8 ohm vs 4 ohm
 

What is the difference between an 8 ohm sub and a 4 ohm sub?
I just purchased a new RF Punch 3812 by mistake (8 indicating 8ohms) instead of a 4 ohm 3412. I installed it and it doesn't sound as loud as i think it should. I was trying to run it with a JBL amp at 4 ohm. I am going to try and return it for the proper one tomorrow without using it anymore but was curious what the difference is. Thanks.

the difference is the ressistance. that may be why it isnt that loud because they arent getting enough power.

Heres the deal: 8 ohm presents twice as much resistance than 4 ohm, so it will take a little more "gain" from a 4 ohm amp to make an 8 ohm speaker sound good. If you had an 8 ohm amp, it would sound okay.

To clarify a bit further, a 4 ohm amp powering a 4 ohm speaker will use about the same amount of electrical power from your car (alternator/battery) as a 8 ohm amp with 8 ohm speaker at same wattage.

The reason you don't want to use 8 ohm speakers with a 4 ohm amp is that for the volume you want, your chips need to run more power through them, which decreases their life, so it's not recommended.

hear a amp bridged with to 8ohm will=a 4ohm load total on the amp and 4ohm will equal a 2ohm load mine you that with 2 subs at hand

u can do this as long as ur amp is 2 ohm stable

most amps get the most power rated at 2ohms (thats if its stable to do so)

You never stated which JBL amp you have. If you have one of the class D's bp150. bp300.1 or bp1200.1 you will need a dual 4 ohm sub
If you have any of the othe BP series amps you will need the 4 ohm sub

Whoa. Where'd TaylorD go? An 8 ohm sub will pull half the current a 4 ohm sub will.
To put it simply, ignore all the marketiing BS, when you bridge a 4 ohm sub, you will THEORETICALLY get twice the power out of your amp as you would with an 8 ohm sub.
When bridging an amp it sees half of the ohms (impedance) of your driver. Basic electical laws. Feel free to abuse me due to the fact I've had a few pops..

Why not buy another 8 ohm sub and wire them up in parallel to your amp? Then you'll have a 4 ohm load and you'll be good to go.

Also becareful with impedance...... the LOWER the impedance (i.e. the lower the number) the more 'work' the amplifier has to do...not the other way around. Running a 8 ohm sub with a amplifier that is rated for 4 ohms won't do any damage.

It's just a matter of impedance matching. Your amp sounds like it's designed with a 4 ohm output impedance, so the maximum amount of power will be delivered to the sub if and only if the sub's impedance is also 4 ohms. With any load but 4 ohms, the power delivered will be less.

(I'll spare everyone the math -- the last time I busted out some theory I killed the thread ;) )

-b

Good call. If the amp isn't designed for lower impedance, it will deliver more power, just for a short time until the protection circuits catch it, or worse yet, smoky time.

Hahahahaha edumacation never hurt anyone ;)

Actually, it'll deliver *less* power -- but more current. Kinda counter-intuitive. But yeah, if the impedance is too low, fuses start blowing.

<geek>

For anyone who's interested, I plotted the voltage, current, and power response of an amplifier with a 4 ohm output impedance vs. varying speaker impedance (boredom and procrastination can do strange things to a person). You can see it here:

http://www.missouri.edu/~bcc5zb/maxpower.gif

You'll notice that the maximum current occurs when the speaker impedance is zero (when the terminals are shorted -- duh). This'll blow fuses. The maximum voltage occurs when the speaker impedance is infinite (on this graph, 20 ohms = infinity :) ) The maximum *power*, which is what we're after, occurs somewhere in the middle -- where the load impedance equals the amp's output impedance -- in this case, at a speaker impedance of 4 ohms.

</geek>

-b

Well... The amp will spill it's guts out to maintain voltage, the current would usually double with half the resistance (ohms law?) Where'd this start? Just get the 4 ohm sub.

Hear hear! Enough of my foolishness!

-b

Cool, thanks! 8)