Electrical Impedance in Car Audio Circuits

Here we put impedance in the spotlight. More accurately, we should refer to it as Electrical Impedance as other kinds do exist. In fact, the word itself is derived from the verb “impede” which means to restrict or obstruct.

Electrical impedance is often mistaken for electrical resistance but differs insofar as it relates to alternating current or a voltage waveform that changes from one state to another as in audio signals. As resistance is directly proportional to voltage, this is not a reliable measure in an audio circuit where the voltage is changing all of the time.

Electrical impedance is crucial when matching speakers to amplifiers. Unlike other specifications we have spoken about in the last few months, it does not necessarily say anything at all about quality of reproduction it simply determines whether a combination of equipment will work together. In a speaker system, impedance is important to system designers. Many a young DIY installer has fallen into the trap of connecting two or more subwoofers together to obtain more noise. This is of course possible but the manner in which they are connected will ultimately affect the impedance of the load presented to the amplifier.

A quick analogy might be helpful here. If you imagine trying to push a small car a short distanced along a flat road, wearing your normal day shoes. It will take effort to get the car moving. You will need traction so that your effort goes into moving the car. Now imagine doing the same thing wearing roller skates. In this case, the high impedance offered by the vehicle will overcome the relatively low impedance offered by the roller skate wearer. The opposite would also be true so, if the car is very, very light, and four people are employed to push it, they will probably all fall on their noses while the car flies forward at a rate of knots.

Audio systems are similar in their response to impedance matching. If the load does not offer enough impedance, the amplifier will fall flat on its nose. This happens when speakers are connected together in parallel – see figure below:

In this case, the two 8 Ohm speakers connected in parallel offer a load of 4 Ohms to the amplifier. If the amplifier is capable of coping with a 4 Ohm load then all will be well and more output will be realised from the amplifier. If not, it will most likely get too hot and switch off. If not properly protected against such eventualities it could even catch fire!

The opposite scenario is where speakers are connected in series:

In this instance, the impedance of one speaker is added to the other. The result will be a larger impedance to the amplifier and reduced output – Like trying to push our car above, with the handbrake on!

Both of these methods are employed on a regular basis but there are pitfalls. Although it would be acceptable to wire multiple speakers in either configuration, this can only be achieved in a safe way with speakers of the same impedance. Close matching is also recommended as even minor variations can lead to the load not being shared equally between speakers. This may detract from performance.

Wiring speakers in parallel to achieve more power is not necessarily recommended and we would always recommend that each speaker receives its own amplifier output if possible. Having said this, if you stick with a high quality manufacturer then you should be in safe hands a suggested by this chart provided by Hertz:

These days, the sheer breadth of available equipment should allow systems to be designed that optimise an amplifier’s output with the load presented by the speaker. This is not only a safer approach, but will often result in higher fidelity.

As always, we recommend you consult with your local FOUR MASTER and preferably have them help design and install your system for you.

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