Wattage and sound

Wattage has to go up by a factor of 10 for volume to double, all other things being equal.

And while all other things are never equal, it's still the case that a high quality 30-50W amp is more than enough to power even the largest gigs . . . just mic it and run it through the PA :)

KP is right...

Volume is referred to as SPL (sound pressure level) and measured in decibels (dB)

An increase of 10dB will sound twice as loud to your ears.

To make a 5 watt amp twice as loud, you'll need a 50 watts. (a 10dB increase)
To make a 50 watt amp twice as loud, you'll need 500 watts. (also, a 10dB increase)

Doubling the power will increase SPL by 3dB (which is perceived by the ear as only a very small difference)

(Double the perceived volume) = (an increase of 10dB (SPL)) = (10 times the power (wattage) needed)

Also, speaker surface area plays a big part in this aswell...

If you use the same head to (seperately) power a 1x12" cab and a 2x12" cab, the 2x12 will be 3dB louder than the single speaker, due to twice the surface area that is moving air.... (assuming efficiency and specs of the drivers/enclosures are identical)
So doubling the number of speakers in your cab will essentially give you the same increase in SPL as doubling the power of your amp.

Hope this helps

J

If you use the same head to (seperately) power a 1x12" cab and a 2x12" cab, the 2x12 will be 3dB louder than the single speaker, due to twice the surface area that is moving air.... (assuming efficiency and specs of the drivers/enclosures are identical)
So doubling the number of speakers in your cab will essentially give you the same increase in SPL as doubling the power of your amp.

There is no free lunch. By the definition of efficiency, the overall dual driver/cab combo would need to have better efficiency than the single driver/cab to be louder. In reality, two drivers in wired-in-parallel have 1/2 the impedance of a single, same type driver. The the two parallel drivers will suck twice the power (twice the current at half the impedance) even though the "volume" setting on the amp does not change.

Thanks very much guys. Just what I was looking for.

Thanks very much guys. Just what I was looking for.

You can determine how loud your amp's speakers will be if you know the speaker's SPL (sound pressure level) or sensitivity rating. Speakers are usually rated at 1 watt power at 1 meters distance.

Everytime you double the watts you get a 3 decibel increase in volume.

So, if you look up your amp's speaker and the manufacturer rates the speaker at 97db @ 1W/1M, this means your speaker will produce 97 decibels volume with only one watt of power applied at one meters distance from the speaker.

Double that to 2 watts and you gain 3 decibels or 100 decibels volume.

Double that 2 watts to 4 watts and now you are at 103 decibels volume.

Double to 8 watts and you have 106 decibels.

16 watts will give you 109db

32 watts- 112 decibels.

64 watts- 115 decibels.

128 watts- 118 decibels.

Jminor said 10 decibels is a doubling of volume. So you can see above that a 128 watt amp would be 9db louder than a 16 watt amp (through the same speaker). This is approx. twice as loud.

It does work out to be approximately 10X wattage to double the volume.

This is why it is often easier and less expensive to purchase a speaker with a higher SPL (sensitivity, efficiency) rating than purchase a more powerful amp.

It is said that 2 identical speakers in close proximity give you about a 3 decibel increase in volume. So a 2 X 12 amp should be very slightly louder than a 1 X 12 with the identical wattage and speaker model.

But high SPL ratings aren't everything. Often speakers with higher SPLs emphasize the high frequencies that require less power. So they are very loud, but they can be harsh. Speakers with huge bottom end low frequencies require more power and will have lower SPL ratings. But they can sound much better. So you have to consider the types of tones you are after. If you want huge bass, a speaker that emphasizes the lows with a lower SPL would be better for you.

The the two parallel drivers will suck twice the power (twice the current at half the impedance) even though the "volume" setting on the amp does not change.

We agree on this point....
By the definition of efficiency, the overall dual driver/cab combo would need to have better efficiency than the single driver/cab to be louder.

I disagree with this... As long as the speakers are in a correctly designed enclosure (based on the Thiele-Small parameters of the drivers) you would definitely notice them to be louder. With twice the power coming from your amp, I don't see how it could not be louder.

The trade off is of course HEAT....
With twice the current running through your amp, you are generating a lot more heat... and amps are designed to operate within a range of loads (impedances)...Exceeding this range will probably damage your amp due to overheating...
This is why you can't just keep adding more and more speakers to your amp.

gnease, I could be wrong but i'd like to know where I'm wrong...

Cheers,

Josh

The the two parallel drivers will suck twice the power (twice the current at half the impedance) even though the "volume" setting on the amp does not change.

We agree on this point....
By the definition of efficiency, the overall dual driver/cab combo would need to have better efficiency than the single driver/cab to be louder.

I disagree with this... As long as the speakers are in a correctly designed enclosure (based on the Thiele-Small parameters of the drivers) you would definitely notice them to be louder. With twice the power coming from your amp, I don't see how it could not be louder.

The trade off is of course HEAT....
With twice the current running through your amp, you are generating a lot more heat... and amps are designed to operate within a range of loads (impedances)...Exceeding this range will probably damage your amp due to overheating...
This is why you can't just keep adding more and more speakers to your amp.

gnease, I could be wrong but i'd like to know where I'm wrong...

Cheers,

Josh

I think you sort of understand the reason. Obtaining the maximum sound pressure level (loudness) is mainly dependent upon the amp's output wattage capability at the load impedance and the cab efficiency. If we assume the dual and single driver cabs are well designed, have the same type drivers, then their efficiencies might be about the same. Let's say it is. Same wattage in => same SPL out. (And this same power occurs at different volume settings on the amp.) So the real question is what max power the amp can put out into the impedances of each cab. That is very dependent upon the amp design. But one thing is clear: If you leave all settings (gains/volume controls) the same and swap and compare SPL. The the dual driver cab will sound louder only because it is sucking more power from the amp at those same settings. But the problem is that you will not be able increase the gains to the same point for both the single and dual driver case. The amp will appear to run out of steam at a lower gain/volume setting (on the amp) for the dual driver case. This is my main point -- you will not be able to "turn up" the amp as high with the dual driver (half impedance) amplifier. Most amps will put out a little more max power into a lower impedance, but not usually twice as much into that half impedance -- there are other losses that work against this: e.g., transformer winding resistances for tube, FET "on" junction resistance in solid state). This is where your heating observation comes in.

So at same settings, dual will sound louder (we agree). But where will the amp top-out into 4 ohms version 8 ohms? That's real ultimate loudness question. Maybe the amp can put 1.5 to 2 dB more level out into 4 ohms. Given our theoretically well-designed cabs, that's the real loudness increase to expect. And it's not really much. If the drivers are not well matched, that could get eaten in loss of efficiency -- easily. The better approach might even be to switch from an 8 ohm single driver to a 4 ohm driver.

On the driving area of the dual versus single: The amp's power is moving a volume of air, not just an area. So the displacement of the driver cones must be considered. If I pump the same power into my single driver as the dual driver cab, the dual drivers will split the power, each getting about half the total. As a result, the paired driver's cones will move only half the distance as the single driver's cone. Until the drivers reach nonlinearity, both cabs will will move the same volume of air: 1 driver's area x 1 unit displacement versus 2 driver's area x 1/2 displacement.

So all I mean by "no free lunch" is that there must be more power for more loudness. Hope that makes sense.

Yes, Gnease is right.
As long as you load the amp with same total impedance ( for example 8 ohm) from the speakers you should get a similar SPL from 1 speaker ( 4 ohm) or 2 speakers ( 2x8 ohms in parallell).
But if you use 2x4 ohm speakers in parallell you will get a 2 ohm load that will draw the double amount of current from the amp. The rated output power on an amp is set by the maximum available output voltage at the maximum available current ( at a given distorsion level) . I you try to draw more current out of the amp (due to a too low load impedance) the output voltage will drop (and distort the output signal) and you will not have more than the rated output power.

Tube amps with pentode or beam power output tubes don't vary their output power much with impedance changes, because the screen grid voltage stays nearly fixed as the control grid voltage and plate current fluctuate, and that's what mainly determines the available power. The tube makers published graphs in the engineering data for various tube types showing power output and levels of various harmonics at different output impedances. The usual output impedances were chosen arbitrarily for the lowest distortion levels, or often for the lowest third harmonic since all even harmonics get cancelled out in push-pull amps. (Usually that's pretty close to the optimal impedance for peak power output, too.) You can usually get away easily with a 2:1 or maybe 4:1 mismatch in output impedance from the rated without hurting anything, unless the output transformer's pretty marginal for the application. You'll just get more distortion, which you may or may not like. But if your amp's OT is marginal, going up on impedance can puncture the insulation in the primary, because it causes wider voltage swings. Going down in impedance won't likely hurt a thing, and a tube amp can run for a while with the secondary of the OT shorted without hurting anything. That won't work with a solid state amp!

Wow, that's some technical reading there. :D

I always heard that 2 identical speakers will divide the power. So, with a 60 watt amp, a single 12" speaker would get 60 watts, a 2 X 12 each speaker would get 30 watts. If we use the imaginary speaker I listed above at 97db @ 1W/1M, the 1 X 12 would produce about 115db, each speaker of the 2 X 12 would produce 112db. But two identical speakers in close proximity give you a 3db gain, so you end up about the same, 115db.

So really, you don't get so much a volume gain, but you do get a fullness with the 2 X 12. This is the big huge tone you get with multi-speaker cabs. It is not volume, it is tone.

I'd just like to mention that I love threads like this :)

I'm tripp'n. You guys are just too cool. Now I know where I am going to submit my techno questions on my tube amps! Oh, if you're pontificating the relationship between spl and wattage its always fun just to measure the sound pressure level and see what you get.