When specifying and installing a reliable, quality sounding AV system we need to take into consideration the cabling and interconnects used by AV hardware, the characteristics of the cables and transport methods, and where each should be used. In this issue we will discuss Audio speaker cabling, and discuss audio signal cabling in the next issue.
Speaker cable is basic copper wire, usually supplied as an insulated pair with a colour or trace to make each conductor distinct. Some are available as double insulated, with a jacket surrounding both pairs, as well as 1 jacket for each conductor, which makes the cable easier to draw, lowers the likelihood of cable damage, and offers a level of fire protection. Speaker cable is also available with multiple pairs inside a common jacket, usually 2 or 4 pairs, to allow for a single cable run for multiple speaker circuits.
Speaker cable thickness, specified as mm2 of the cross section of the conductor, should be selected based on the power of the amp and speaker combination, and the cable run distance from the amp to each speaker location. The higher the power, the thicker the cable, similarly the longer the cable run the thicker the cable should be to combat the power losses in the cable. For Low ohm speaker systems a 4.6mm cable will usually be suitable for higher power, or for medium powered systems with longer cable distances. For constant voltage systems 1.8mm cable is the standard unless dealing with very high power (1000+plus) or extremely long cable runs, greater than 840m. This is a table to serve as a rough guide for selecting speaker cable, giving the maximum cable run length of any given gauge based on a maximum power loss of 0.5dB.
Speaker cable runs should be kept as short as possible, as additional length presents a loss of power and system efficiency. A Drop in efficiency requires an amplifier to run harder to achieve the same sound output as a shorter cable run, meaning higher power consumption, greater heat generation and shorter equipment life.
When running any audio cable, best practice is to avoid crossing over high current AC cabling, power or otherwise, to reduce the likelihood of inducing ‘noise’ into the audio cable. Whilst this is far less likely in a speaker cable, as opposed to a signal cable, it is still a good practice to adhere to. Likewise, avoiding kinks and hard turns will protect the cable against breakages in the jackets or internal conductors. When internal conductors break or kink, it reduces the efficiency of the cable and thus the system, in a similar way to running additional unneeded cable length.
Speaker cable can be terminated in many ways depending on the speaker or equipment type. Any instance where bare copper is inserted into a socket or dry join connector, the copper conductors should be ‘tinned’ or covered with solder first to safeguard against breakage of the copper conductors.
Consumer products often feature push down connectors, commercial products use screw down terminals or screw on “phoenix” connectors, and professional products require the speaker cable to be terminated to ruggedized connector. Where a speaker cable junction is needed, as in most constant voltage speaker systems, the use of ‘cable protect’ style terminal blocks will negate the need of tinning the speaker cable, which is difficult and time consuming when working in roof cavities.
One core of a two core pair of speaker cable should have a ‘trace’, line marking, or both cores should be 2 distinctly different colours. These features allow you to identify which core is connected to which terminal on each end of the cable, with the terminal options being commonly referred to as Positive (+) and Negative (-). Whilst connecting these terminals differently at either end of a speaker cable is unlikely to damage any equipment, it will adversely affect the sound quality of the system as one or more speaker drivers will be ‘out of phase’ with the others which will reduce sound quality, particularly at lower frequencies. The predominant thought in relation to a cabling standard is that the Trace should be connected to the Positive (+) terminal of all equipment where a trace is present. Where the identifier is cable colour, the brighter colour goes to the Positive (+) terminal, for example positive would be red on a red and black pair. Often these are interpreted differently, so it is advised to ensure all employees understand the standard as it is set, adhere to it, and document any variances, such as additions made to existing systems.
In our next article we will discuss Audio source equipment wiring, balanced vs unbalanced signals, mic level vs line level, and which cable to use in each scenario.
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