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A Question on Speaker Driver Efficiency

I have been tweaking my guitar amps, by upgrading the speakers.

I installed a larger speaker (was 8" now 10") in my bass amp, but I made sure it was very efficient - net result
- not only is the bass much deeper sounding,
- but because the new driver was more efficiant I now play at a lower volume.

So I am now considering upgrading my other amp (i.e. used for my 6 string) and got to thinking about building a new cabinet that houses two speakers.

I know that connecting the speakers in ...
- series will double the impedance, i.e. 2 x 4 ohms would have an onverall impedance of 8 ohms
- parallel will halve the impedance, i.e. 2 x 16 ohms would have an onverall impedance of 8 ohms

But what I have not been able to get my head around is...
- what will each connection method (i.e. series or parallel) have on the "combined" sensitivity rating?
- e.g. if both speakers are rated at 96db sensitivity, will the overall sensitivity change due to the connection method or remain at 96db?

Since I can get 4 ohm or 16 ohm drivers - which connection method would be best? series or parallel?

in case it is a factor
- the amp is 15 watts into 8 ohm
- I am looking at employing two identical drivers each rated at 96db sensitivity
- 96 db (or higher) is the target for the combined sensitivity

Any help is appreciated - Many Thanks Steve
williewonka

Showing 10 responses by erik_squires

erik_squires
16,592 posts
 
Sorry, wrong is wrong. And with that, I have to move on. I have better things to do with my time.

And with that, I’m done giving out free lessons in first year electrical engineering. Find someone else to insult.

Well I must say some days a much more interesting here than others. :)

E
erik_squires
16,592 posts
 
Al - Yes indeed. These are all just rewrite.  Since power is proportional to the square of the voltage:

(0.5 x 0.5) = 0.25

Now, calculating power difference: 

10 log (0.25) = -6 dB

Calculating voltage difference:

20 log (0.5) = -6 dB

In either case, the SPL at a reference distance, measured in dB, changes in proportion to the power OR voltage when either is expressed as dB assuming there is no compression in the driver. 

Said another way, for a single driver:

Delta V dB = Delta W dB = Delta SPL dB

That's what's so cool about dBs! 

Best,

E
erik_squires
16,592 posts
 
@cj1965

Where in my post did I mention power? I mentioned the relationship between output dB and input voltage.

Having said that, as @almarg has alluded, both formulas are true. Much like

P = V x I

and

P = (V x V) / R

They are mere re-writing of each other. Again, I encourage you to grab XSim to validate any formulas. It is pretty accurate. 

Best,

E
erik_squires
16,592 posts
 
Thanks @almarg - I should have said "voltage" a few more times I think. 

Best,


E
erik_squires
16,592 posts
 
Also, we are so far from the OP's original topic, maybe we should let this thread just die. :) 
erik_squires
16,592 posts
 
For a single driver : The change in dB in input voltage is equal to the change in output SPL in dB.

Voltage dB can be calculated in this fashion: 

dB = 20 x log ( Vnew / Vold )

in the case of half the voltage:

20 x log ( 1 / 2 ) ~= (- 6 dB)

Also, you can use this handy web gizmo:

http://www.sengpielaudio.com/calculator-gainloss.htm
erik_squires
16,592 posts
 
I encourage anyone wanting to follow along, to grab XSim crossover simulator, and examining the Frequency Response, Watts Dissipated and Impedance charts. 

http://www.diyaudio.com/forums/software-tools/259865-xsim-free-crossover-designer.html

The "blank" driver is an ideal 8 Ohm driver with an 80 dB efficiency. This will put to rest any questions about power, efficiency and sensitivity you may have. 

Best,

E
erik_squires
16,592 posts
 
Hi OP! 

XSim will help you evaluate the impedance and relative output. You can try making up circuits of various speaker drivers, and the output volume AND impedance will change when you do. :) So it gives you immediate feedback to possible choices. 

Notes:

Sensitivity: Volume in dB at 2.83V. 

Efficiency: Volume in dB at 1 Watt. 

For 8 Ohms, this value is the same. 

Best,

E
erik_squires
16,592 posts
 
Double check the sensitivity on each! :) It is very unusual that the same driver has the same sensitivity regardless of impedance! It's not impossible, but usually manufacturers cut the sensitivity by 3dB as impedance doubles. 

Yes, your math is correct.  Again, I encourage you to grab XSim Crossover Simulator for PC or equivalent to try these ideas out. 

Best,

E
erik_squires
16,592 posts
 
Hey Willie,

You know, you could grab a speaker crossover simulator and try this out for free, it may help you. I use XSim. There you can use the virtual 8 Ohm drivers to explore various different wirings and output. 

Yes, the amp matters. If it's a tube amp, you probably want to shoot for 8 Ohms. 

If we compare a single 16 Ohm driver vs. 2 in parallel, you get 8 Ohms, but +6 dB in output. 

If you wire 2 x 16 Ohms, you end up with 32 Ohms, and no gain in sensitivity. 
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