Handbook for Sound Engineers

Transmission Techniques: Wire and Cable 405

laboratory. Table 14-9 shows some standard numbers as

a point of reference.

14.3.2 Wire Insulation Characteristics

The key difference between rubber compounds and plas-
tic compounds is their recyclability. Plastic materials can
be ground up, and re-melted into other objects. Polyeth-
ylene, for instance, can be recycled into plastic bottles,
grocery bags, or even park benches. And, should the
need arise, these objects could themselves be ground up
and turned back into wire insulation, or many other uses.
The term thermoplastic means changed by heat and is
the source of the common term plastic.
Rubber compounds, on the other hand, are ther-
moset. That is, once they are made, they are set, and the
process cannot be reversed. Rubber, and its family, is
cured in a process sometimes called vulcanizing. These
compounds cannot be ground up and recycled into new
products. There are natural rubber compounds (such as
latex-based rubber) and artificial, chemical-based
rubber compounds such as EPDM (ethylene-
propylene-diene monomer).
The vast majority of wire and cable insulations are
plastic-based compounds. Rubber, while it is extremely
rugged, is considerably more expensive that most plas-
tics, so there are fewer and fewer manufacturers
offering rubber-based products. These materials, both
rubber and plastic, are used in two applications with
cable. The first application is insulation of the
conductor(s) inside the cable. The second is as a jacket
material to protect the contents of the cable.

14.4 Jackets

The jacket characteristics of cable have a large effect on
its ruggedness and the effect of environment. A key con-
sideration is often flexibility, especially at low tempera-
tures. Audio and broadcast cables are manufactured in a

wide selection of standard jacketing materials. Special

compounds and variations of standard compounds are

used to meet critical audio and broadcast application

requirements and unusual environmental conditions.

Proper matching of cable jackets to their working envi-

ronment can prevent deterioration due to intense heat

and cold, sunlight, mechanical abuse, impact, and crowd

or vehicle traffic.

14.5 Plastics

Plastic is a shortened version of the term thermoplastic.

Thermo means heat, plastic means change. Thermoplas-

tic materials can be changed by heat. They can be melted

and extruded into other shapes. They can be extruded

around wires, for instance, forming an insulative (non-

conductive) layer. There are many forms of plastic.

Below is a list of the most common varieties used in the

manufacture of wire and cable.

14.5.1 Vinyl

Vinyl is sometimes referred to as PVC or polyvinyl chlo-

ride, and is a chemical compound invented in 1928 by

Dr. Waldo Semon (USA). Extremely high or low tem-

perature properties cannot be found in one formulation,

therefore, formulations may have 55°C to +105°C

(67°F to +221°F) rating while other common vinyls

may have 20°C to +60°C (4°F to +140°F). The many

varieties of vinyl also differ in pliability and electrical

properties fitting a multitude of applications. The price

range can vary accordingly. Typical dielectric constant

values can vary from 3.5 at 1000 Hz to 6.5 at 60 Hz,

making it a poor choice if high performance is required.

PVC is one of the least expensive compounds, and one

of the easiest to work with. Therefore, PVC is used with

many cables that do not require high performance, or

where cost of materials is a major factor. PVC is easy to

color, and can be quite flexible, although it is not very

rugged. In high-performance cables, PVC is often used

as the jacket material, but not inside the cable.

14.5.2 Polyethylene

Polyethylene, invented by accident in 1933 by E.W.

Fawcett and R.O. Gibson (Great Britain), is a very good

insulation in terms of electrical properties. It has a low

dielectric constant value over all frequencies and very

high insulation resistance. In terms of flexibility, poly-

ethylene can be rated stiff to very hard depending on

molecular weight and density. Low density is the most

Table 14-9. Dielectric Constant

Dielectric

Constant

Material Note

1 Vacuum By definition

1.0167 Air Very close to 1

1.35 Foam, Air-filled Plastic Current technological limit

2.1 Solid Teflon™ Best solid plastic

2.3 Solid Polyethylene Most common plastic

3.5–6.5 Solid Polyvinyl Chlo-

ride

Low price, easy to work

with