Specifications

Bosch Security Systems | 2007-08 | 9922 141 50672en
Plena Loop Amplifier | Installation and User Instructions | Design and planning en | 16
2.3 Induction loops
2.3.1 Introduction
When you make an induction loop, you must take a
number of parameters into consideration. However,
sometimes there are special situations, which make the
design and planning of the induction loop even more
important. A number of potential problems and
solutions will be discussed later.
2.3.2 Position
For the best audio quality and the smallest variation in
the magnetic field strength, the distance between the
induction loop and the listening plane must be between
12 and 15% of the width of the room (refer to
figure 2.7).
For example, in a room with a width (W) of 10 m, the
induction loop should be installed 0 to 0.4 m below or
2.4 to 2.8 m above the floor for the best audio quality
and the smallest variation in the magnetic field strength.
Typically, you will install the induction loop in the floor
or in the ceiling of a room. When the distance between
the floor and the induction loop is too small (less than
8% of the width) or too large (more than 20% of the
width), refer to figure 2.8. The figure 2.8 shows the extra
power that the loop amplifier needs to make the correct
magnetic field. The numbers next to the curves show
the distance from the floor to the induction loop in % of
the width (W) of the room.
2.3.3 Wire diameter
For the best audio quality, the DC (direct current)
resistance of the induction loop must be between 1 and
3 Ω. The DC resistance depends on the wire diameter
and the wire length. Do as follows:
1 Calculate the wire length. The wire length depends
on the size of the induction loop.
2 Use figure 2.9 to get the allowed wire diameter.
For example, in a rectangular room with a width (W) of
10 m and a length (L) of 30 m, the wire length is 80 m.
According to figure 2.9, the wire diameter must be
between 0.77 and 1.34 mm. Thus, you can use AWG 20
wire or a wire with a standard diameter of 1.00 mm.
2.3.4 Magnetic field strength
For the best audio quality, the vertical component of the
magnetic field must be 100 mA/m ± 3 dB at 1.2 m
above the floor in the area that is surrounded by an
induction loop. The strength of the magnetic field
depends on the electric current through the induction
loop. Peaks in the strength of the magnetic field must be
less than 400 mA/m at 1.2 m above the floor in the area
that is surrounded by the induction loop.
2.3.5 Connection
Refer to section 4.1 for instructions that tell you how to
connect an induction loop to the loop amplifier.
2.3.6 Configuration
Refer to section 5.2 for instructions that tell you how to
configure the electric current through the induction
loop.
figure 2.7: Position
Loop
H
W
L
12..15% W 12..15% W
Loop