Handbook for Sound Engineers

1546 Chapter 42

mic the actors, talkers, or singers. A sound pickup that
does not reject the reverberant field and extraneous
noise, and/or has distorted sound will not be useful for a
system for the hearing impaired.

42.2.1 Magnetic Induction Loops

Magnetic induction, sometimes called a loop system, is
one of the oldest but still useful systems. The principal
advantage of this system is that it can operate directly
into the user’s hearing aid without the need for a
portable receiver as required by all other systems. A
loop of wire is wrapped around the seating area, usually
under the carpet, and connected to an amplifier. The
electrical current flowing through the loop will create a
magnetic field (as the primary of a transformer) that can
be picked up by a hearing aid equipped with a T-c o i l (T
as in telephone). About 60% of the hearing aids in the
United States have T-coils for magnetic coupling with
the earpiece of a telephone. Portable receivers are avail-
able for use by patrons who do not have an aid with a
T-coil.
There are, however, several problems with the loop
system. Most buildings have other magnetic fields that
will be picked up by the T-coil. Ordinary electrical
wiring will radiate a large 60 Hz field throughout the
room, so the T-coil and the portable receivers are
designed to have no response in the low-bass region in
order to avoid the 60 Hz hum. There are other
power-line-related noises that cannot be filtered
out—motors, dimmers, and fluorescent lamps being the
most common. The size and shape of the loop and the
amount of nearby steel in the building or in the seats
will affect the strength and uniformity of the magnetic
field. Simultaneous use of loops in adjoining rooms is
often a problem because of crosstalk between the
systems. Quality of reception is also dependent upon the
quality of construction of the T-coil in the hearing aid.
All these factors combine to provide reception that
sometimes has poor sound quality, is noisy, and varies
in volume depending on the location of the listeners and
how they turn their heads.
The limited statistics available indicate that of the
people needing hearing assistance, only about 20% are
actually wearing aids, and only 60% of those aids are
equipped with T-coils, which suggests that only 12% of
those needing help are able to make use of a magnetic
loop through their hearing aids. It has been argued that
the majority of those without T-coils actually are young
children and the very old; active adults are most likely
to have T-coils. Despite the comparatively limited avail-
ability of T-coils and the several substantial limitations

of the magnetic loop system, it remains popular and

enjoys the vocal support of many of those 2.5 million

people who have T-coils.

The cost of a magnetic induction loop system is

largely the cost of the amplifier and the labor of

installing the wire loop. The receivers are inexpensive.

Until recently, the magnetic loop was the least costly

system. However, advances in solid state electronics

have made the AM and FM broadcast systems very

competitive in price. Where large areas are to be

covered, the magnetic loop is probably not as cost effec-

tive as a broadcast system.

42.2.1.1 Loop Design Criteria

The international standard for the magnetic field

strength of a loop system with an input signal of normal

speech level is 0.1 A/m. Magnetic field strength H =

0.1 A/m in SI units or 0.125 Oe in cgs units. This field

strength produces an audio voltage in the T-coil about

equal to the output of the hearing aid’s microphone at

normal speech levels, Fig. 42-1. This eliminates the user

having to make volume control adjustments when

switching between microphone and T-coil. Also, this

field is strong enough that noise and interference prob-

lems are minimized, yet it is not so strong as to overload

the hearing aid amplifier.

The field strength should be as uniform as possible

over the coverage area. An achievable criterion for

uniformity is a maximum variation of ±3 dB in the

audio output signal.

System design is based on the vertical component of

the magnetic field, ignoring the horizontal field compo-

nents for three reasons:

Figure 42-1. Typical hearing aid response. (From

Reference 1.)

40

30

20

10

0

100 200 500 1 K 2 K 5 K 10 K

Inductive input/

acoustic output

Acoustic input/

acoustic output

Frequency–Hz

Arbitrary dB