Handbook for Sound Engineers

Assistive Listening Systems 1547

• The vertical field strength predominates over most of
  the loop area, Fig. 42-2.


• The T-coils in hearing aids are typically positioned to
  be most sensitive to the vertical field.


• Rotating the hearing aid about the vertical axis (as in
  turning the head) results in no change in the pickup
  of the vertical component, whereas the pickup of the
  horizontal component changes from zero to
  maximum to zero with such rotation.

42.2.2 Loop Location and Size

The field strength produced by the loop will vary in
intensity from the edge of the loop to the center, Fig.
42-3. The range of variation is dependent on the area
and shape of the loop and the listening height, which is
the vertical distance between the plane of the loop and
the receiver. This interrelationship is expressed as the
relative listening height and is determined by

(42-1)

where,

hr is the relative listening height,

h is the listening height,

A is the area covered.

The normalized field strength along the diagonal of
loops of various shapes and the corresponding range of
acceptable values for hr are shown in Fig. 42-4.

By application of Eq. 42-1 and the hr values found in

Fig. 42-4, it is possible to design a loop of acceptable

shape, area, and listening height. The penalty for an

inequality in Eq. 42-1 is degraded uniformity of field

strength, as can be seen in Fig. 42-4.

If the loop is to be placed at floor level (h=48

inches for seated listeners), square loops falling within

the acceptable hr range will vary from 28 ft × 28 ft to

38 ft × 38 ft. A rectangular 1:4 loop may range in size

from 24 ft × 96 ft up to 32 ft × 126 ft. Smaller loop

dimensions will require a smaller h; larger areas need a

larger h.

As h grows larger, the field-distorting effect of steel

in the building structure and in the audience chairs

Figure 42-2. Field strength along the diagonal of a square

loop. (From Reference 2.)

20

10

0



10



0 0.2 0.4 0.6 0.8

dB

Relative distance from center

Vertical component

Horizontal component

hr h

0.5A

=---------------

Figure 42-3. Vertical field strength along the diagonal of

a square loop. (From Reference 2.)

Figure 42-4. Vertical field strength along the diagonal of

rectangular loops. (From Reference 2.)

20

10

0



10



0 0.2 0.4 0.6 0.8

dB

Relative listening distance from center

Relative

listening

height

0.05

0.2

0.15

0.2

0.3

20

10

15







0 0.5 0.4 0.15 0.20

dB

0.25

Range for variation

between 4 dB and 6 dB

Loop side

ratio

1:1

1:28

1:2

1:4

Relative listening height–hr