridge (an electrical bridge circuit used to measure resistance).
Somehow Metz perceived that impedance measurement could be applied to clinical measurement of the ear. He was fortunate in that a physics professor at the university’s Institute of Biophysics, Dr. V. Thorsen had actually visited Waetzmann in Breslau and seen the bridge in action.
In collaboration with Thorsen and a technician named V. Thygesen, Metz succeeded in constructing a modified Schuster bridge and so began his pioneering work on the measurement of aural acoustic impedance in 1939. Performing extensive studies of acoustic impedance on human ears, he determined the acoustic absorption and the phase characteristics of normal and pathological ears. As early as 1942, Metz was able to publish his first results in a paper given to the Danish Society of Otology.
Work
continues despite World War II
At the same
time, Van den Borg was screening schoolchildren using an audiometer together with a device called the
pneumophone (which caused air pressure variations in the middle ear and was developed by Van Dishoeck
in the 1930s). He found that over 10% had negative middle ear pressure combined with varying degrees
of hearing loss. As early as 1942, assessment of middle ear function was being proposed for hearing
screening in schools.
In October 1943, just as the Nazis were preparing to intern all Jews in Denmark, Otto Metz had the good fortune to be one of the more than 6,000 Jews spirited across the sea to Sweden by the Danish Resistance. Thus escaping capture, Metz was able to continue his pioneering research at the University Hospital of Lund.
After returning safely to Copenhagen in 1945, Metz formulated the basic principles of tympanometry in his dissertation of 1946: “The acoustic impedance measured on normal and pathological ears”. This constituted the earliest substantial set of acoustic impedance measurements in normal and pathological ears – and obtained using a mechanical bridge.
Continuing this work at Rigshospitalet, Metz also published the seminal “Threshold of reflex contractions of muscles in the middle ear and recruitment of loudness” in the Archives of Otolaryngology in 1952. This was the first study of the acoustic stapedius reflex in patients with ear disease.
In fact, much of what is today employed clinically in middle ear measurements can be attributed to the ingenuity of Dr. Otto Metz – and to the Danish researchers he inspired around him.
The
Metz acoustic bridge
While research continued
apace in the 1950s, the acoustic bridge was difficult to use, and only 6 units were ever constructed.
Metz’s first bridge was destroyed during a sabotage operation carried out by the Danish Resistance in
1943 – the device was in a workshop in the Radiometer factory where it had been sent for repair. Four
more devices were produced for research carried out at Rigshospitalet and Lund by, among others, Otto
Jepsen and K.A. Thomsen.
A
decisive advance in technology
Two other key
names in the history of immittance measurements are Dr. Knud Terkildsen of Rigshospitalet and Engineer
Scott-Nielsen of the Hearing Center in Copenhagen. Terkildsen had recognized the limitations of the
mechanical acoustic bridge, particularly the fact that it was not possible to achieve an airtight seal.
At this point, acoustic impedance measurement was missing the additional dimension of ear canal pressure. Work done by K.A. Thomsen had demonstrated that, by measuring impedance as a function of ear canal pressure, it was possible to calculate the impedance of the middle ear without the contaminating influence of the ear canal.
To achieve this, and to measure middle ear pressure, an airtight system was required. This inspired Terkildsen and Scott-Nielsen to design and build the electroacoustic bridge and, using a prototype, Terkildsen and Thomsen were able to publish the first findings of the application that came to be known as tympanometry in 1959. The paper was entitled “The influence of pressure variations on the impedance of the human ear drum. A method for the objective determination of the middle-ear pressure” (J. Laryngol. Otol. 73:409). It featured for the first time ever an illustration of what would later be known as a tympanogram:
The Madsen Electronics’ ZO61 comprised both the bridge circuitry necessary for the detection of stapedius muscle contraction, and circuitry for measuring the both dimensions of the static impedance of the middle ear system – the vector magnitude and the phase angle. Additionally, a calibrated air pump permitted the study of how middle ear pressure changed under positive and negative air pressure. A graph of these changes was referred to as a “tympanogram”.
Scott-Nielsen and Terkildsen with the ZO61
In 1960, Terkildsen and Scott-Nielsen published a description of the electroacoustic bridge in the Archives of Otolaryngology and, at the same time, entered into a constructive collaboration with Poul Madsen, who was able to convert their lab model into a working instrument (the ZO61). Actually, it was this opportunity that persuaded Madsen to abandon his stagnating business in Odense and move to Copenhagen where he established Madsen Electronics. The company set up an R&D facility and commenced manufacture of the ZO61 as well as the OB 1 audiometer.
A slow start
Now
that the theory had been published and an instrument was commercially available, the stage was set for
the news about this new clinical application to gradually spread around the world of audiology and otology,
slowly at first but with steadily gathering momentum.
James Jerger visited Copenhagen after the 1960 International Congress of Audiology in Bonn, West Germany, and was shown the ZO61 “impedancemeter” by Ewertsen. Duly impressed, Jerger returned to Northwestern University in Evanston, Ill., and predicted that the method would probably have “a very significant impact on audiometric evaluation”. After moving to the VA in Washington D.C., Jerger purchased what was probably the first Madsen ZO61 to come to the USA.
In 1961, at an audiology congress in Paris, K.A. Thomsen gave a presentation about impedance measurement and middle ear pressure – only 25 attended whereas the subsequent banquet in Versailles attracted 1200 participants.
Meanwhile, in the USA, Zwislocki was following a different path, building on the Schuster concept, and designing the first commercially available mechanical acoustic bridge. This was then manufactured by Grason-Stadler in 1962.
Growing
interest in impedance measurements
While, from
the outset, impedance measurement promised a speedy technique for assessing middle ear function, it
is doubtful whether it would have gained such rapid acceptance in Europe and the USA without the remarkable
and dedicated efforts of Terkildsen, Madsen and Scott-Nielsen. In the 1960s, they traveled the world
holding numerous seminars on impedance measurements. In fact, the demands of constant travel proved
too much for Scott-Nielsen, who preferred to stay at home – instead Terkildsen found an able young engineer
named Poul Osterhammel to clarify the complex scientific theory involved.
Their combined efforts, as well as the advent of clinical instrumentation in the 1960s, created a great deal of interest in the technique and led to many studies and papers on the effects of specific pathological conditions on tympanograms. Different tympanometric patterns were described for patients of different ages and different ear diseases – and using different probe frequencies.
Jerger published a number of influential papers including “Clinical Experience with Impedance Audiometry”, which reported results on more than 400 patients obtained using a Madsen ZO70 (essentially the same as its predecessor, but without the phase angle measure). Jerger reports that “Impedance audiometry … has become, in our clinic, a routine part of the audiologic assessment
of every patient. We frankly wonder how we ever got along without it.”
Applying
theory in the clinic
However, the varying tympanometric
patterns were not well understood until a group of physicists under Vanhuyse at the University of Antwerp
published a study relating tympanometric patterns with the physics of the middle ear (Vanhuyse, Creten,
and Van Camp 1975. On the W-
notching of Tympanograms. Scandinavian Audiology, 4, 45-50). The so-called Vanhuyse model supplies the basis for the clinical interpretation of tympanograms.
Although much has since been learned about the acoustic immittance characteristics of normal and pathological ears, tympanometry today is not substantially different from the technique first used by clinicians in the late 1960s and early 1970s. In fact, tympanometry has evolved into a routine clinical tool and is now used in most audiologic evaluations and screenings.
Nowadays, the vast majority of audiologists routinely use acoustic immittance measures in their daily practice. Acoustic immittance instruments and procedures have become commonplace in audiology, ENT and pediatric clinics – they are also widely used in school screening programs.
Of course, the instrumentation has changed a lot since the ZO61, but the fundamentals of the immittance technique haven’t changed much since those early days.
Perhaps the most noteworthy part of the history of immittance measurement is following the cross-fertilization between different scientific and medical disciplines, and observing how, yet again, the very different talents of the scientist and the engineer can combine to solve problems and transform theory into practice. To the benefit of all.
Notes
on terminology
From the days of Metz, the term
“impedance” (meaning resistance to the flow of sound pressure waves) has been widely used in the context
of tympanometry. There is, however, a good case to be made for using the reciprocal term “admittance”
instead.
More recently, the generic term “immittance” has been coined to embrace the various quantities: acoustic impedance, acoustic admittance, acoustic compliance, equivalent air volume and acoustic reflex threshold.
Consequently, the term immittance has today largely supplanted impedance, which accounts for the interchangeability of the two terms – especially in descriptions of the history of this application.