Loudspeaker impedance is not very well understood by many people. In this article, I’m going to go into some more detail regarding this fairly important specification of a loudspeaker. I’m also going to explain how this relates to picking a proper amplifier for driving the speakers.
Loudspeaker impedance is expressed as a number and the unit Ohms. Commercially available loudspeakers are typically rated at 4 or 8 ohms. However, they also loudspeakers available which are rated as low as 2 ohms and as high as 32 ohms. Now what exactly does that number represent? If you have ever used resistors while working or perhaps you are familiar with electronics then this unit represents a ratio. The ratio of how much voltage is required to exert a certain amount of current within the resistor is expressed in ohms. The same goes for loudspeakers.
You should understand that loudspeakers are made up of a diaphragm which is moved by a coil which is suspended in a magnetic field. When there’s current being exerted inside the coil, there will be a physical force driving the diaphragm. The larger the current inside the coil the larger the force. Simply put, the impedance describes how much voltage has to be applied across the terminals of the loudspeaker in order to exert a certain amount of current in the loudspeaker coil.
If you have a loudspeaker that has a fairly high impedance such as a 16 ohms speaker, you will notice that you might not be able to use any amplifier on the market to drive the speaker to high volume. That is because only an amplifier has a large voltage swing will be able to induce enough current inside the speaker. So if you’re not sure about what power amplifier you have and what voltage swing is available then you should contact the manufacturer of the amplifier. There are many amplifiers on the market which are powered by a wallwart power supply. In that case you can easily determine the voltage swing that the amplifiers able to provide.
An amplifier that uses a 12 V supply voltage, for example, will be able to output a voltage swing of 12 V at best. However, car amplifiers are an exception because they transform the supply voltage internally to a higher voltage. So given the voltage swing you should choose a speaker that has an adequate impedance. That means don’t choose a speaker that has a very high speaker impedance. Otherwise you will not be able to drive the speaker to full wattage. Then again, don’t pick a speaker with a very low speaker impedance. That is because amplifiers usually don’t tolerate speakers with a very low impedance. That could actually burn out the power stage of the amplifier. Also, the lower speaker impedance reduces the amplifier efficiency.