Mark Ross, Ph.D.
T-coil plays an important role in helping people keep in touch over the
telephone and use assisitve listening devices to their fullest. This is
a topic that has been discussed several times before, but bears
repeating because of its importance for consumers. Here, Dr. Ross talks
about the Tcoil, what is new, the history of telecoil standards and why
they are needed, and, for those readers who want to know the
technological aspects of T-coils, he provides it.
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 1947,
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.
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 percent of the households in the
United States possessed a telephone. Probably, this percentage is 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.
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 percent reported satisfaction with their
ability to hear on the phone. Clearly, there is much room for
improvement in telephone communication.
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 wire loops in large areas (e.g. rooms,
auditoriums) or small areas (TV listening), or via a neckloop plugged
into either infrared 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.
the potential value of T-coils with telephones and as assistive
listening device receivers, it would seem logical that they be given the
same careful scrutiny that the microphone responses of hearing aids
receive. But this is not the case. It wasn't until 1976 that the
profession developed its first official standard with which telecoils
were to be measured. This required testing only at one frequency (1,000
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 which shows the lack
respect for t-coils at that time.
wasn't until 20 years later (1996) 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 (1,000
Hz, 1,600 Hz and 2,500 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 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.
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.
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.
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
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 Tcoil 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.
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 the microphone through the
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 microphone, the hearing aid receiver, individual variations
in ear canals or the influence of an earmold. While necessary, they are
electronic measures only.
measures 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 person with hearing loss. 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.
difference between these three procedures (programmer, coupler, and
real-ear) was well demonstrated in an article by Hawkins and Cook in the
July 2003 issue of The Hearing Review. It turns out that the graphs
displayed on the programming device tend 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.
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 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.
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, 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 ot this augurs well for acceptable speech
perception through the T-coil.
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 1991. 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.
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.
Issues and Variables
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 appear to be only marginally larger or costly.
I see no reason why amplified T-coils should not be used routinely.
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.
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 be positioned horizontally in the hearing aid
with 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.
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 ioops. Hearing aid users often find that when
listening through a ioop, they experience a change in the signal
strength as they move their head slightly to one side or other or up and
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 with all types of loops.
difference between telephone and loop listening is the effective
frequency range amplified by the hearing aid. The frequency range of
most telephones falls between 300 and 3,400 Hz. What this means is that
no matter how "hi-fl" the T-coils 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.
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 6,000 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.
the last several years, another type of Tcoil has been introduced,
termed the "touch-less 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.
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
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.
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.
has been clear-cut progress in the physical dimensions and performance
of Tcoils. According to one manufacturer of T-coils (Tibbetts indusuy)
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).
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 listening to sound through the
telephone and through ALDs is every bit as important as hearing sound
signals transmitted through the air.