RangOH 
07062010 03:47 PM 
The Truth About Ohms...
I figured some folks could use a little good info in between all of their misinfo....
Quote:
The "Real Deal" with 8 ohm drivers, by npdang  founder of diyma.com
I've seen many people say, "Well I want to use this speaker, but I'm put off by the fact that it's 8 ohms."
Let me explain why it's ok to use an 8 ohm speaker, and why it could actually be better than a 4 or 2 ohm speaker.
First of all, using a higher impedance than what your amp is nominally rated for is always ok. In fact, if your amp is rated for 4 and 2 ohm impedances, typically you will get less power into 8 ohms meaning that your amp will run cooler and more efficiently at higher impedances. It won't hurt your amp, and in fact it's actually much better for your amp's longevity.
Now, you're probably saying how is getting less power out of my amp a good thing? Think of your amp's power reserves as your bank account. Just because you have 100 dollars in your bank account doesn't mean you have to spend it all. Quite the opposite. Wouldn't you rather spend LESS and get MORE? That's exactly what you're doing when you use less power from your amp, and get the same amount of output from a high efficiency speaker.
Remember, most speakers are rated at 2.83V. A 4 ohm speaker rated for 90db spl at 2.83v is really being rated at 2 watts! Whereas an 8 ohm speaker rated for 90db spl at 2.83v is only being rated at 1 watt. You can do the math for yourself, Power = Voltage^2 / resistance. At 2 watts, we can assume that same 8 ohm speaker is actually rated at 93db spl (remember, every doubling of power gives you a theoretical 3db gain in spl). So using that 8 ohm speaker will give you the same amount of output, at half the power required as a 4 ohm speaker. Your amps run cooler and draw less power from your vehicle's charging system, your speakers run cooler, and everyone is happy!
Ok, now let's look at another example of an 8 ohm versus 4 ohm voice coil. Typically, the efficiency of a speaker is given by:
Efficiency = ( B^2 * L^2 ) / ( R * Sd^2 * Mms^2 )
B = magnetic field strength
L = length of wire
R = resistance
Sd = surface area
Mms = mass
So for your 8 ohm voice coil, using the same wire as a 4 ohm voice coil, you would need twice the L or length to get an 8 ohm impedance. That makes sense doesn't it? A longer wire will have more resistance. Now, looking at the formula above, doubling L actually causes your efficiency to rise, even though the impedance also rises. So in this very oversimplified example, raising the impedance actually causes efficiency to go up and lowering the impedance actually causes a loss of efficiency.
What's important to remember is that it's the overall output and efficiency of the speaker that's important, not the impedance. A high impedance, high efficiency driver can get just as loud off a small amount of power as a low impedance, low efficiency driver that sucks a ton of power! Just because you have a 100 watt amp doesn't mean you have to use all 100 watts.... it's all about being efficient.
I also found this excellent post by Dan Wiggins over at carstereos.org:
"I think one thing to consider is that going to a higher impedance voice coil will result in better packing of the voice coil, meaning a higher crosssectionalarea of copper in the flux, for a given mass.
If you take a given driver, and simply swap out voice coils, you end up with more efficiency as you increase the impedance. Take a voice coil, say 2" diameter, 1" winding length, 24AWG 4 layer, and swap it with a 2" diameter, 1" winding length, 27AWG 4 layer, and you double the impedance, but the efficiency also goes up  less mass and better packing density."
It's because the moving mass has dropped, and if desired  because of the thinner wire diameter which packs in tighter  you can put more layers in the voice coil and potentially raise the BL."

The following was Taken from the manual of a Zed Audio Amplifier...
Quote:
I cannot resist getting into this subject. Scenario: Kid
buys gizzilion watt amplifier with manufacturer’s specifications of x watts into 4 ohm, y
watts into 2 ohm and z watts into 1 ohm. Well let’s take a wild guess and I bet you all that
the vast majority will drive the amplifier into 1 ohm. Why? Easy, he feels that he is getting
his “moneys worth” by doing this. OK let’s see what reality is. Said amplifier is maybe
rated at say 600w into 4 ohm, 1000w into 2 ohm and 1500w into 1 ohm. We will assume
that the speaker impedance is resistive. The difference between the 4 and 2 ohm power is
2.2dB, the difference between the 2 and 1 ohm power is 1.76dB and the difference
between 4 and 1 ohm power is 3.9dB. So what do these numbers tell us. First if the
speaker is 4 ohms vs 2 ohms, no way you can hear any difference. Same issue
comparing 2 as to 1 ohm. I grant you that between 4 and 1 ohm there is almost 4dB
difference BUT at what price. Two issues. Low impedance loads affect the amplifier’s
sound quality adversely and this is quite easy to prove. Load an amplifier with a 4 ohm
woofer and listen. Add parallel resistors to the speaker to make the amplifier “think” that it
is driving a low impedance speaker and listen to the difference in sound quality. You will
be surprised. This test is not 100% valid in fact as the resistive loads added, let the
amplifier off the hook as the reactive components of the load are confined to the 4 ohm
part. The idea of this test is to keep the loudness about constant but load down the
amplifier.
The average person can just perceive a 3dB (doubling of power) difference and to
actually double the sound pressure on your eardrum, you require TEN TIMES the
amplifier power. Yes TEN TIMES no kidding!
The other consequence of driving amplifiers into these ridiculously low impedances is that
the amplifier is stressed substantially more as compared to when driven into more sane
impedances. Efficiency drops as the losses in the output stages increase dramatically
even with “super efficient” class D amplifiers.
It serves ONLY to boost the ego of the owner of the car so he can quote numbers to his
buddies.

moral of the story, quit torturing your amps.
