Dr. Ross on Hearing Loss
More on T-Coils
by Mark Ross, Ph.D.
This is a topic that has been discussed several times before, but bears repeating because of its importance for consumers. Plus, there always seems to be something else to add. To review very briefly, a T coil is a hearing aid component that responds to magnetic fields rather than sounds as do microphones. Unfortunately, T coils have not really received the attention they deserve from the hearing aid industry or hearing aid dispensers. From the time in l947, that they were first described as a way of listening to telephones by accessing the magnetic leakage around them (an accident of telephone design at the time), T coils have been very much a secondary and peripheral concern among audiologists and the hearing aid industry. One wonders why. Is it not just as important, and sometimes more so, for a person with a hearing loss to understand conversation on the telephone as it is when listening through the hearing aid microphone? In our modern society, telephone communication is no longer a luxury as it was many years ago. As of 1990, over 95% of the households in the U.S. possessed a telephone. Probably, this percentage is even higher now. People do more than just stay in touch by using the phone; they acquire information, make appointments, share confidences, and use it as a lifeline in the event of emergencies.
It’s not as if people with hearing loss have not been voicing their concern about their problems with telephones. In an article published November 2002 in The Hearing Review, Sergei Kochkin reports on the results of one of his surveys of hearing aid users, this one concerning what they desire from their hearing aids. Among other findings of the survey, eight out of every ten consumers would like to hear better on the telephone. In fact, only 38% reported satisfaction with their ability to hear on the phone. Clearly, there is much room for improvement in telephone communication.
Now, however, given the increased use of T coils as a convenient assistive listening device (ALD) “receiver”, T coils have even more of a role to play than they did when their use was limited to telephones. Using T coils in this fashion has also been discussed in these pages and also deserves a bit of repetition. Since a T coil detects magnetic fields (including unwanted electromagnetic interference), it can pick up such fields generated from a wire loops in large areas (e.g. rooms, auditoriums) or small areas (TV listening), or via a neckloop plugged into either Infra-Red or FM assistive listening devices. The magnetic fields that T coils pick up from these sources are basically sound signals coded in a different form. The job of the T coil is to help convert these magnetic fields back into sound.
Given the potential value of a T coils with telephones and as an assistive listening device receiver, it would seem logical that they be given the same careful scrutiny that the microphone response of hearing aids receive. But this is not the case. It wasn’t until l976 that the profession developed its first official standard with which telecoils were to be measured. This required testing only at one frequency (1000 Hz) with a weak and unrealistic magnetic field input, while the tester was required to rotate the hearing aid on a flat board (in which a wire loop was embedded) until a maximum response was obtained from the hearing aid (set on “T” and connected to a coupler). This was all that was deemed necessary to evaluate T coils at the time - which serves as an indication of the lack of respect in which they were held.
It wasn’t until 20 years later (l996) that new official standards were adopted with which to evaluate telecoils. In this standard, a stronger magnetic field was utilized (simulating the strength of a hearing aid compatible – HAT – telephone) and three frequencies were utilized (1000 Hz, 1600 Hz and 2500 Hz) instead of one. In this procedure, the average sound output generated at the three frequencies with the magnetic field input is compared to that obtained with an acoustic input of 60 dB input at a standard gain setting. The idea was that these two figures should coincide. That is, the output with the magnetic field should equal the output obtained with the microphone. If that measured with the T coil is less than that occurring in the acoustic condition, then the implication is that the T coil strength has to be boosted. This comparison is called the STS (for simulated telephone sensitivity). Hearing aid dispensers should be queried on the STS of the aids they dispense; any one that doesn’t know what it is has something to learn. This bit of assertiveness may also help sensitize some dispensers of hearing aids to pay more attention to T coils.
The new standard is an improvement, but it is still not good enough. It still does not include the kind of comprehensive tests with which the microphone response of hearing aids is evaluated. It does not, for example, require a full frequency response (that is, measuring the amplification pattern across the entire frequency range of the hearing aid). It is very possible that the details of the frequency responses obtained with a microphone and that measured with the T coil would be very different, in spite of similar averages at the three standard input frequencies. Audiologists would never fit a hearing aid without knowing the specific frequency response, usually aiming at some given “target” response, so why is it considered less important to do the same with T coils? The same amplification target should be achieved whether a person listens through a microphone or a T coil.
I’d like to emphasize and restate this point. Hearing aid dispensers spend much of their time when fitting hearing aids trying to ensure that an appropriate amplification pattern has been selected. The programming directives they use always include electroacoustic targets for them to aim at. These targets may be “fine-tuned” later as a person actually experiences the amplified sound the hearing aid delivers. These steps are exactly what are needed to try to ensure that a hearing aid is fit appropriately. But this same logic applies to T coils as well. Only it is almost never applied to them. Hearing aid dispensers have no idea what pattern of amplified sound is being produced by the T coil in the ear canals of their clients. Most will explain and perhaps demonstrate the use of a T coil, but they depend entirely on subjective responses from their clients. Subjective impressions are important and necessary, but, as with conventional hearing aid selection, they must be paired with more objective measures.
One of the most important tools in the audiological armamentarium at the present time is a probe microphone system that measures the “real-ear” response of hearing aids. What this entails is the placement of a very fine flexible tube alongside the earmold and extending it a few millimeters past the tip into the ear canal. The tube leads to a microphone and a sound wave analyzer. What a probe-mike device does is measure the spectrum of sound in the ear canal produced by a hearing aid. It is the most accurate estimation we now have of the sound signals that actually reach a person’s eardrum as it reflects all the electronic and acoustic pathways from microphone through the earmold.
Most hearing aids today are programmable. It should be noted that the graphs shown on a programming screen reflect the relative changes produced by programming modifications, not the sound pressures actually existing in the real ear. These graphs do not reflect the acoustic influence of the hearing aid microphones, the hearing aid receiver, individual variations in ear canals or the influence of an earmold. While necessary, they are electronic measures only. Acoustic measures, on the other hand, utilize a coupler into which the hearing aid is connected and include the influence of the entire hearing aid, not just the electronics. However, this coupler is meant to simulate an “average” ear and not specific individuals. Still, coupler measures can be considered one step forward into the acoustic reality faced by a hearing-impaired person.
Real-ear measures, on the other hand, take reality one step further by including everything along the acoustic pathway that may affect the performance of a hearing aid, from the microphone right to an individual’s ear canal and eardrum. The difference between these three procedures (programmer, coupler, and real-ear) was well demonstrated in an article by Hawkins and Cook in the July issue of The Hearing Review. It turns out that the graphs displayed on the programming device tends to overestimate the amplified sound, particularly at the higher frequencies, that hearing aid wearers actually receive. Plus, as we would expect, individuals varied quite a bit in the extent of this overestimation. Using this measure only, as is frequently done, may not be an accurate representation of how the hearing aid actually performs in a person’s ear. In reality, all three measures have a contribution to make and all may be necessary. But what does all this have to do with T coils? Actually, quite a lot.
In a recent survey, more than half the audiologists reported that they employed real-ear acoustic measures when fitting hearing aids. Probably a higher percentage take coupler measures although this was not specifically cited in the study I reviewed. However, less than five percent evaluate the performance of T coils with real-ear tests. I myself do not know of a single hearing aid dispenser (audiologist or hearing instrument specialist) that measures the electroacoustic performance of T coils in the real ear.
In some ways, however, the direct measures of T coil performance may be even more important than looking at the microphone response of a hearing aid. Why? Because we have long known from a body of research that goes back many years that once a hearing aid is switched from microphone to T coil that all bets are off insofar as the electroacoustic characteristics are concerned. Studies have shown (using couplers or a mannequin head with a microphone for an eardrum) that T coils can affect the response of a hearing aid in unpredictable ways. Sometimes, there would be a reduction of low frequency energy and sometimes this reduction would appear at the higher frequencies. Or there would be an energy peak in the middle frequencies. None of this augurs well for acceptable speech perception through the T coil.
The trouble is that there is no easy way to measure real-ear performance with T coils. Existing probe-microphone systems do not possess the same capability to measure real-ear T coil responses as they do with microphones. The last time such measures were reported in the literature (to my knowledge) was in l991. The co-author (Alison Grimes) tells me that she concocted a Rube Goldberg array to make these measures, but evidently it worked fine. There should be simpler ways now. Recently, at the RERC at Gallaudet and at the University of Connecticut, several colleagues (Matt Bakke and Kathy Cienkowski) did it with me with several of my hearing aids. After some missteps (always happens!) and some Rube Goldberg innovations of our own, we were able to visualize just what kind of amplification pattern the T coils were producing in my ear canal. Two of them were quite different, which I had suspected but couldn’t be sure. It has helped me decide which aids to use on T coil. In other words, this test can provide information useful for consumers.
Other Issues and Variables
T coils also introduce some issues and variables that do not occur with microphones. Weak ones are a perennial complaint of many hearing aid users. This is one of the most frequent complaints I hear from consumers (besides not knowing that they have one, how to switch it on, or how to use one effectively with a telephone or loop system). There are two ways that T coil strength can be improved. The first, of course, is to obtain an aid with a stronger T coil. T coils are available that come with a pre-amplifier. These can boost the signal by 20 dB or more compared to an unamplified T coil and appears to be only marginally larger or costly. I see no reason why amplified T coils should not be used routinely.
The strength of T coils can also be boosted through programming. A recent article in Seminars in Hearing describes how one of the memories in a multiple memory aid can be devoted to the T coil (many, if not most, modern hearing aids provide for more than one memory). This permits its electroacoustic performance to be controlled independently of the microphone. Of course, there are limits to what can be achieved through programming and the cautions expressed above still apply: what one sees in the programmer is not necessarily what occurs in the real-ear.
An issue with T coils that never occurs with microphones is their physical positioning. T coils operate by converting a magnetic field into an electrical current in the coil (the process is termed “induction”). As discussed in this journal in the past (both by me and by others) in order for T coils to respond maximally to telephones, they should positioned horizontally in the hearing aid in respect to the telephone. Used with a loop (neck or floor loop), on the other hand, maximal response occurs when the T coil is positioned vertically in the hearing aid. This is one of the great unknowns in hearing aids. No standard, no specifications that I know of, prescribe the physical positioning of a T coil in a hearing aid. And the smaller the aid, the more “custom built”, the less information available regarding the specific positioning of the T coil.
People who use T coils know how important positioning is. In using a telephone, we often rotate it around the hearing aid in order to find the point where we pick up the loudest signal. A similar experience can occur with loops, particularly floor loops. Hearing aid users often find that when listening through a loop, they experience a change in the signal strength as they move their head slightly to one side or other or up and down. Hopefully, the near future will see a new type of T coil, one that is relatively insensitive to its physical positioning within the hearing aid. Several industry sources state that they are in the process of developing just such an omnidirectional T coil. A successful effort would permit hearing aid wearers to use their T coils with equal facility on the telephone as well as all types of loops.
Another difference between telephone and loop listening is the effective frequency range amplified by the hearing aid. The frequency range of most telephones fall between 300 and 3400 Hz. What this means is that no matter how “hi-fi” the T coil or the hearing aids, speech sounds of higher or lower frequencies cannot be delivered to the ears of a hearing aid user. This will be true no matter no matter how people couple their hearing aids to the telephone (acoustically or inductively). For people familiar with the language this is rarely a problem, except in a few instances. For example, people can’t hear the difference between “Ross” and “Roth” on the phone, because the distinguishing information between /s/ and /th/ is located at frequencies above 3000 Hz.
The situation is different when the same T coil is used to access sound signals from a loop. Ordinarily, the upper frequency range of transmitting loops is similar to that of hearing aids (about 6000 Hz). While this means, at least theoretically, that greater listening fidelity is possible through loops than with telephones. It also raises the possibility of unwanted noise at the higher frequencies when a telephone is being used. Everything seems to be a trade off.
In the last several years, another type of T coil has been introduced, termed the “touchless T coil” (described in an earlier issue). It works by automatically switching on whenever a telephone is placed next to the hearing aid. When moved away from the telephone, the T coil is turned off and the microphone is again activated. This operation can be more than just a convenience for some people. There are many people who have difficulty manipulating the T switch and thus find telephone communication a continued challenge. The major problem with this new development is that neck and floor loops do not activate the touchless T coil and thus people who want to use their hearing aids with neck or floor loops cannot. However, I understand that this will soon be rectified, that a newer line of touchless T coils will provide for a manual override switch. This will permit the use of the hearing aid both for telephones and as an assistive listening device receiver.
In my judgment, T coils should routinely be included in hearing aids. While their inclusion may add a bit to the size and cost of hearing aids, the potential advantages of T coils are immense. It is true that people who wear the smaller in the ear hearing aids are able to use a telephone directly and do not need a T coil. For them, acoustical coupling works just fine. But acoustical coupling will not help these people hear electromagnetic signals emanating from loops. As loops become more common (not just in large areas, but in many smaller applications as well), the advantages of having an ALD receiver located in one’s hearing aid will be more and more apparent. And as more and more hearing aids include T coils, one can be certain that more and more loops will be installed, in all types of locations.
There has been clear-cut progress in the physical dimensions and performance of T coils. According to one manufacturer of T coils (Tibbetts industry) even more progress is now being made with new concepts, such as T coils that will cancel distant electromagnetic interference without affecting performance of the desired magnetic fields (that emanating from a telephone or a loop). Excellent T coils are here and improving all the time. What is lacking is the recognition by the professionals and hearing aid industry that T coils merit the same attention and respect that microphones receive. Whatever evaluation techniques are deemed necessary for analyzing the microphone performance of a hearing aid should also be conducted via T coils. In short, it must be kept in mind that for consumers listening to the telephone and through ALD’s is every bit as important as hearing sound signals transmitted through the air.