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Topic: Understand Ohm's Law for Car Audio

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Wiring Car Subwoofers & Car Speakers Ohm's Law

It seems like one of the most common questions we receive is: What are Ohmís? And how do it relate to a car audio system? The most basic definition of Ohmís law states that the current through a conductor between two points is directly proportional to the potential difference (i.e. voltage drop or voltage) across the two points, and inversely proportional to the resistance between them. WellÖÖWhat exactly does all that mean?

I find that the easiest way to understand Ohmís law is by a visual image. So letís say the diagram below represents a circuit. First you would look at potential voltage which is represented as a body of water in the diagram. The voltage will always be the same in a car audio application. Twelve volts would be your potential for any circuit in a standard car audio system. Next the two different pipes represent resistance, notice they are different sizes. The resistance can greatly vary and that will determine how much current (amperage) can flow though any given circuit. Resistance is measured in Ohmís. A very high resistance allows a small amount of current to flow. A very low resistance allows a large amount of current to flow. The larger pipe represents a lower Ohm load than the smaller pipe. Next you can see the water flowing though the pipes. That represents current (amperage). The larger pipe is able to carry more amperage than the smaller pipe since it is rated at a lower resistance. As pictured you can see much more water coming out of the larger pipe. That is representing power which is measures in watts.


Ohm Diagram

These are only the basics but for most car audio applications it is enough to understand how and why you should wire an amplifier or sub in a certain configuration †In a car audio application you should always run your equipment at the lowest possible Ohm load that an amplifier, head unit or speaker can safely handle. All amplifiers will feature a rating which will usually look something like this.

  • 500 Watts x 1 @ 4 Ohm
  • 1000 Watts x 1 @ 2 Ohm

Now this is really important when purchasing equipment. To get the maximum output out of the amplifier rating listed above you want to make sure it is wired at a 2 Ohm load, running the same amplifier at a 4 Ohm load will result in a 50% loss in potential power. The same applies the other way around. You would not want to run this amplifier lower than a 2 Ohm load otherwise you can potentially burn it up since it would be running at twice the recommended power rating and capabilities. Most amplifiers have a thermal protection circuit however, it is still possible to damage the amplifier. Now if you look at my diagram above you can really see how it works. The larger pipe above would represent a 2 Ohm load whereas the smaller pipe would be 4 Ohm. Every time an Ohm load is divided by 2 the current will double in amperage. Every time an Ohm load is multiplied by 2 the current will be cut by half.

Next we will look at subwoofers and how to wire them properly to the amplifier. Using the same amplifier ratings from above we now know that we want to wire the amp at a 2 Ohm load. So how do we do that? This is where things tend to get a little trickier however it is not as hard as it may seem. There are two terms that you will need to know when wiring subs or speakers, Parallel and Series. Lets look at each example before we figure out exactly how to wire our amplifier example from above.† Just remember wiring in series it is used to INCREASE the Ohm load and parallel is used to DECREASE the Ohm load. So an example of a series circuit can be viewed below.

Wired in a Series

When wiring in series you are increasing the Ohm load by hopping from one voice coil to the next. So as you can see pictured the positive hops to negative on the opposite voice coil. To add this up you take the 4 Ohm load of each coil and add them together. So if you are wiring two Single 4 Ohm subs together you would add the two voice coils together sp 4 Ohm + 4 Ohm equals an end result of 8 Ohm. Moving on an example of a parallel circuit can be seen below:

Wired in a Parallel

When wiring in parallel you are decreasing the Ohm load and wiring the voice coils together.† So you would run a lead from the positive voice coil on each sub to the opposite voice coil. In essence all the positives are connected on a single wire (conductor) and the same for the negatives. So if wiring two Single 4 Ohm subs you would take 4 Ohm and divide it by the number of voice coils. So 4 Ohm divided by two voice coils will equal a 2 Ohm load.

Okay now as you saw above the single 4 Ohm subs wired in a parallel resulted in a 2 Ohm load, just what we needed for the amplifier example from above. But the dual 2 Ohm subwoofers came to a 1 ohm load that's too low, what do we do? You will normally have to use a combination of series and parallel to achieve the correct Ohm load. Letís say you are running the amplifier I mentioned above. That amplifier will need to run at a 2 Ohm load to achieve maximum power. So if you decide you wanted to run two subwoofers on that amplifier you would need to run either two single 4 Ohm subwoofers or two dual 2 Ohm subwoofers. The two single 4 Ohm subs are easy to wire and you can see that in the diagram above. However, the dual 2 Ohm subs will use a combination: first we wire each subwoofer in a series and then wire them together in parallel at the amplifier. How do we do that? Lets go over this in a diagram. Think of it as a two step process this will make it easier to figure out. So first you would take the subs and wire them in series like below.

Wired in a Series

Ok so that is first step. So now you have two Dual 2 Ohm subs wired in series. This increases the Ohm load to 4 Ohm at each sub. Now we need to wire them in parallel at the amplifier. Check out the diagram below.

Wired in a Parallel

As you can see we now used a combination of first wiring each sub in series, and then we wired the subwoofers in parallel at the amplifier. This would be a typical installation and can be found in many installs. These are just a few examples but once you have the basics down you can figure out much more complex systems using the same methods.

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