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Choosing the Right Audio Cable...

Determining what cable to use for different aspects of a system can sometimes be challenging. However, there are a few rules of thumb to follow when choosing what cable to use. First and foremost, cost does not determine quality. A lower cost cable may actually be more suitable for a given signal type and usage than a more expensive cable of the same type. In addition, there are many key distinctions among similar looking cables.

> The number of conductors
> The size and construction
> The geometry between pairs
> Shielding
> Insulation
> Jacketing

All of these parameters combined help determine the cables intended use. Because there are many different cable types, this article is limited to low voltage audio cables. Future articles will cover video and data cable selection criteria.

Number of Conductors -

When determining how many conductors are necessary, you are limited to either unbalanced or balanced. With an unbalanced connection at least a single conductor and a ground is required and with a balanced connection at least two conductors with or without a ground conductor is required. If interfacing between balanced and unbalanced, you require at least two conductors. However, complexity arises when mixed formats must be connected. With balanced and unbalanced terminals, there are numerous likely interconnection schemes. Ground loops and shorted outputs are possible problems.

Gage -

There are two major considerations for gage. First, is the conductor large enough to carry the necessary amount of current without overheating? Second, does the conductor offer an adequately low resistance over length so that losses are acceptable.

Conductor Construction -

The construction of the conductor relates to the cables flexibility as well as ease of termination. A solid center conductor is less flexible and more susceptible to stress over time. Cables with solid center conductors are better suited for permanent installation where the cable would remain in a raceway. This would limit the cables movement and reduce stress from flexing. A stranded center conductor is more flexible than a solid center conductor and offers a longer flex life. Cables using stranded center conductors are more appropriate for portable use, so the cable may be flexed often.

Insulation -

Not only is insulation a flame retardant, it protects the inner conductors from abrasion. However, its most crucial function is separation and symmetry. The spacing of the conductors determined by the insulation is known as the dielectric constant. It is the dielectric constant that determines the capacitance between signal carrying conductors as well as the shield. These capacitances combined with the natural resistive properties of the conductor(s) constitute a low pass filter. This means the larger the capacitance per foot, with a longer cable, the more high end roll off experienced. The shorter the run the less critical it becomes for analog audio. However, for runs over 100 feet a low capacitance cable should be used.

Shielding -

There are three main types of shields used in cable construction: braided, spiral, and foil. A braided shield offers the best structural integrity. They are flexible and have a long flex life. Braided shields are usually specified with a coverage area; however, this does not denote the amount of attenuation the braid offers, rather the physical coverage area of the braid. The number of strands, the grouping of those strands, the number of crossovers per inch, and the angle determine the braided shields effectiveness. In addition, a braided shield offers a lower resistance path to ground. This makes cables with a braided shield more appropriate for portable use. A foil shield can achieve 100% coverage and minimizes electromagnetic interference, particularly radio frequency interference. Again, the 100% is related to physical coverage and does not mean faultless EMI shielding. Foil shields are also more flexible than braided shields. However, the flex life for a foil shield is shorter than a braided shield. This makes foil shielded cable more suitable for permanent installation. Foil shields are usually combined with a bare drain wire to lower resistance and provide a simple termination point for the shield. A spiral shield has great flexibility and is easily terminated. However, over time flexing the spiral can spread it and cause a gap in the shielding. This leads to a short flex life, which makes a cable with a spiral shield appropriate for permanent installation.

Jacketing -

The jacket of a cable serves as a bundle and physically protects the inner conductors. This makes the criteria for materials used unique. In certain cases, the jacket material may be required to meet standards set by a governing body such as underwriter’s laboratory (UL). In this case, ratings are assigned to the jacket material which relate to its flammability. Cables with a UL rating of CMP, CL3P, and CL2P are considered to be plenum rated. This means that the cable is suitable for use in a plenum, which is an environmental air space or duct. Cables that have a UL rating of CMR, CL3R, and CL2R are considered suitable for riser use. This means that the cable is can be used in vertical riser shafts from floor to floor in a building. A cable with a rating of CM is considered suitable for general use. This means a cable is suitable for general purpose use, with the exception of risers and plenums. Each rating utilizes different mixtures of materials to craft the jacket. Plenum cables tend to be the least flexible, riser rated cable are flexible, but tend to have a memory, and general cables tend to be the more flexible. One of the most commonly used types of jacket material is PVC or polyvinylchloride. PVC is available in numerous blends customized to many applications. It resists most solvents, oil, flame, and sunlight. Although, standards like these have been set you must consult your local authority to determine the appropriate cable rating for the job.

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