Compound speaker sensitivity

[sanjit]

Hi,
I did the following calculation to find out the total power needed for an 8 driver sealed box using 4 pairs of Isobaric Push-Pull drivers. The original power requirement was 76w for 1 driver and with 8 drivers it comes to 19W. Is this correct? I would very much appreciate a reiew of the calculation and some enlightenment on what the correct power requirement would be. Thanks in advance :-)
Let Listening distance be Dlistening in Meters (m) = 5
Let Reference Distance be Dreference in Meters (m) = 1
Let Room Gain be Groom in dB = 6
Let Reference Sensitivity be RSens = 90
Let Speaker Sensitivity be LSens in dB (1W/1m) = 89.2
Let Head Room Required be H in dB = 10
Let Power Required = Pmax in Watts (W)
Then Pmax=〖10〗^(((Rsens-Lsens+20.log⁡〖(DListening/DReference)+H-Rgain〗 ))/10)
Substituting values gives Pmax=〖10〗^(((90-89.2+20.log⁡〖(5/1)+10-6〗 ))/10) = 76W
This is for Dayton Audio RSS315HO-44 12” woofer with 8 Ω impedance
Now, If I want to use a total of 8 drivers connected as below:
(a) 2 drivers in Isobaric push-pull connected in series – giving impedance of 16 Ω and sensitivity change of -3db
(b) Connect 2 Isobaric push-pull series connected pairs (a) in parallel – giving impedance of 8 Ω and sensitivity change of +3db
(c) Connect 2 Isobaric push-pull series parallel connected pairs (b) in parallel – giving impedance of 4 Ω and sensitivity change of +6db
In all, 8 drivers connected in this way gives an impedance of 4 Ω and sensitivity of 89.2+6=95.2 db (1W/1M).
Pluging this new sensitivity into the Pmax equation I get Pmax = 19W! I.e I can get a 19w amp to drive 8 drivers to give me the target SPL
This sounds too good to be true. Hence the request for a review.
Sanjit

[nandababaji]

Hi Sanjit, yes that's correct- when you use 2 drivers you gain 3 db and 6db for 4 drivers. As you are using 8 drivers in isobarik configuration you will have 6db sensitivity gain, as a pair of isobarik drivers has all of the same characteristics of one driver, except the VAS is effectively halved, and impedance is either halved (parallel connection) or doubled (series connection) In effect by using a larger box you are getting a sensitivity increase of 6db, in comparison with a single driver. The isobarik configuration allows you to use a box half as big as usual (for a given f3), but half as sensitive- that is an isobarik pair gives 4 ohms for the same voltage sensitivity as a single 8 ohm driver. I hope this all makes sense to you!

[sanjit]

Thanks MLN :-)