Key Elements: To Screen or Not To Screen
Screens (or shields) inevitably change the way a cable works as the screen is in close proximity to the core and therefore electro-magnetically coupled to it. Usually this has an adverse effect on propagated signals.
In many applications, balanced line level audio and data circuits can be used without a screen, which leads to far better data transmission, greater audio phase accuracy and lighter, cheaper cable. An example is the millions of kilometres of un-screened telephone cable around the world carrying several terabytes of data at high speed!
Screens are essential in sensitive applications; for microphones, unbalanced circuits or in close proximity to noise-generating environments such as lighting rigs. Some speaker and power cables have to be screened when used in sensitive environments such as shipping and aviation in order to prevent possible interference with radar and other communication systems.
Screens make cable stiff. Stainless steel braid is stiff and heavy but rugged and most effective as its ferrous element kills electro-magnetic interference. Aluminium foil is a good general purpose screen but is more transparent to higher frequency interference. Copper braid works well and makes very flexible cable. Both copper braid and aluminium foil screens are light but copper is more expensive.
There are some sophisticated flexible screens used by the military that provide solid electro-magnetic shields, but these require very special (read expensive!) materials.