8 ohms vs 4 ohms

[Billy K]

I have a Hartke Hydrive 115c which is 4 ohm speaker. I bought a Hartke XL 4X10 cab and it is 8 ohm.The combo amp does not have a external speaker jack so my choice is run one or the other.I was told the 15 would be louder than the 4x10's. I don't understand that.Can someone explain that?When the cab arrives I guess I will see which sounds better.

[lbwdog]

Originally Posted by Billy K

I was told the 15 would be louder than the 4x10's. I don't understand that.

The thinking behind that statement comes from the fact that the 4 Ohm 15 has less resistance than the 8 Ohm 4x10, but it's not necessarily true. The sensitivity of the drivers and cab design are also major contributing factors.

[JMac4strngr]

Think 40 inches,versus 15. I would think that the four tens would move more air making them seem to have more volume, however I could be wrong.

[billfitzmaurice]

Originally Posted by Billy K

.I was told the 15 would be louder than the 4x10's. I don't understand that..

I don't understand it either.
The person who told you that probably assumes that the higher power into the lower impedance load will make the 15 louder. That assumption is incorrect. Output level is determined by impedance load and speaker sensitivity. In most cases an 8 ohm 410 will have higher sensitivity than a 4 ohm 115, negating the effect of the lower impedance of the 115. Chances are their relative output levels will be about the same.

OTOH a 4 ohm 115 and 8 ohm 410 is a serious mismatch. What limits speaker output is the total driver displacement. A 410 typically displaces around 400cc, a 115 around 300cc. If you have the 115 at 4 ohms it's taking 2/3 the system power while only providing 2/5 the system displacement. In layman's terms the 115 will fart out while the 410 is coasting. Even with both cabs the same impedance a 115 won't keep up with a 410.
A good match is a 115 and 210. You usually don't see it done that way though, because it doesn't look as pretty as two cabs that are the same size.

[KJung]

Originally Posted by billfitzmaurice

I don't understand it either.
The person who told you that probably assumes that the higher power into the lower impedance load will make the 15 louder. That assumption is incorrect. Output level is determined by impedance load and speaker sensitivity. In most cases an 8 ohm 410 will have higher sensitivity than a 4 ohm 115, negating the effect of the lower impedance of the 115. Chances are their relative output levels will be about the same.

OTOH a 4 ohm 115 and 8 ohm 410 is a serious mismatch. What limits speaker output is the total driver displacement. A 410 typically displaces around 400cc, a 115 around 300cc. If you have the 115 at 4 ohms it's taking 2/3 the system power while only providing 2/5 the system displacement. In layman's terms the 115 will fart out while the 410 is coasting. Even with both cabs the same impedance a 115 won't keep up with a 410.
A good match is a 115 and 210. You usually don't see it done that way though, because it doesn't look as pretty as two cabs that are the same size.

+1

[Billy K]

thanks Bill make sense now. The Hartke tech gave me the info. Sounds like I use one or the other in fact I can not use them together because there is no speaker jack which is a good thing.Might have to get a head. Wife not gonna like that.

[Jason P Bass]

Originally Posted by JMac4strngr

Think 40 inches,versus 15. I would think that the four tens would move more air making them seem to have more volume, however I could be wrong.

Wow.

More like 4x10 = PI * 5^2 * 4 ~= 314 in^2
While the 1x15 = PI * 7.5^2 ~= 177 in^2

There is much more speaker surface area in a 4x10, but we can get the math right while we're at it!

[JMac4strngr]

Originally Posted by Jason P Bass

Wow.

More like 4x10 = PI * 5^2 * 4 ~= 314 in^2
While the 1x15 = PI * 7.5^2 ~= 177 in^2

There is much more speaker surface area in a 4x10, but we can get the math right while we're at it!

Thanks for the correct formula for the displacement of speakers. I never really thought of speakers that way, but it makes since. I usually use formulas like that for figuring pump metering displacement....... So that useless day job knowledge could come in handy for somthing......

[silky smoove]

Originally Posted by JMac4strngr

Thanks for the correct formula for the displacement of speakers. I never really thought of speakers that way, but it makes since. I usually use formulas like that for figuring pump metering displacement....... So that useless day job knowledge could come in handy for somthing......

FYI: Thats not a formula for displacement. It's a formula for surface area relative to a flat plane the size of the speakers' nominal cone diameter.

[NoGraveConcern]

Originally Posted by silky smoove

FYI: Thats not a formula for displacement. It's a formula for surface area relative to a flat plane the size of the speakers' nominal cone diameter.

Right, and to get the true surface area of the cone you need the driver's sd Thiele & Small Parameter which looks something like:

15 = 856.0 cm2
10 = 350.1 cm2 times 4 = 1400.4 cm2

[playerunk]

Originally Posted by NoGraveConcern

Right, and to get the true surface area of the cone you need the driver's sd Thiele & Small Parameter which looks something like:

15 = 856.0 cm2
10 = 350.1 cm2 times 4 = 1400.4 cm2

and then comes xmax

[NoGraveConcern]

Ok so 856 x .59 x 2 = 1010.1 cm3 (3015)
1400.4 * .42 x 2 = 1176.3 cm3 (2510 II)

[silky smoove]

Originally Posted by NoGraveConcern

1400.4 * .42 x 2 = 1176.3 cm3 (2510 II)

...ahem....

(350.1 x 4) * .42 x 2 = 1176.3 cm3 (4 2510 II's)

I was feeling snarky

[Pbassred]

Bill is right. Speaker area is immaterial. The lower impedance of the 4ohm 15inch driver aught to consume about 2db more power. Whether it converts that into sound, depends on the efficiency of the drivers. It's not unheard of for one driver to be 10db more efficient than a different brand. Then you have the effect of 2 identical speakers side by side ( I forget the name) which can make 3db difference.

But ALL of this is spurious rubbish if you are making BAD sound. If you're not getting the tone that is right for YOU, making it louder is irrelevant. In fact pushing your stage volume up lowers the bands ability to hear each other and creates feedback.

Get your sound right. If the audience needs more VOLUME, then you must be playing a gig that needs PA support.- which is fairly cheap.

[rptrsn2]

Originally Posted by billfitzmaurice

I don't understand it either.
Even with both cabs the same impedance a 115 won't keep up with a 410.
A good match is a 115 and 210. You usually don't see it done that way though, because it doesn't look as pretty as two cabs that are the same size.

thats what i run foe exactly that reason, i found a 410 drowns a 115 even if cabs are like brand and meant to be run together by the manufactuer. send bacy the 410 and get a 210,if you can add a cab. if you can,t and are not happy with the 115, dump the combo, buy a head and run whatever speaker config you want.