Handbook for Sound Engineers

436 Chapter 14

The current through the system is found with

(14-21)

or in this case

The power to the 50: load would be found with

(14-22)

or in this case

or 26% less power than assumed.

Only 11.7 W are lost to the line, the other 14 W
cannot be taken from the amplifier because of the
impedance mismatch. While high-power amplifiers are
relatively inexpensive, it is still practical to use heavy
enough wire so the amplifier can output almost its full
power to the loudspeakers. Table 14-31 shows the char-
acteristics of various cables which could be used for
loudspeaker wire and Table 14-32 is a cable selection
guide for loudspeaker cable.

The following explains how to use Table 14-29:

1. Select the appropriate speaker impedance column.


2. Select the appropriate power loss column deemed
   to be acceptable.


3. Select the applicable wire gage size and follow the
   row over to the columns determined in steps one
   and two. The number listed is the maximum cable
   run length.


4. The maximum run for 12 AWG in a 4:loud-
   speaker system with 11% or 0.5 dB loss is 140 ft.

Table 14-31. Frequency Limitations for 33 ft (10 m)
Lengths of Cable with Various Loads

Cable Type Upper Corner

Frequency, kHz

Resonant

Measurement

Phase (°)

2 :

Load

4 :

Load

4 “F

Load

4 :

Load

No. 18 zip cord 75 136 35 3

No. 16 zip cord 61 114 32 2

No. 14 speaker cable 82 156 38 2

No. 12 speaker cable 88 169 40 2

No. 12 zip cord 55 106 32 4

Welding cable 100 200 44 2

I P

R

= ---

I P

R

= ---

86.2

58

= ----------

=1.21 A

PI=^2 R

PI=^2 R

1.21

2

= R

=74.3W

Braided cable 360 680 80 1

Coaxial dual cylindrical 670 1300 112

Coaxial RG-8 450 880 92

Table 14-32. Loudspeaker Cable Selection Guide.

Courtesy Belden.

Power (%) 11% 21% 50%

Loss (dB) 0.5 1.0 3.0

Wire Size Maximum Cable Length–ft

4 : Loudspeaker

12 AWG 140 305 1150

14 AWG 90 195 740

16 AWG 60 125 470

18 AWG 40 90 340

20 AWG 25 50 195

22 AWG 15 35 135

24 AWG 10 25 85

8 : Loudspeaker

12 AWG 285 610 2285

14 AWG 185 395 1480

16 AWG 115 250 935

18 AWG 85 190 685

20 AWG 50 105 390

22 AWG 35 70 275

24 AWG 20 45 170

70 V Loudspeaker

12 AWG 6920 14,890 56,000

14 AWG 4490 9650 36,300

16 AWG 2840 6100 22,950

18 AWG 2070 4450 16,720

20 AWG 1170 2520 9500

22 AWG 820 1770 6650

Table 14-31. Frequency Limitations for 33 ft (10 m)

Lengths of Cable with Various Loads (Continued)