Reference Laboratory Looking Ahead in Pro Audio
Shields, insulating materials and their properties

BRAIDED a braided shield consists of two groups of thin wires wowen respectively in clockwise and counterclockwise fashion. The angle between two crossing strands is called the braid angle. Generally offers the best highfrequency shielding performance and protection from radio-frequency interference (RFI). This is due to the very high electrical conductivity of the braid, and to its low-inductance, self-shorting configuration. Moreover, the braided construction offers the highest structural integrity, good flexibility and flex life.

LAP spiral-wrapped copper serve shields are more inductive than braided shields, as they resemble a long coil of wire when extended. This can compromise highfrequency shielding and is not recommended when effective shielding above 100 kHz is required. Spiral shields are relatively inexpensive and easy to terminate, while guaranteeing the highest flexibility.

FOIL SHIELDS Foil-shielded cable is very heavily used for permanent installation work and for portable multipaire "snake" cables. The light weight and slim profile makes foil very advantageous in applications involving pulling cable into conduit and permanent installations. The thin mylar/aluminum foil guarantees 100% coverage ratio and offers excellent RFI and EMI shielding properties, but is not suited for portable applications due to its shorter flex life. Cables with foil shields generally feature a drain copper wire used to terminate the shield and give extra mechanical strength to the cable.

CONDUCTIVE PLASTICS Cables can feature conductive textile or plastic shields for 100% coverage, with copper drain wires or very low-coverage copper braid added for ease of termination and low DC resistance. While this type of construction is very flexible, its shielding effectiveness suffers greatly as frequency increases, offering very little effect above 10 kHz because of its low conductivity. Conductive plastic is therefore used in conjunction with traditional braided or spiral shields where flexibility should not compromise RFI and handling noise immunity.

FILLERS, TNT TAPE & HANDLING NOISE The triboelectric effect that causes impact-related "slapping" noise as the cable hits the stage or is stepped upon during use is related to capacitance, specifically the change in capacitance that takes place as the insulation or dielectric is deformed. This causes it to behave as a crude piezoelectric transducer, a relative of an electret condenser microphone. Handling noise is best addressed by using soft, impact-absorbing insulation and jacket materials in a very solid construction with ample cotton or plastic fillers to insure that the cable retains its shape. TNT tape wrapped around the outer shield greatly enhances solidness and electrical parameters uniformity along the cable.

INSULATION MATERIALS depending on the specific application cables might employ a combination of different insulation materials to meet both electrical (i.e. capacitance) and mechanical (flexibility) demands. PVC, polyethylene (PE), and polypropylene (PP) are typically used. Reference cables employ special mixtures guaranteeing minimal shrinkage of the insulating sleeves during soldering operations.

INSULATION MATERIALS PROPERTIES
 PVCPEPPTEFLONPOLYURETHANE
Temp. range [C]-20/80-50/70-20/80-40/200-40/80
Flame resistanceGoodPoorPoorExcellentPoor
Water resistanceGoodExcellentExcellentExcellentGood
Abrasion resistanceGoodFairGoodExcellentGood
Low temp. flexibilityFairExcellentGoodExcellentGood
Solvent resistancePoorFairGoodExcellentExcellent
Mineral oils resistancePoorPoorPoorGoodExcellent



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