Cab ohms vs. amp watts

[boingoman]

I think I'm following you here. let's say one power amp is driving one enclosure @150 watts. Then we stack another enclosure powered by the same amp, and maintain the wattage ouptput of the amp @ 150 watts. We gain only 3db acoustically, so to gather 6db we`d have to double the amps output to 300 watts?

Do you mean each speaker gets 75W? Or 150? If you mean each gets 75W, Then yes, you would have to double the power to each speaker to 150W to get 6 db vs. one speaker getting 150W.

Because I was right, before, but for the wrong reasons.

The volume added by stacking two cabs getting 150W each vs. 1 cab getting 150W is 6 db. But not for the reasons I stated. I misspoke in my last post. And was corrected by a colleague. In only a semi-harsh way. Sorry for the misinformation.

The average volume increase comes from acoustic coupling. The sound waves from each cab reinforce each other, adding together. This is where the extra average volume comes from. This only works up to a certain frequency, based on the distance between the box and driver centers. Basically the closer they are, the higher the coupling goes, frequency wise.

I won't get into the math, but two bass 4X10 cabs right next to each other, getting 150W each, will be 6 db louder than one at 150W, up to around 300 hz.
This is around the D (7th fret) on the high g-string on an electric bass.

With two 4X12 cabs, the maximum frequency where coupling stops is lower by a few hertz, because the centers are farther apart. This brings you to around the D note right below the high E string on an electric guitar.

In both cases the 6db volume increase due to acoustic coupling covers the majority of the range of both instruments.

So if you double the amount of speakers, and also double the amount of power to those speakers, and take advantage of the coupling, your net increase is 9 db.

If the cabs are moved far enough apart so no coupling occurs, the increase in volume will be negligible at any given point in the room, though coverage will be more even.

BTW for there to be no coupling down to 50 hz, the cabs need to be about 10 feet apart, center to center.

 

[Ed Dixon]

Do you mean each speaker gets 75W? Or 150? If you mean each gets 75W, Then yes, you would have to double the power to each speaker to 150W to get 6 db vs. one speaker getting 150W.

Because I was right, before, but for the wrong reasons.

The volume added by stacking two cabs getting 150W each vs. 1 cab getting 150W is 6 db. But not for the reasons I stated. I misspoke in my last post. And was corrected by a colleague. In only a semi-harsh way. Sorry for the misinformation.

The average volume increase comes from acoustic coupling. The sound waves from each cab reinforce each other, adding together. This is where the extra average volume comes from. This only works up to a certain frequency, based on the distance between the box and driver centers. Basically the closer they are, the higher the coupling goes, frequency wise.

I won't get into the math, but two bass 4X10 cabs right next to each other, getting 150W each, will be 6 db louder than one at 150W, up to around 300 hz.
This is around the D (7th fret) on the high g-string on an electric bass.

With two 4X12 cabs, the maximum frequency where coupling stops is lower by a few hertz, because the centers are farther apart. This brings you to around the D note right below the high E string on an electric guitar.

In both cases the 6db volume increase due to acoustic coupling covers the majority of the range of both instruments.

So if you double the amount of speakers, and also double the amount of power to those speakers, and take advantage of the coupling, your net increase is 9 db.

If the cabs are moved far enough apart so no coupling occurs, the increase in volume will be negligible at any given point in the room, though coverage will be more even.

BTW for there to be no coupling down to 50 hz, the cabs need to be about 10 feet apart, center to center.

Sorry, but you're still wrong. Answer is 3db

The following link has a simple table to explain this.


http://www.safetyline.wa.gov.au/institute/level2/course18/lecture54/l54_05.asp


See the first entry in the table. Also to quote from text below the table:

"You will notice that adding two sounds of equal sound pressure level gives an increase of 3 dB."

Ed

 

[Ed Dixon]

And if you want to see the math for it, go here:

http://www.safetyline.wa.gov.au/institute/level2/course18/lecture54/l54_13a.asp

Ed

 

[Ed Dixon]

Some other references that say the same thing:


http://www.menlh.go.id/apec_vc/osaka/eastjava/noise_en/1/page3.html

http://www.csgnetwork.com/decibelamplificationcalc.html

http://www.ccohs.ca/oshanswers/phys_agents/noise_basic.html


Ed

 

[boingoman]

Genelec website:
When two subwoofers are positioned close to one another mutual coupling is the fortunate by-product. This is due to the long wavelengths, associated with low frequencies, causing constructive superimposition. For mutual coupling the subwoofers must be place within ½ a wavelength of one another (85Hz upper crossover frequency ½ wavelength is approx. 2m). For example, two subwoofers give a 6dB increase in acoustical output at the listening position - see table below.

Total number of subwoofers SPL increase compared to a single subwoofer
1 0.0 dB
2 6.0 dB
3 9.5 dB
4 12.0 dB


Mix magazine's SR editor:

Coupling, the mechanism by which a line array works, is familiar to anyone who has assembled large quantities of speakers into systems. Coupling between transducers occurs when the distance between their acoustic centers is less than half a wavelength. The best case for 12-inch speakers, with their drivers nearly touching, can provide coupling up to about 550 Hz. For 10-inch speakers, coupling can occur up to 630 Hz, and for 8-inch speakers, coupling can extend up to 800 Hz. (This is why simply putting a hand's width of space between adjacent speakers in a traditional array attenuates the mids.)

 

[Ed Dixon]

What we are dealing here with is basic sound level issues. The oriignal question dealt with impedance load, and has since been answered. Follow up quiestions deals with sound levels.

There are always other factors that can affect the overall result, but that is not the question here. Things like corner placement, on floor or above the floor, inside, outside, etc. There are also factors like cabinet design, out of phase operation, distance, etc. Those are all interensting subjects, but not the question at hand.

When two identical sources are involved, the sound level change is 3db. The equations say that. The various tables on the Internet say that. The expects say that. That is a fact.

Ed

 

[boingoman]

You have to look at the whole thing as a system, you are right. All those websites you listed deal with separate sources, not a single system.

A bass rig with two identical speakers is treated as one source, not two different sources, as long as the speakers are close enough. As long as they are within a certain distance, they are treated as one source. Move them far apart, they are considered separate sources, and the 3db rule applies, as the outputs are out of phase enough they do not couple.

My answer was based on the system of two 4X10 cabs stacked, each receiving 150W, as opposed to one, receiving 150W. The system will indeed have 6 db more output. The amp is now putting out twice the power as it was into one cab.

Toki, your assessment was right, and for the right reasons. Another apology. I thought I was wrong, I wasn't. Again.


I trust JBL, EAW, SPL, Genelec, and a whole slew of speaker manufacturers who deal with this stuff in the real world every day.

And I spent last night confirming this to myself with measurements.

I ran a 100hz tone through my subs, and calibrated each for identical SPL output. Placing them together resulted in a 6 db increase in SPL at a distance of 1.5 meters from the boxes versus the output from one at the same distance. It was as close to a free field as I could get, the subs on a scaffold 6 feet off the ground outside. This frees them from any boundary effects down to about 30 hz.
My subs don't go much higher, and I didn't run my tops up there to check any higher frequencies. But the effect was confirmed for me.




The math even works, looked at as a single source system.

Hooking up two cabs, both at 150W, vs. one at 150W, You have doubled the speakers in the system, and the power to that system.

(Yes, I am retracting my correction. I was recorrected after talking to another person who reminded me to look at things as a system.)