driver sensitivity

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2 drivers will simply require double the enclosure volume....

Also unless you are bi amping you really need to use a tweeter that is more efficienct than the woofer/s not less or you'll lose 2 much power as heat....

remember 2 woofers in the same enclosure will theoretically increase the efficiency by 3dB per Watt.
Although in general it makes sense to have a tw more efficient than the wf (if not for else, because you can attenuate the tw and so increase the power applicable to the loudspeaker) there are acouple of general considerations to make.

First, when values are close (i.e. 90 and 92) the declared SPL of the speakers means near to nothing, being a kind of 'average' spl often enhanced by mkt department of the speaker company.
To understand if the 'marriage' makes sense it takes to see and compare the graph of the speakers response, paying attention that the graphs are comparable.
(normally the spl curve is traced at 2.83V/1mt on IEC baffle, but some manufactorer uses a different setup. Apropos, when measure is taken on IEC baffle there is almost always a dip around 400 Hz, due to the baffle diffraction effect.)

Second, in your case if you plan to wire the woofers in parallel you will have the midbass section at 98 db (92 + 3 for doubling the area + 3 for cutting impedance by 2), and the difference becomes too big to have a chance!
By the way you'll have more than a few problems in finding a dome tweeter with that efficiency, we're already in the horns domain.

Which speakers have you chosen? maybe we could tell you a bit more knowing some more details, it's very difficult to say something helpful on generic cases, the field is too large!
Thanks guys for your help, just a little more info, I am looking at the SEAS H624 woofers. they are rated at 92dB with a VAS or 45.1, if i have to double the volume for 2 drivers then i am going to have a huge box for a centre channel, can you 2 plz recommend well priced (middle of the range price) drivers for a centre channel, only Audax, Seas, Scanspeak, Vifa and Focal are available to me, and whats a good tweeter going around for not too much money?
Ok Griff, the H624 graph is visible at line up.htm, click on H624

The curve is traced at 2 volt (= 0.5W/8Ohm) and 0.5 mt distance.
To normalize at the standard 1W/1mt reference you should subtract 3 db (+3 for doubling the power and -6 for doubling the distance)

With this normalization you see that the SPL curve is approx

Hz 200 500 1k 2k 5k 10k
SPL 85 89 92 91 90 83

with a peak of 94db at 4k
The dip at 1k is probably due to some resonance of the measurement setup (see correspondant peak in impedance curve), while the reduced emission at low freq is related to the 4PI measurement (= you can expect that in normal room condition the low freq will increase of 3-4 db)

Now it should not impossible to match this speaker with a 90db tweeter (but to be sure we should see the tw response graph...), more difficult if you plan to match 2 H624 in parallel with one tw.
If you want to stay on a MTM design you may think to wire the H624 in series, their SPL will be unchanged vs one unit (add 3 db for doubling the radiation area and subtract 3 for cutting power to half)

About wolrking volume, I don't see big issues: a quick look shows that you may have a 15 lt QB3 reflex with F-3 at 50 Hz or 19 lt reflex with QB3 around 42 Hz. With 2 speakers you'll have 30 or 38 lt, that's not too much.

By the way, I don't know this speaker but his 'daddy' (P17RCY) was one of the best sounding 6" ever built (.. at human prices)
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