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what is the advantage of having 2 identical drivers in a speaker? do you get twice the volume?

That is the common assumption!

IF ONLY that was the actual result!

The real result is an increase in gain along with a degradation in performance that seriously offsets the increase in gain.

The superposition of spaced sources results in serious destructive frequency dependent spatial polar lobing that appears as a 'comb filtering' pattern in the frequency domain. And this is very audible.

And this occurs even if both drivers are ideal with a perfectly cardioid polar plot and a frequency response that is flat from DC to gamma rays!

A single source is ALWAYS preferable to multiple sources covering the same passband.

Sources superpose in the manner displayed below modeled in EASE with respect to degree (% wavelength) of separation. Note also that the orientation of the lobes rotate accordingly with relation of the spaced sources (horizontal spacing is displayed. For example, vertical spacing would rotate the diagrams by 90 degrees...).
 

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There's a good discussion of the advantages of the D'Appolito configuration in Speaker Building 201; too long to quote here, but it deals directly with smoothing the lobing patterns of drivers, which occur in all speakers.
So, there are arguments on both sides, positive and negative. This is a great example of how all designs are a compromise of various choices.

Tom
 

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...The real result is an increase in gain along with a degradation in performance that seriously offsets the increase in gain.

The superposition of spaced sources results in serious destructive frequency dependent spatial polar lobing that appears as a 'comb filtering' pattern in the frequency domain. And this is very audible....
Beware of conventional wisdom, as it's not always applicable.

Case in point: comb filtering. It's ubiquitous. Think in terms of your 2 ears... any single source produces a comb filter signal to your brain. If that's an objectionable idea, then think "phantom center channel" - 2 spaced sources (L/R speakers) producing the same program, so it images in the center. Enough said.

So, superposition of spaced sources will result in wave interference effects, every time. The seriousness of any negative impacts is as debatable as the value of any positive impacts, until you specify an application.

If the application is an MTM in a D'Appolito configuration, used vertically, then the "serious destructive frequency dependent spatial polar lobing" aims nulls at the floor and ceiling. as shown here:
http://www.zaphaudio.com/BAMTM-polar-1000-2000-100step-red-blue.gif

Note the strong nulls at +/- 40 degrees, up to -30dB deep through the crossover region (1K to 2KHz plotted at 0.1KHz steps). In many cases, the reduction of output toward ceiling and floor has a beneficial effect on sound quality, reducing the impact of room acoustics. Note that this vertical polar response is accompanied by a very smooth and uniform horizontal polar response (which Zaph didn't post). The vertical response also explains why no one recommends conventional MTMs in horizontal applications (which will have "serious destructive frequency dependent spatial polar lobing" because it's turned 90 degrees from where it works well).

To the OP's question, dual drivers in parallel gain 3dB in output due to 2x surface area, and 3dB in voltage sensitivity due to reduced impedence (8 ohms in parallel is 4 ohms), which draws 2x more current at constant voltage, eg @2.83V. You only see a total of 3dB in power sensitivity, because it adjust the voltage for constant IV=power.

Have fun,
Frank
 
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