3 way with differing impedence drivers

[Jason Schultz]

I'm thinking of designing a new 3 way speaker. A WMT, where the efficiency of the Woofer is 91 dB ,
the mid is 88 dB
Tweeter is 91dB
To get the Mid up to 91 dB I thought i could use 2 mid-range drivers. However this would mean that the mid section is 4 ohms and not 8.
Can sombody explain to me why this is not a good idea ?
What are the effects of doing this?

Thanks in advance guys

[BoomieMCT]

This is only a bad idea if your amp can't drive a 4 ohm load or if you are using tubes (which like a flat impedence).

One thought - putting two 88dB drivers in parallel will make the mid's efficiency 94dB. If 91dB is the goal, you can then add a 2 ohm resistor in series so you have 91dB and a 6 ohm load. Most 8 ohm recievers can drive 6 ohms.

One thing to be aware of is that published efficiencies are averaged over the the driver's bandwidth. They are often not perfectly flat or exactly where you might think they are. Make sure to test the drivers in baffle before building the x-over.

[Jay_WJ]

Using drivers with different impedance levels does not cause any problem.

BTW, I don't think you'll need two midrange drivers. If the woofer sensitivity is 91 dB, you'll get only 86 to 87 dB due to baffle step loss. You'll possibly even need to pad the midrange.

-jAy

[Jason Schultz]

I am finding this hard to beleive. I have not read anything in depth about baffle step gain/loss. Please correct me but are you saying that the woofers response will be reduced by 4-5 dB because of this effect. Is this only relevant to anechoic measurements or should i be compensating for this in my crosover design?

[Jason Schultz]

Thanks for your reply this helps alot. I read a similar thread recently about published driver efficiencies and how they can be misleading.
Regarding the 2ohm resistor to reduce efficiency and raise impedence: should this be added before the cross over or after it. Or do i add a 2 ohm resistor to each positive cable after it forks to go to each mid driver. please pardon my ignorance

[denizen23]

I purposely used 'mismatched' drivers, to obtain optimal tonal balance. The crossover simply needs to be designed accordingly.