Handbook for Sound Engineers

1548 Chapter 42

becomes more pronounced. This field distortion is
manifest by dead spots within the loop. At its worst, the
entire system may be rendered useless.

The great listening height required of large loops
also presents architectural problems. Often the only
practical place to locate a loop is at floor level, either
below the floor or under the carpet. Where it is not
feasible to locate the loop far above (or far below) floor
level, the single, large loop can be broken up into a
number of smaller loops that can be sized to locate at
floor level. Because the vertical field strength rapidly
falls to a minimum above the conductors, it is important
to locate the loop wires in aisles or other areas that do
not require coverage. For multiple loops, the current in
parallel conductors of adjacent loops must flow in the
same direction, Fig. 42-5.

Unfortunately, multiple loops will almost always
have poorer uniformity than a single loop of the same
size as one of the multiples. There is a special design
technique for achieving a more nearly constant vertical
field strength when using multiple loops. It involves the
use of two sets of overlapping loops that are driven with
electrical signals 90 degrees out of phase. This complex
procedure is described by Bosman and Joosten.^3

42.2.2.1 Loop Current

Once the size and location of the loop are fixed, the
required current in the loop can be calculated. The
strength of the magnetic field is directly dependent on
the current in the loop. The required current, I, in a
single-turn loop is

(42-2)

where,

0.1 A/m is the field strength criterion,

A is the loop area in square meters or square feet,

D is the loop diagonal in meters or feet.

The terms containing hr are a correction for the

distance of the listener from the plane of the loop and

are obtained from Fig. 42-6 by going vertically from h

to the line and horizontally to the correction distance.

If a multiturn loop is used, the required current in the

loop is

(42-3)

where,

I is the current from Eq. 42-2,

n is the number of turns.

42.2.2.2 Loop Impedance

Wire size and number of turns in the loop must be

selected to handle the required current safely and to

control the range of variation of impedance across the

audio band. A loop can be designed to provide the

required magnetic field strength by using a relatively

small wire with one turn or by using a larger wire with

several turns. In the first case the loop impedance would

be mainly resistive; in the second, it would be heavily

inductive.

The impedance increases with frequency because of

the inductive reactance of the loop. This increase is

limited by adjusting wire size and the number of turns

so the impedance at 1000 Hz is no more than three

times the impedance at 100 Hz. This moderately rising

Figure 42-5. Multiple-loop current flow diagram.

Current flow

Figure 42-6. Graph for obtaining distance correction.

I

0.1A/m*SA

2 D

=-----------------------------u 12 + hr^2 u 1 +hr^2

* 0.0305 A/ft in English units

1.7

1.6

1.5







0 0.1 0.2 0.3 0.4 0.5





[1 + 2

hr

2 ] 1 + 2

hr

2

hr

IM I

n

---=