Quick quiestion on External Cabnets

[Liko]

Re: Quick quiestion on External Cabnets

If I was to run my Jet city JCA2212 into either a 2x12 or another 1x12 as a extention to the 1x12 in the combo, how much louder would it be?

I hear stories of guys running 5 watters into 4x12 cabs and getting giggible levels, where a 50watt amp would normally be used.

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Lot of variables involved. Technically, by adding a 1x12, you're cutting the power each speaker receives in half, but doubling the total surface area of the speaker cones pushing air. Considering just these two things, it's a wash.

However, all things are not equal. One other major thing that changes is the total impedance of your speaker system. By adding a second cabinet in parallel, you reduce the impedance of the speaker circuit, therefore given the same voltage produced by the amplifier, more current (and thus more power) flows to the speakers. If both speakers are the same impedance (say 16 ohms), adding the second one cuts impedance in half (1/Rt = 1/R1 + 1/R2, so 1/16 + 1/16 = 1/8), so ideally given the same voltage, double the current flows, and watts equals volts times amps so we get double the power. In reality it's not quite double, but it's close enough to say that you can expect about a 3dB increase in volume by adding a second cabinet of equal speaker surface area and impedance to your existing rig.

So really, what you're doing by taking a 5W 1x12 guitar combo and running it through an additional 1x12 is turning it into a 10W head (or, if the amp was rated 5W at 8 ohms and you had been running it at 16ohms, you're turning a 2.5W head into 5W).

Another huge variable is speaker sensitivity. This is a measure of how efficiently the speaker will transform the power you give it into acoustic sound waves. The number you get is the sound pressure level you'd experience standing one meter away from the speaker while it is being given a signal of 1WRMS power at the speaker's resonant peak frequency. Double the wattage, you get a 3dB increase in SPL all other things being equal. Double the distance, you get a 6dB decrease. This means that a 97dB-sensitivity speaker sounds half as loud as a 107dB-sensitivity speaker. If the speaker cabinet you add is more sensitive than the one in the first cabinet, volume will increase by more than 3dB because the speaker is more efficiently transforming the power it gets into sound.

Lastly, sensitivity isn't constant. As guitarists, we're familiar with the concept of "peaking" or "clipping". Typically we think of this being a property of the transistors (tubes or solid-state) in the amplifier, where given a stronger input the output does not increase by a proportional amount, because the transistor simply cannot provide it. However, a similar thing happens with speakers; as the power increases, the forces within the speaker chassis designed to keep the voice coil from shooting the cone out into the audience begin hindering the cone's movement and so the marginal increase in volume doesn't match the marginal increase in power. This is more of an issue when running speakers much closer to their peak power rating than you'd ever get with a 5W amp and a garden-variety Eminence or Celestion, but even at lower power levels, if double the power doesn't equal double the volume, then spreading that power across more cones will get you more volume than trying to push it all through one cone.

[IM4Tone]

Re: Quick quiestion on External Cabnets

Lot of variables involved. Technically, by adding a 1x12, you're cutting the power each speaker receives in half, but doubling the total surface area of the speaker cones pushing air. Considering just these two things, it's a wash.

However, all things are not equal. One other major thing that changes is the total impedance of your speaker system. By adding a second cabinet in parallel, you reduce the impedance of the speaker circuit, therefore given the same voltage produced by the amplifier, more current (and thus more power) flows to the speakers. If both speakers are the same impedance (say 16 ohms), adding the second one cuts impedance in half (1/Rt = 1/R1 + 1/R2, so 1/16 + 1/16 = 1/8), so ideally given the same voltage, double the current flows, and watts equals volts times amps so we get double the power. In reality it's not quite double, but it's close enough to say that you can expect about a 3dB increase in volume by adding a second cabinet of equal speaker surface area and impedance to your existing rig.

So really, what you're doing by taking a 5W 1x12 guitar combo and running it through an additional 1x12 is turning it into a 10W head (or, if the amp was rated 5W at 8 ohms and you had been running it at 16ohms, you're turning a 2.5W head into 5W).

Another huge variable is speaker sensitivity. This is a measure of how efficiently the speaker will transform the power you give it into acoustic sound waves. The number you get is the sound pressure level you'd experience standing one meter away from the speaker while it is being given a signal of 1WRMS power at the speaker's resonant peak frequency. Double the wattage, you get a 3dB increase in SPL all other things being equal. Double the distance, you get a 6dB decrease. This means that a 97dB-sensitivity speaker sounds half as loud as a 107dB-sensitivity speaker. If the speaker cabinet you add is more sensitive than the one in the first cabinet, volume will increase by more than 3dB because the speaker is more efficiently transforming the power it gets into sound.

Lastly, sensitivity isn't constant. As guitarists, we're familiar with the concept of "peaking" or "clipping". Typically we think of this being a property of the transistors (tubes or solid-state) in the amplifier, where given a stronger input the output does not increase by a proportional amount, because the transistor simply cannot provide it. However, a similar thing happens with speakers; as the power increases, the forces within the speaker chassis designed to keep the voice coil from shooting the cone out into the audience begin hindering the cone's movement and so the marginal increase in volume doesn't match the marginal increase in power. This is more of an issue when running speakers much closer to their peak power rating than you'd ever get with a 5W amp and a garden-variety Eminence or Celestion, but even at lower power levels, if double the power doesn't equal double the volume, then spreading that power across more cones will get you more volume than trying to push it all through one cone.

You have some facts mixed in with some fallacies. Without critiquing the entire post, let me just highlight one: You have described what occurs in a solid state amp NOT in a tube amp.

[SlyFoxx]

Re: Quick quiestion on External Cabnets

I dunno. Got the same result a moment ago with recorded drum samples via the fx return. A tick above 90 dB from each hit with two speakers. 94dB with four.

With a little research I've found it's the phenomena of "mutual coupling". In a nutshell speakers grouped closely together and reproducing the same signal reinforce each other up to around 500 hz (that's about B on the seventh fret of the high E string) to the tune of up to 6 dB.

[Liko]

Re: Quick quiestion on External Cabnets

You have some facts mixed in with some fallacies. Without critiquing the entire post, let me just highlight one: You have described what occurs in a solid state amp NOT in a tube amp.

I'm unclear. You're saying that in a tube amp, Ohm's law doesn't hold, so a speaker system with half the impedance gets the same current and therefore the same power? That makes no sense. I agree that tubes don't act ideally (not nearly as close to the ideal transistor behavior as solid-state anyway), but unless the tubes are providing a self-limiting effect that keeps the impedance of the speaker circuit roughly constant, or the amp design only works at a single nominal impedance so adding cabs requires changing the impedance of the ones you have, more cabs equals less impedance equals more speaker circuit power.

[dystrust]

Re: Quick quiestion on External Cabnets

I'm unclear. You're saying that in a tube amp, Ohm's law doesn't hold, so a speaker system with half the impedance gets the same current and therefore the same power? That makes no sense. I agree that tubes don't act ideally (not nearly as close to the ideal transistor behavior as solid-state anyway), but unless the tubes are providing a self-limiting effect that keeps the impedance of the speaker circuit roughly constant, or the amp design only works at a single nominal impedance so adding cabs requires changing the impedance of the ones you have, more cabs equals less impedance equals more speaker circuit power.

I was under the impression that a tube amp's output transformer performs essentially that function.

[IM4Tone]

Re: Quick quiestion on External Cabnets

I'm unclear. You're saying that in a tube amp, Ohm's law doesn't hold, so a speaker system with half the impedance gets the same current and therefore the same power? That makes no sense. I agree that tubes don't act ideally (not nearly as close to the ideal transistor behavior as solid-state anyway), but unless the tubes are providing a self-limiting effect that keeps the impedance of the speaker circuit roughly constant, or the amp design only works at a single nominal impedance so adding cabs requires changing the impedance of the ones you have, more cabs equals less impedance equals more speaker circuit power.

Ohms' law ALWAYS is a law, it always 'holds.
In a solid state amp, halving the resistance will double the current, hence doubling the wattage. When you're near max for the amp (the OP was talking about very low powered amps) this doubling of current will most likely burn up the amp.
In a tube amp, halving the speaker load is accompanied by changing taps (via an impedance switch or automatically by the way a second speaker output jack is wired) which changes the turn ratio of the output transformer, which in-turn changes the reflected impedance that is seen by the tubes. Wattage may change slightly due to the specific design of the amp, but nowhere the magnitude of 2X.
The law of conservation of energy is also always a law. You cannot create energy via a passive device (speaker), only by putting out more energy from the energy source (the amp).

[skyydogg01]

Re: Quick quiestion on External Cabnets

Remember, too, that Sensitivity is measured at a certain frequency ... like 1 kHz ... not through the entire frequency range.