Speaker Impedance Varies!

Huh... The impedance's on the plate ready to be served.

I guess most of you while you're reading this don't know that your 8ohms speakers are also 4ohms,6ohms,10ohms,and maybe 50ohms aswell.

No? That's what i guessed... Continue reading...
Most of the manufacturers are rating their speakers with the most common values that are 4ohms,6ohms,8ohms and 16ohms and those values are either useless DC or Nominal-Average AC Measurements.

The Audio Signal is made by thousands of frequencies in sine waves that are passing through AC (Alternate Current),and the impedance of the coils of the speakers varies to the frequency.

To make you understand,here i have a Tangband woofer driver.
It's rated at 70w (long term) and it has 8ohms Nominal Impedance on AC,6ohms on DC (the one you can measure with a multimeter) which is useless because you can't use a speaker on DC unless you want to blow it,7.9ohms on 100hz,and about 75ohms on 38hz (Resonance Frequency).

So as you can understand Nominal or Average Impedance is only a rate to help you design a Crossover Network or take a taste in order to find an Amplifier that can drive the corresponding load.

High loads can lead the Amplifier to Overheating or serious damage.
This is happening because if we have an Amplifier that can provide 100w on 8ohms only,if you put 4ohms load it will ask the Amplifier to give the double power which can't be done.

Here's a Calculation:
28.4V x 28.4V / 8ohms = 100w
28.4V x 28.4V / 4ohms = 200w

The rated load of 4ohms might be minimum low like 2ohms or high as 20ohms,so if you want to get an Amplifier for high load,be sure to be aware that your load might be worse than you expected.

Most of the big modern Stereo Amplifiers that can provide a decent amount of power will even work at 2ohms without any problem,and most of the 5-7Channel Amplifier will work too (at least in 2Ch Stereo Mode),so if you are looking an Amplifier to drive heavy load,look for PA Power Amplifier or something BIG for Home Use.

Known myths of the Impedance: The speaker will be better than another if its Impedance is lower or higher.

Actually... Not...
Power Rating,Impedance or Loudness aren't making a speaker good.
We (at least i hope) all know that we judge a speaker from its overall Quality and nothing like the Impedance.

However High Impedance in some cases can admit that the Coil of the Speakers is cheap,but there's cases like in Car Audio in which some speakers are made to have High Impedance (as low as 2ohms) and huge Wattage Rating so they can suck up more power from the Amplifiers making the owners thinking there speakers are loud,but that's another story which is explained in the post called "Explaining The Watts"