Calculating Impedance for Subs/Speakers and Amp Matching

This article assumes that you are using Parallel Wiring technique, which is the right way to wire your system.  It does not apply to subs/speakers that are wired using the Series Wiring technique.

There are 2 very important pieces of information you need to match subs/speakers with an amplifier:

  1. RMS wattage rating
  2. Impedance

Matching subs/speakers based on per channel RMS wattage is pretty simple.  Ensuring you have a perfect match based on impedance is a bit more involved.  Here is some info that will help you.

The math behind these calculations is pretty simple if each sub has the same Impedance (i.e. 2 subs each rated at 4 ohm impedance):

Ending Impedance of subs = Impedance of a single sub  / # of coils in all subs

  • Ex. You have 2 single voice coil subs and each has an Impedance of 4 ohms:
    • 4 ohms / 2 coils  = 2 ohms, for these subs you can go with a 2 ohm or 1 ohm stable amp.
  • Ex. You have 2 dual voice coil subs and each has an Impedance of 4 ohms:
    • 4 ohms / 4 coils = 1 ohm, for these subs you can only go with a 1 ohm stable amp, if you add a 2 ohm stable amp via parallel wiring, you burn up the amp!

The Ending Impedance is then used to find either a 1 ohm or 2 ohm stable amp.  Simply put, if the Ending Impedance of the subs is 2 ohms, you need a 2 ohm stable amp.  You could use a 1 ohm stable amp but you will not get all the potential power that amp is capable of producing.  The reason is simple, at 2 ohms the amp is experience greater resistance and hence pushes out less power.  You may be familiar with Amp rating info such as: 500 WATTS X 1 @ 4 OHMS, 1000 WATTS X 1 @ 2 OHMS, 2000 WATTS X 1 @ 1 OHM.  As you can see as the ohms decrease the amount of power pushed by an Amp increases.  So getting back our example where we need a 2 ohm stable amp, we could buy a 1 ohm stable amp but we could get a fraction of the actual power it is cable of.  Essentially, we would be buying a bigger amp than we really need.

In the event the Ending Impedance of the subs is 1 ohms, you can only use a 1 ohm stable amp.  If you try and use a 2 ohm stable amp it will be susceptible to overheating and going into Thermal Protection mode or it could burn out.

The last step, once you do the work to learn the stable impedance rating needed for your subs, you simply match up the total RMS wattage of the subs with the total RMS output of the amp that has the requisite stable impedance rating.

Matching Mid-Range or High-Range Speakers and Amplifiers

If you’re wiring mid-range or high-range speakers (i.e. “mids” and “highs”), you can use the same simple formula but substitute coils for speakers and take into consideration the number of channels on the amp.  Nearly all mid-range and highs amps are 2 ohm stable per channel so to get the maximum amount of safe stable power, without over heating the amp, your goal is to achieve an Ending Impedance of 2 ohms per channel.

Ending Impedance of Speakers = Impedance of a single speaker  / # of speakers

  • Ex. You have 2 mid-range speakers, each has an Impedance of 4 ohms:
    • 4 ohms / 2 speakers = 2 ohms, so on a 2 ohm stable 2 channel amp, you can add 2  4 ohm speakers per channel for a total of 4 speakers.  On a 2 ohm stable 4 channel amp, you can add 2  4 ohm speakers per channel for a total of 8 speakers.
  • Ex. You have 4 mid-range speakers, each has an Impedance of 8 ohms:
    • 8 ohms / 4 speakers = 2 ohms, so on a 2 ohm stable 2 channel amp, you can add 4 8 ohm speakers per channel for a total of 8 speakers.  On a 2 ohm stable 4 channel amp, you can add 4  4 ohm speakers per channel for a total of 16 speakers.

Again, nearly all mid-range and highs amps are 2 ohm stable per channel so when connecting speakers you want to ensure your ohm load does not drop below 2 ohms per channel.

  • Ex. You have 4 mid-range speakers, each has an Impedance of 4 ohms:
    • 4 ohms / 4 speakers = 1 ohms

So if you connect 4 speakers each with an impedance of 4 ohms with a 2 ohm stable amp, you will either overheat and burn out your amp or it will constantly go into Thermal Protect mode.

It is advised that if you don’t know what you’re doing or have questions, ask an expert.  If you ever have any question about what speakers go with which amp, feel free to call us at Five Star Car Audio, 216-475-5868 or come see us at 17170 Broadway Ave Maple Heights OH 44137.

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