# the sum of dB

Discussion in 'Amps and Cabs [BG]' started by Musicfreak1988, May 29, 2005.

1. ### Musicfreak1988

Feb 2, 2005
Belgium
Hello,

I was wondering, two cabs of let's say 8 ohm make a total load of 4 ohm, but how does it work with dB? I can't imagine that 101 + 102 dB is 51,5 nor 203 dB...

2. ### Petebass

Dec 22, 2002
QLD Australia
Add a speaker of equel sensitivity and you get an extra 3dB.

So if one cab is 101dB, and the other is 102dB, adding them together will give a sensitivity rating somewhere between 104 and 105dB.

3. ### Tash

Feb 13, 2005
Bel Air Maryland
assuming the speakers are of equal area.

4. ### illidian

Jul 2, 2004
As Tash said, adding another speaker cabinet doesn't do this automatically.

If you have a 8x10 and add a 2x12, you won't get 3dB of volume just from the cone area. You may get 3dB by the extra wattage and the extra cone area, but not necessarily.

5. ### resolGuest

Feb 21, 2005
why does it seems that the odds are always stacked against the bassist...WHY!..WHY!!!

6. ### i_got_a_mohawk

Its basically if you double the speaker surface area and each cab has the same amount of wattage to each cab as the single cab had (ie doubling the output wattage) you get 3dB more right?

( just checking this is right )

7. ### illidian

Jul 2, 2004
Yup.

200 watts to a 4x10 will give an ( x ) dB rating. Add a second identical 4x10, and those 200 watts with eight tens will give an ( x + 3 ) dB rating.

8. ### steveksux

Mar 23, 2005
Detroit area, Troy, MI
Nope. Unless you provide 200W to each of the 4x10's.

If you're saying the same 200w into 2 4x10's = 3dB louder than 200W into 1 4x10, no, each 4x10 gets 100W, is 3dB quieter, so the 2 together = same volume as before.

You generally get a boost from the 2nd cab because you get more watts out of the same amp since the ohm load goes from 8 to 4 ohms.

Watts X Efficiency of cab= volume output. # speakers is irrelevant. Size of drivers= irrelevant. Those details are accounted for in the efficiency rating.

2 or 20 identical cabs with the same efficiency, doesn't matter.

You're not saying a 20watt amp with 10,000 cabs will be louder than a 2,000 watt amp with 2 cabs are you?

we're mixing up sensitivity ratings (db/W m ) and output (dB).

A 101dB/W@1m cab with another 101dB/W@1m cab gives you.... a 2 cab system of 101dB/W@1m. You get more cone area, but with 200W total input, you get the exact same output. Taking a 101dB/W@1m cab with a 102dB/W@1m cab gives you somewhere around a 101.5dB/W@1m system. Give it 200W and you'll get slightly less output than into the 102dB/W@1m cab by itself.

Typical amp: 200w@ 8 ohms, 300w @ 4 ohms. So 1 speaker gets 200w, but running 2 cabs, each cab gets only 150W. So adding the 2nd cab may not get you 3 dB extra since the amp power doesn't double. But you do get extra output (volume, loudness) because of additional watts input.

Randy

9. ### OstinatoGuest

Feb 7, 2005
Toronto ON
A difference of 3db is very noticeable., IMO. Decibels are measured logarythmically, like earthquakes.

10. ### Musicfreak1988

Feb 2, 2005
Belgium
Maybe it's more easy for you guys if you know the specific amp and cabs... I was thinking of a GK 1001RB-II (750W + 50W for the bi-amp system) and a 115SBX and 410SBX cab (which are 101 and 102 dB). Thanks for the replies so far. Sam.

11. ### illidian

Jul 2, 2004
I would guess about 104dB, +/- 1 dB.

12. ### i_got_a_mohawk

more in depth, but what i thought, cheers

13. ### steveksux

Mar 23, 2005
Detroit area, Troy, MI
The 102dB cab will seem exactly as loud as the 101dB cab with the same wattage going in. You can't hear 1dB difference.

So 50W going to the 4x10 will be as loud as when 50W is going to the 1x15.

When 750W is going to the 1x15, and 50w is going to the 4x10, the 1x15 will be 12dB louder than the 4x10.

The 1 db difference is sensitivity is overwhelmed by the 12dB of power available.

Randy

14. ### Musicfreak1988

Feb 2, 2005
Belgium
Actually, the 750W are for both cabs and the 50W coming from the bi-amp system are for the horns... I'm planning to buy this to replace my Roland DB-700. Not because I dislike it, but when you have to compete with a loud drummer and 4 marshall cabs connected to full tube guitar heads... Yeah well, you get the picture...

15. ### steveksux

Mar 23, 2005
Detroit area, Troy, MI
We're way off in the weeds here...

what's the original question? 101dB(@1W@1m) + a 102dB(@1W@1M) cab=???

a 2 cab system with around 101.5dB (@1W1M) sensitivity.

Efficiency doesn't "add" exactly. What's your gas mileage if you get 30mpg and your wife gets 15mpg? 45mpg?

Somewhere between the 2 cabs (or cars) is the correct answer.

The 750W into both cabs would give you the same output volume if you had 750W into either one of the cabs (ok, 1dB louder on the 102dB/w/m cab).

But since amps that put out 750w into 4 ohms only put out 450-500 W into 8 ohms, the 2 cabs will be louder than 1. Because of extra power, not more speakers, not more cone area.

Call it 2dB louder. 10x log(750/450)=2.2dB

Unless you have a tube amp with output transoformer with different impedance taps. Then you get 750W at whatever impedance tap you have available.

Randy

16. ### Musicfreak1988

Feb 2, 2005
Belgium
Man it seems that I need some extra math lessons ... Just a joke... I get what you mean but triple hooray for who invented those formulas

17. ### steveksux

Mar 23, 2005
Detroit area, Troy, MI
Yup, you were fine, I wasn't trying to go geek on you. I think someone else sounded like they were saying doubling cone area=3dB increase by itself, no extra power required. That's what that was directed at.

Randy

18. ### i_got_a_mohawk

Sorry, i think i may have thrown ppl off track, i reread my post and being late n all last night i didnt put it down the way i intended

19. ### Marcus Willett

Feb 8, 2005
Palm Bay, FL
Endorsing Artist: Bag End - Dean Markley - Thunderfunk
Fletcher-Munson curve, baby. Unfortunately for us bassists, human ears are not very sensitive to low frequencies. We're not terribly sensitive to high ones either, but consider:

5KHz to 20KHz, a bandwidth of 15,000 cycles is only 2 octaves. The last octave in particular is in an area that our ears are not too sensitive to, but guitarists can do fine with a cab that only goes to around 5K. If they lose much of that last octave (10KHz to 20KHz) because of our ears, the guitarist is fine.

40Hz to 160Hz is also 2 octaves, a bandwidth of only 120 cylcles. As we get closer to either end of our more-or-less 20Hz to 20KHz hearing range, our ears get less and less sensitive. The drag is as bassists, we need that bottom octave (40Hz to 80Hz, or 30Hz to 60Hz for us 5 stringers), we can't lose it, so we need every increasing power to sound as loud as our guitarist brethren, regardless of the actual acoustical energy being generated.

In brief, yeah...the odds are always stacked against the bassist.
Kinda sux, but oh well.

20. ### Masher88Believe in absurdities and you commit atrocities

May 7, 2005
Cleveland, OH
So, what are the benefits of adding another cabinet if you aren't getting a very big increase in "loudness"? I guess if you have 2- 8 ohm cabs and a S.S. amp, your amp would see a 4 ohm load and roughly double the power output? What if you had a tube amp? I have read that they give out the same wattage no matter what ohms you feed it. Is this true? I'm so confused! Here's an example: I have an Avatar 4x10 and a 2x10 and a Mesa 400+. Both cabs are 4 ohms. Am I better off just running the 4x10 at 4 ohms? Or using both cabs for 2 ohms? I've never done an A-B comparison to see which is louder in reality...I'm just asking "theoretically". Maybe I'll try it out at practice tonight.