How Open Canal Amplification Was Discovered

By James R Curran

If you were asked to name the most significant developments in hearing aids over the last fifty years, which ones would you consider? Digital signal processing? The custom ITE/ITC family? The directional microphone? First fit algorithms? Technologies for feedback control? They are all very worthy of inclusion, but there are those who would place the introduction of CROS amplification very near the top of that list. In point of fact, the CROS concept, introduced over fifty years ago, was the impetus for a revolution in the thinking of hearing care professionals of the day, and it spawned any number of understandings over the next decades that remain an influence in our modern approach to fitting.

Recall that CROS is an acronym for Contralateral Routing of Signals, a hearing aid system first recommended (and still fitted today) for unilateral hearing losses where the patient’s hearing is good on one side and a loss is on the other. Originally conceived for use with eyeglass aids, a microphone in the temple of the unaidable side picked up the signal that was transmitted by a thin wire/cord connected to the circuitry and receiver in the other temple. The amplified sound was delivered by a tubing to an open ear, obviating the use of standard earmolds. Later, the industry developed wireless CROS instruments that did away with the need to use wires and cords to connect each side.

In 1970, Al Dunlavy, a hearing aid dispenser in Manhattan wrote an article for Audecibel, a publication of the National Hearing Aid Society,¹ with the title, CROS: The New Miracle Worker. Why would he call CROS, of all things, a miracle? And was it really? This article deals with the specific and unique application of the CROS aid that he wrote of that was never originally intended, but that eventually became its most significant form of usage, i.e., a solution to the problem of feedback.

Until the advent of CROS fittings, problems with feedback dogged the industry. Today open canal fittings are routine, seldom requiring a second thought to feedback issues. Feedback cancellation algorithms make bilateral high frequency fittings a walk in the park. One can literally grab a couple of unoccluded ear buds from off the shelf and fit without ever giving a thought to the issues faced years ago.

The Harvard Report on Hearing Aids

To get a fuller appreciation of the impact of CROS on the practices of the day, we can go all the way back to 1947, about the time audiology began. That year a famous research monograph on hearing aids was published, referred to as the Harvard Report.² At the time the PsychoAcoustics Laboratory at Harvard University was the single most influential research center in the United States on matters auditory and acoustic. The report recommended that a flat or 6 dB per octave slope frequency response was adequate for the majority of patients who needed a hearing aid, and it severely criticized other methods of fitting, implying they were a waste of time.

At about the time of the Harvard Report, Raymond Carhart, generally considered the “father of audiology” in North America, published procedures for selecting the appropriate hearing aid.^3,4,5 Although criticized by the Harvard Report, this (comparative) method gained much ascendancy in the university clinics. Aids were pre-selected from clinic stock for inclusion according to the best judgment and preferences of the professional. Only body aids were available then, and during the evaluation, the instrument’s case was placed on a (baffle) board alongside the patient. The patient was tested in a sound field, usually but not always, with stock molds, and often, but not always, without venting. Feedback problems were not a big issue unless the loss was substantial, for the aid’s microphone and receiver were at a good distance from each other. Importantly, conventional wisdom at that time held that on average, aided word recognition scores were not expected to exceed the unaided score, which served as a target. The best performing aids were those that provided aided scores approximating the unaided score obtained under circumaural earphones, for it was expected by comparison, that the degraded signals provided by the hearing aid would result in lower scores.⁶

The Harvard Report recommendations led the early audiology world astray for years. Fitting hearing aids with a flat response or a 6 dB per octave response on patients having other than flat or moderately sloping threshold configurations led to many dissatisfied, poorly performing patients. And easily accessible information regarding the effect of the earmold on the amplified response was, for all intents and purposes, nearly nonexistent.

Fitting problems multiplied when the first BTE and eyeglass aids reached the market in the early 1950s. The two transducers were positioned much closer to each other in head worn instruments, and manufacturers had a difficult time keeping receiver vibrations from spilling over into the microphone. Further, wider bandwidth was possible with head worn aids, and this increased the probability of acoustic feedback problems. The result was a high incidence of internal and external feedback issues. One could use full shell earmolds having minimal or no venting in order to eliminate external feedback, but that exacerbated the occlusion effect for many patients. Professionals were fitting rather unrefined aids with little or no understanding of earmold acoustics to patients who, then as now, invariably presented with losses having a high frequency component. The usual outcome was frustration on the part of the professional, and dissatisfaction on the part of the patient.

It is no wonder that hearing aid fitting became one of the least desirable aspects of audiology during those years. Few if any students opted for making hearing aids the major focus of their studies; in fact it was regarded as somewhat déclassé if one did, and pity the brave instructor who taught amplification, for reliable facts were few and far between.

The Beginning of Wisdom

Fully five years prior to Dunlavy’s article mentioned above, Earl Harford, a professor at Northwestern University, began to document the advantage of the CROS concept and reported it to the scientific community.⁷ He and his colleagues published a series of studies in the professional journals exploring its potential and its benefits.^8,9,10,11,12 Almost immediately professionals recognized that CROS was not just a solution for unilateral hearing loss, but rather, because the microphone and receiver were on separate sides, it was possible to provide high gain, high frequency amplification without encountering feedback for patients with high frequency losses. Since nearly all fittings in those days were monaural anyway, every patient who presented with a bilateral (or unilateral) sloping high frequency loss was a candidate, and was assured of a nearly perfect fitting in at least one ear using an open mold. It was finally possible to deliver the satisfaction that the hearing aid advertisements promised.

In one fell swoop this unique CROS application dealt with a number of issues. Papers began to appear in the audiological literature showing that aided discrimination scores actually did improve markedly with open canal amplification compared to scores that had been obtained under earphones or with occluded earmolds.^13,14,15,16 This was a surprise to many for although it was known that test scores varied as test conditions changed (talker, level, transducers, test stimuli, etc,) for some reason this understanding had never fully registered in the case of hearing aid fittings. The improvement in scores resulted from, 1) the high frequencies receiving markedly greater amplification than had been possible heretofore, 2) the high frequency amplification bandwidth being significantly wider than was previously achievable, 3) the occlusion effect being virtually eliminated, and, 4) as a bonus, upward spread of masking effects being reduced due to the absence of amplification in the low frequencies. These results set in motion countless research studies over the years dealing with the benefits and usefulness of high frequency amplification and its contribution to word recognition in both children and adults, and produced many studies dealing with the effect of the earmold/coupling on the frequency response.

Achieving Miracles

It is instructive to visit the steps of hearing aid dispensers who were fitting CROS hearing aids, (prior to the introduction of wireless CROS). First, the patient had to be wearing zyl (special plastic) eyeglasses, or the patient was persuaded to purchase a pair. If he/she did not wear glasses, they were asked to get a pair with plain glass lenses. The frames had to have so-called “standard hinges” because the graduated temple terminations furnished with the eyeglass hearing aids were only available with this type of hinge. Then a small circular motor-tool saw blade was used to cut a trench across the back of the plastic frame, from hinge to hinge. A very thin plastic cable containing two or three extremely fine wires was placed in the trench and covered over with a plastic sealant. After it had dried, the inside covers of both hearing aid temples were removed, and the fine wires were soldered to the microphone on one side and to the circuitry and receiver on the other. The temple covers were then reglued or screwed back into place. The eyeglass temples and frame were heated, bent and adjusted so that the patient was comfortable with the glasses. A shaved-down pipe cleaner was inserted into a length of earmold tubing and bent to the right shape for secure placement in the ear canal. The tubing was heated with a blower until it set. If needed, the hearing aid’s response could be manipulated somewhat by changing the depth of the tubing in the ear canal or by using tubing with different exterior dimensions.

Why would dispensers go through such a complicated, lengthy and convoluted process? The answer is that they never had so many grinning, enthusiastic, happy customers. Handholding just about disappeared if the patients were fitted with CROS; most old and new customers experienced wearing success right out of the box. Even with all the rigamarole that attended CROS installation and fitting, countless professionals routinely chose to recommend and to fit them. To them, fittings without feedback problems were indeed miracles. In the early 1970s, the records show that in some years CROS fittings accounted for nearly 20% of all head worn aids. By 1974, Harford and Dodds¹¹ suggested that CROS fittings had probably reached close to 40% of all recommendations in University audiology clinics.

The CROS concept and the children that it spawned (IROS, BiCROS, Hi-CROS, etc) became a somewhat neglected fitting option in ensuing years, as custom ITE aids grew in importance. The wonderful solution to feedback issues that CROS provided was essentially forgotten, and CROS was seen again solely as an application suitable for fitting unilateral losses. The advantage of an open canal fitting however, never disappeared, and when it appeared feasible again as a result of modern feedback control methods, the miracle happened all over again.

During an audiology convention some years ago, a speaker remarked to the audience that the open canal technology of today shouldn’t be confused with the old CROS and IROS fittings of years ago. The speaker was in error, of course, for it’s the same idea. Today’s professionals are standing on the shoulders of some very tough and committed professionals who developed the original technique, changed a lot of widely held assumptions, and brought to the fore many of the important understandings we hold today about providing acceptable amplification for high frequency losses.

References.

1. Dunlavy, Alfred R. (1970) CROS: The New Miracle Worker. Audecibel, Fall, 141-148.
2. Davis, H., Stevens, S.S., Nichols, R.H., Hudgins, C.V., Peterson, G., Marquis, R.J., Ross, D.A. (1947) Hearing Aids: An experimental study of design objectives. Cambridge: Harvard University Press.
3. Carhart, R. (1946). Selection of hearing aids. Arch Otolaryng, 44, 1-18.
4. Carhart, R. (1946). Tests for the selection of hearing aids. Laryngoscope, 56, 780-794.
5. Carhart, R. (1950). Hearing aid selection by university clinics, Journal of Speech and Hearing Disorders, 15 ,106-113.
6. Hirsh, l.J. (1952) The Measurement of Hearing. New York: McGraw-Hill.
7. Harford, E., & Barry, J. (1965). A rehabilitative approach to the problem of unilateral hearing impairment: the contralateral routing of signals (CROS). Journal of Speech and Hearing Disorders, 30, 121-138.
8. Harford, E. (1966). Bilateral CROS. Two-sided hearing with one hearing aid. Archives of Otolaryngology, 84, 426-432.
9. Harford, E. (1967). Innovations in the use of the modern hearing aid. International Audiology, 6, 311-314.
10. Harford, D. (1968). Recent development in the use of ear–level hearing aids. Maico Audiological Series, V5, #3, 10-13.
11. Harford, E., and Dodds, E. (1974). Versions of CROS hearing aids. Archives of Otolaryngology, 100, 50-57.
12. Dodds, E., & Harford, E. (1968). Modified earpieces and CROS for high frequency hearing loss. Journal of Speech and Hearing Disorders, 11, 204-218.
13. McClellan, M. (1967). Aided speech discrimination scores in noise with vented and unvented earmolds. Journal of Auditory Research, 13, 93-99.
14. Green, D., & Ross, M. (1968). The effect of a conventional versus a nonoccluding (CROS type) earmold upon the frequency response of a hearing aid. Journal of Speech and Hearing Research, 11, 638-647.
15. Hodgson,W., & Murdock, C. (1970). Effect of the earmold on speech intelligibility in hearing aid use. Journal of Speech and Hearing Research, 13, 290-297.
16. Jetty, A., & Rintelmann, W. (1970). Acoustic coupler effects on speech audiometer scores using a CROS hearing aid. Journal of Speech and Hearing Research, 13, 101-114.