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SOUND CABLE

Wire and cable are important factors in sound system performance. Any noise interference in the system is quickly attributed to the quality and type of the cable installed. In the extreme, an entire audio system's integrity and performance may be degraded or silenced by faulty cable. The choice of cable is the vital link between costly audio hardware.
 
All wire and cable are not the same! Looks alone don't tell the story. Two similar looking cables may have completely different electrical and physical properties.
 
Physical Properties of Sound Cable
Many questions are posed regarding "twisted" or "cabled" products and their use on a given application. All installations are unique and may require different cable products. However, here are some basic guidelines:
 
Twisted (cabled) vs. straight lay (zip)
Charges from electric motors, fluorescent lighting ballast and other sources may be coupled onto conductors to produce electromagnetic interference (EMI). Standard shielded cables do not offer enough protection. Using twisted conductors on a balanced line with several different types of conduit or armor can best eliminate this type of noise. It is also a good idea to run cables an acceptable distance away from any potential source of EMI. Twisted wiring should always be run through the walls for speaker and component wiring. Do not run wiring parallel to AC and RF lines.
 
The "lay length," or length between twists on two or more conductors, is essential for the rejection of EMI. The shorter the lay length, the better the magnetic noise rejection. Zip cables are not recommended for in-wall installations. Their inherent straight lay design offers minimal protection from EMI.
 
 
 
 
Stranded vs. solid conductors
Over the years, stranded conductors have become an audio industry standard. Due to their flexibility, stranded cables are easier to pull and terminate. They also reduce potential conductor breakage when repeatedly flexed. The increased surface area they provide may be a factor in overall frequency response.
 
Gauge size
Gauge size has an important impact on the performance of the system. Not only does it effect the current loss on long runs, the gauge size can easily impact the cost of the installation. Always check with a systems specialist or equipment manufacturer for the recommended gauge size.

 

 

 

Shielded vs. non-shielded
Electrostatic sparks or spiking from motors, neon, fluorescent lighting and other sources can couple onto adjacent cables causing noise. Electrostatic charges are most disturbing on microphone or line signals because these signals are often amplified. Shielded cables or protective conduit provide an acceptable solution to electrostatic noise in many instances.

 

Physical Properties of Shielded Cable
Shielded cables should be considered for installations in areas near dimmer panels and light switches, in parallel runs, near neon or fluorescent lights and near power cables.
 
Some frequencies used for radio communications have a tendency to become coupled onto conductors to produce radio frequency interference (RFI). Depending on the level of interference, shielded cables or conduit offer excellent protection against this common noise interference.
 
National Electrical Code Article 640
NEC Article 640 covers "equipment and wiring for audio signal generation, recording, processing, amplification and; distribution of sound; public address; speech input systems; temporary audio system installations; and electronic organs or other electronic musical instruments." (This article has undergone an extensive revision for the 1999 addition).
 
Glossary of Terms
Baseband Signal: The most basic non-modulated form that audio signals take is called baseband. Baseband signals are the raw form of low frequencies before they modulate to the carrier. These frequencies are often lower than line level and sometimes include direct current. Cables used on baseband signal systems may include shielded twisted pair for audio or 75 ohm coax for video.
 
Broadband Signal: Broadband technology involves multiplexing or modulation of several different signals over high-frequency transmission lines. Many times one cable type is used to transmit from several to hundreds of different channels at a time. Demodulators are used at the receiving end to extract the individual signals from the trunk line. Common cable types are coax, UTP, Fiber and "hybrid" composite cables.
 
Crosstalk: Crosstalk is an interference caused when signal frequencies from one line are coupled onto an adjacent line. Crosstalk may cause noise in the system and is often times easily prevented by use of twisted pair cabling.
 
Impedance: Impedance is the total electrical opposition a circuit, cable or component offers to alternating current (AC). Expressed in ohms, impedance contains both resistance and reactance.
 
Line Level Signal: Low level signal associated with audio applications. Microphone cables are common line level signal carriers.
 
Noise: Electrostatic Interference: Electrostatic sparks or spiking from motors, neon, fluorescent lighting and other sources can become coupled onto the cables causing noise. Electrostatic charges are most disturbing on microphone or line signals because they are often amplified. Shielded cables or protective conduit provide an acceptable solution to electrostatic noise in many instances.
 
Microphonic Noise: Capacitance changes between inner conductors or conductors and the shield can cause interference. This internally produced noise is best corrected by using a cable with a stable dielectric insulating material. A tightly braided shield is also recommended to keep outside interference from entering the cable.
 
Electromagnetic Interference: Electric motors, fluorescent lighting ballast and other continuous sources of interference may be coupled onto conductors to produce electromagnetic interference (EMI). Standard shielded cables do not offer enough protection. Using twisted conductors on a balanced line, several different types of conduit or armor raceways can effectively eliminate this type of noise. It is always a good idea to run cables an acceptable distance away from any potential source of EMI.
 
Radio Frequency Interference: Some frequencies used for radio communications have a tendency to become coupled onto conductors to produce radio frequency interference (RFI). Depending on the level of interference, shielded cables or conduit offer excellent protection against this common noise interference.
 
Ground Loops: Current flowing through improperly grounded shielded cables will cause noise in the system. The only solution is to provide proper grounding of the system.
 
Resistance: The resistance to the flow of current is expressed in ohms. Every material offers some resistance or opposition to the flow of electric current. The size and type of material in a circuit is chosen to provide the lowest resistance possible. Copper is an excellent conductor for the price. The larger the gauge (AWG), the lower the resistance.