if i was to invest in 2x12s instead of 4x12s, would that let me turn my amp up louder

[dassy]

if i was to invest in 2x12s instead of 4x12s, would that let me turn my amp up louder......there for making the tubes to kick in cuz of the volume loss of the 4x12 to the new 2x12s?


or does the science of this work a different way?





someone please shed some light on how this works : )

 

[TexRoadkill]

Your amp will work harder driving 4 speakers than 2 but it depends on the OHM rating of the cabs. 4 is probably better if you want to push it.

 

[dassy]

hi tex,

really, if i get 4x12s my amp will have to actually be turned up louder to get to the same volume as 2x12s?

just outta curiousity, could you explain why so i can understand the science of that so i can help to get a overall better understanding of this,

 

[JR#97]

loudness and wattage output are two different things. I can't remember all of the mathmatical specifics... ohms, parallel vs series, etc...

a 4x12 doesn't work as hard as a 2x12 because of the coupling effect of speakers. so theoretically, you could get more "loudness" out of a 4x12 than a 2x12.

 

[dassy]

i a little confused, please bare with me

HI JR#97,

I do understand that 4x12 would be louder than 2x12s,



but I am wondering if I would have to turn the volume knob up higher with the 4x12s or 2x12s, therefor running the tubes hotter,



also I wanna know why, so I understand the concept of this better. thanx!

 

[TexRoadkill]

Knightfly or Barefoot and some others would be better suited to give you the lowdown on the mathmatics and science. I'm just a button monkey.

Given 4 equal speakers: With only 2 speakers you push the speakers harder to get the same real world SPL as 4 speakers but the amp should have to work a bit harder with 4 speakers to get that same level of SPL. So with 4 speakers the amp works harder and the speakers work less.

I could be completely wrong and, if so, somebody please correct me.

 

[Peter Maydew]

Are you saying that you want to turn your amp up high, to get the sound you want, but don't actually want it to be too loud? If that's the case, and assuming everything else is unchanged, a 2 x 12 would be quieter - but not by very much. If you're after true overdriven amp sounds at lower volume levels, you need a power soak or something like that (not sure what's available nowadays).

The science:

Your amp has to be matched to the speakers, and it isn't a good idea to mismatch it although that will cut down the output. It may also change the sound and/or cause unreliability. Once your amp has a matched load - 4 ohm speakers for a 4 ohm amp, etc. - it will produce a certain maximum output power. From there, the thing that determines the loudness of the sound you and your audience hears is the efficiency of the speakers. The design of the drive units and the design of the cabinet will have a bearing on this; but using the same type of speaker drive units, a 4 speaker cabinet can move more air than a 2 speaker cabinet. That means higher efficiency and more loudness for the same volume setting.

The power soak is effectively a speaker that makes no sound: so the amp has to work just as hard and stays matched to its load, but the efficiency is reduced.

 

[rambo]

Hi dazzy
it's quite simple.... if you want to move a lot of air to build up soundpressure, the more conearea the better.
A single speaker of 1 square foot has to pump more than 10 speakers with 10 sq ft.
So for hi soud level more speakers are better.

rgds

 

[vintagedrive]

dassy, not really sure I understand what you are trying to accomplish. If your wanting to make your amp louder, go with the 412's or are you trying to get a loud overdrive sound with your tube amp? You can turn your amp as loud as you want, be with 112 speaker or 2- 412 cabinets, as long as the power and Ohms rating is correct.

 

[Matt303]

I'm not getting all this?

My understanding is that an amplifier is rated to operate at a particular load (usually 8 ohm or 4 ohm) the lower the impedance the higher the current (Ohms Law)and therefore the hotter the unit runs. i.e. An amplifier optimised for 4 ohm operation is given a de-rated value for 8 ohm operation. Running at lower than the recommended impedance will theoretically deliver more power but will also result in amplifier damage so generally not a good idea.
The trick would seem to be wiring the speaker combinations to deliver the correct impedance. Two Speakers would need to be 8 ohm rated to deliver 4 ohm in combination.
By wiring in parallel you half the impedance, by wiring in series you double it.
Two 4 ohm speakers will only allow 8 ohm in combination as they would have to be wired in series, parallel combination would yield 2 ohm (too low for most amps)

For combinations of 4 speakers, 8 ohm units can't combine to give 4 ohms overall, the best is back to 8 ohm i.e. 2 in series to 16 ohm added to the other two in series parrallels to 8 ohm again.
4 ohm combinations of 2*2 in series paralells to 4 ohm.

The rated impedance will therefore maximise the amplifier efficiency.
I'm dont think more speakers = more volume, I'm not sure of the physics but presumably the combination of driver and cabinet optimised to provide the highest pressure wave over the required frequency range is what you're after i.e. a sub bass folded horn enclosure is very loud when supplying bass but would not be effective in amplifying a guitar.
I guess the combination of speaker and cabinet build and design efficiency is what you pay the big money for, the only independent variable would seem to be selecting the optimum impedance for your amplifier.

 

[barefoot]

Knightfly or Barefoot and some others would be better suited to give you the lowdown on the mathmatics and science...

Thanks for the vote of confidence Tex. I'll do my best.

This is a slightly tricky question because it depends on the type of amplifier. Solid state amplifiers act approximately as voltage sources. In other words, they supply a certain voltage regardless of the load. So the amplifier output power is dependent on the load. A solid state amp spec might, for example, read 100W into 8 Ohms and 200W into 4 Ohms.

Tube amps, on the other hand, work most efficiently when the output tubes drive a relatively specific load via the output transform. In order to maintain the maximum output power, tube amps usually come with impedance selectors so the amp can deliver its full power into either a 4, 8 or 16 Ohm load. So a 100W tube amp delivers about 100W into any impedance
speaker as long as it is connected to the right transformer tap. Unlike a solid state amp, connecting a speaker with a lower impedance than the rated output tap actually tends to decrease the output power of the amp.

So what does this all have to do with how loud a 2x12 or 4x12 will sound?

First of all, let's specify that the 2x12 is wired as 4 Ohms and the 4x12 is wired as 8 Ohms and that both cabinets are using the same model drivers with the same sensitivities in the same type of cabinet (closed or open back). In other words the sound pressure levels (SPL)measured at a certain distance for a given input voltage swing are the same for all the individual drivers.

Doubling the surface area of a speaker increases its sensitivity by +6dB. So a 4x12 has a +6dB sensitivity gain over a 2x12 due to its increased speaker area. For a given input voltage however, the voltage drop across each driver in a 4x12 is only half that of the 2x12 because the 4x12 has twice the impedance. So, for a given input voltage, each driver in the 4x12 is putting out -6dB lower SPL than each driver in the 2x12. The +6dB gain from increased surface area is offset by the -6dB loss due to the higher impedance. So the voltage sensitivities of the 2x12 and the 4x12 are the same.

Now we know that a solid state amp supplies a given Voltage Swing regardless of the load. Therefore, it will treat the 4 Ohm 2x12 and the 8 Ohm 4x12 equally with respect to voltage. So the SPL of the two cabinets will be the same if driven by a solid state amp.

A tube amp, as we discussed, supplies a given Power regardless of the load. 20 Volts (RMS) is needed to drive 100 Watts into a 4 Ohm speaker. But 28 Volts (RMS) is needed to drive 100 Watts into an 8 Ohm speaker. That's a 3dB difference. So a tube amp supplies +3dB higher voltage to an 8 Ohm speaker than it does to a 4 Ohm speaker assuming they are connected to the correct transformer taps. Now remember that the speaker SPL output depends on the voltage and our 8 Ohm 4x12 and 4 Ohm 2x12 have equal voltage sensitivities. Therefore, the 4x12 will be +3dB louder than the 2x12 if driven by a tube amp because it supplies higher voltage to the 4x12.

In a nutshell:
All things being equal - same drivers, same type of cabinet - a 4x12 will be about +3dB louder than a 2x12 when driven by a tube amp.


Wooohh, for some reason that felt like a dissertation! I hope I explained it clearly

barefoot

 

[Matt303]

Wooohh, for some reason that felt like a dissertation! I hope I explained it clearly

You explained it very well, I didnt appreciate the effect of the valve output transformer.
In general terms you say that with all else being equal, more speakers = more output pressure when supplied with the same source electrical power ?

 

[barefoot]

...In general terms you say that with all else being equal, more speakers = more output pressure when supplied with the same source electrical power ?

Yes. But to be honest, I skirted past some details in my explanation for simplicity’s sake. So, let’s see if we can come up with a simple rule of thumb.

The real relationship is this:

[b]Change in SPL = 20*Log[sqrt(Change in Area)*sqrt(Change in Power)][/b]

and if you break out your calculator you’ll find that

20*Log[1] = 0dB
and
20*Log[sqrt(2)] = +3dB
and
20*Log[2] = +6dB

if we combine this equation with the power equation

P = I*V = V^2 / R

with a little algebra we can come up with some rules for the SPL change when moving from a single driver to two drivers wired in either parallel or series.

[b][u]SLP Change Going from 1 Speaker to 2[/u]

[color=blue]Tube Amp (Fixed Power)[/color]
Series:   2*Area, 1*Power   =>  +3dB
Parallel: 2*Area, 1*Power   =>  +3dB

[color=blue]Solid State (Fixed Voltage)[/color]
Series:   2*Area, 0.5*Power =>   0dB
Parallel: 2*Area, 2*Power   =>  +6dB[/b]

Going from a 2x12 to a 4x12 is analogues to going from 1x12 to a 2x12 wired in series.

barefoot

 

[barefoot]

Just a note:

Remember that these are rules of thumb and depend on the specific amplifier.

Some solid-state amps don’t have enough current capacity to act as true voltage sources and may not work as predicted with low impedance loads.

Some tube amps may have enough reserve collector current such that one could, for example, run a 4 Ohm 2x12 on the 8 Ohm tap and achieve approximately the same +6dB gain over a 1x12 as a solid state amp.

barefoot

 

[TexRoadkill]

By adding more speakers you can increase volume with no cost to the power amp? That seems like that would violate some laws of thermodynamics.

 

[barefoot]

By adding more speakers you can increase volume with no cost to the power amp? That seems like that would violate some laws of thermodynamics.

Yes. The reason is that you gain efficiency by adding more cone area.

Put out your hand and fan it in the air. You can do quite a bit of work without much result. Now grab a sheet of paper and fan that in the air. Even if you just put forth the same effort, you will move much more air.

In a way you can think of it as the extra speakers help match the amplifier output impedance with the air.

barefoot

 

[Matt303]

Thanks for the note barefoot. Your assumption is therefore that providing the total impedance is within the contstraint specified by the amplifier design the system can deliver sufficient current to be considered a near ideal voltage source, by this equation, with a set circuit impedance increasing surface area will result in a net logarithmic gain in SPL.
Have I got this right?
I'm interested in your comment about valve amplifiers, why is it that an AC30 at just 30W is so loud, does it have something to do with a valve ciruits capability to produce impulsive power?

 

[barefoot]

....I'm interested in your comment about valve amplifiers, why is it that an AC30 at just 30W is so loud, does it have something to do with a valve ciruits capability to produce impulsive power?

As we said, for a tube amp 30W is 30W regardless of whether we are connecting a 4, 8, or 16 Ohm speaker as long as we use the correct transformer tap. So the reason the AC30 is so loud probably has little to do with the amplifier. It’s most likely the speaker.

Speakers, like many other things, are designed using a set of parameters which are tradeoffs of one another. One tradeoff, for example, is bass extension and box size. Holding everything else the same, one can lower the potential cutoff frequency of the driver my giving it a looser suspension. The looser suspension, however, requires a larger volume of air behind it. So, all else being equal, deeper bass means a bigger box.

There are also tradeoffs between sensitivity, bass extension, and power handling. One can design a speaker driver that achieves a high sensitivity (dB’s per Watt) at the expense of bass extension and power handling. This is likely the tradeoff made in the AC30 speakers. Each speaker only sees 15W and the AC30 is a rather bright amp as I recall. So, I imagine the speakers in the AC30 are very sensitive with low power handling capacity.

barefoot