series connection question

[JC951t]

Hi Guys,
I was wondering. Assuming I connect two 4 ohm
90db woofers in series to create 8 ohm, would the sensitivity remain the same or would it drop by 3db ?

Thanks

[Ron E]

stays the same

[pjpoes]

Doubling the woofers has the effect (regardless of how they are wired) of gaining 3db's of efficiency (for a given wattage, how much output you get), and then the manner in which its wired will effect the sensitivity (Output for a given voltage). If you wire two drivers in series (say two 4 ohm drivers) you will gain 3 db's in efficiency and lose 3db's in sensitivity. If you wire two drivers in parallel, you gain the 3db's in efficiency and 3db's in sensitivity (Though the last 3db's is arguably cheating as it was done at the expense of needing more current from the amplifier.

I think this issue can be confusing when designing a speaker if you don't understand the difference between voltage sensitivity and efficiency. Voltage sensitivity is useful in that placing a 4ohm driver and an 8ohm driver together (Say a 4ohm woofer and 8ohm tweeter with crossover), the amplifier will see a 4ohm load below the crossover point, and an 8 ohm load above it (simplistically speaking). Since its best to design a speaker with a benign impedance load and relatively linear phase (The amplifier then can work most efficiently "seeing" the most ideal load), if you wire woofers in series as mentioned, you are not gaining anything as far as the basic design is concerned. However you are now sharing the load between two woofers (This is essentially how the efficiency was increased), and so the system will be capable of playing louder with less compression, while also presenting a benign load.