I am trying to use multiple small drivers with sensitivity = 87 and get the total sensitivity to 93, so I thought about using 4 drivers oriented in the usual way, but that results in too big of a Vas and I can't make a box that big. I don't have enough space to make do isobaric, so the question is:
If I put 4 drivers in a box like so:
> >
< <
where < and > mean polarity
so that there are 2 on each opposite wall, what is the new Vas and sensitivity?
What about for
< <
< <
Does the second case imply Vb = inf, with +3dB?
If I put 4 drivers in a box like so:
> >
< <
where < and > mean polarity
so that there are 2 on each opposite wall, what is the new Vas and sensitivity?
What about for
< <
< <
Does the second case imply Vb = inf, with +3dB?
There is no free lunch.
It's a bit hard to figure out what your 4-driver setups are supposed to look like.
Regardless of whether the drivers are mounted on different sides of the box, or facing in or out, all the cones have to move together, either "into" or "out of" the enclosure. The Vas will always be 4x that of one driver.
Assuming you wire the drivers so that the total impedance is the same as a single driver, the efficiency will increase by about 6 dB, at low frequencies anyway.
Take a look at this thread:
http://www.diyaudio.com/forums/multi-way/4372-multiple-drivers-increase-efficiency.html
It's a bit hard to figure out what your 4-driver setups are supposed to look like.
Regardless of whether the drivers are mounted on different sides of the box, or facing in or out, all the cones have to move together, either "into" or "out of" the enclosure. The Vas will always be 4x that of one driver.
Assuming you wire the drivers so that the total impedance is the same as a single driver, the efficiency will increase by about 6 dB, at low frequencies anyway.
Take a look at this thread:
http://www.diyaudio.com/forums/multi-way/4372-multiple-drivers-increase-efficiency.html
Indeed, there is no free lunch. Just physics 
Basically, there is a trade-off between sensitivity and low frequency extension which depends on cone area and box volume.
If you want to maintain the same low frequency extension but increase efficiency by 6 dB, you'll simply need four times the Vb. There's no way around it.
A very small driver can never be efficient at low frequencies, nor can a small box be. A large Vas is not the problem. On the contrary, a large Vas simply means a soft suspension, which in itself is a good thing for low frequency extension in a small cabinet. It usually also means a low driver Fs, which is what you want. That way, the cabinet compliance will dominate the response, not the driver compliance.
I know a nice tool which calculates the theoretical maximum efficiency for a given F3, Qtc and box size, but it's in German...
Basically, there is a trade-off between sensitivity and low frequency extension which depends on cone area and box volume.
If you want to maintain the same low frequency extension but increase efficiency by 6 dB, you'll simply need four times the Vb. There's no way around it.
A very small driver can never be efficient at low frequencies, nor can a small box be. A large Vas is not the problem. On the contrary, a large Vas simply means a soft suspension, which in itself is a good thing for low frequency extension in a small cabinet. It usually also means a low driver Fs, which is what you want. That way, the cabinet compliance will dominate the response, not the driver compliance.
I know a nice tool which calculates the theoretical maximum efficiency for a given F3, Qtc and box size, but it's in German...
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