# Force canceling sub - would two 9" drivers equal the output of a single 18" driver?

#### jaminjames

Force canceling sub - would two 9" drivers equal the output of a single 18" driver?

Long time lurker, first time poster.

I'm DIYing two subs to even out some modal issues in my studio control room. My current monitors are Barefoot MM35s - 3 way monitors that use dual opposing subs. Low end is tight, articulate and lovely. There is hardly any cabinet vibration.

I've been thinking about using a force cancelling design for my DIY subs. If one enclosure hold two 9" drivers, would that equal the output of a similarly speced 18" driver?

#### BJosephs

The increase in cone area is what matters for low end spl and while a doubling of drivers does double cone area, that isn’t the same thing geometrically as a single driver of twice the diameter. If a 9” driver has ~60in2 of area, a pair has 120. A single 18” has ~254in2. Please check my math.

I recently bought new monitors for my home studio and the Barefoots were on my short list. I ended up getting a steal on a new set of KH310s. I’m sure the Bf are awesome.

#### CharlieLaub

Paid Member
Long time lurker, first time poster.

I'm DIYing two subs to even out some modal issues in my studio control room. My current monitors are Barefoot MM35s - 3 way monitors that use dual opposing subs. Low end is tight, articulate and lovely. There is hardly any cabinet vibration.

I've been thinking about using a force cancelling design for my DIY subs. If one enclosure hold two 9" drivers, would that equal the output of a similarly speced 18" driver?

Assuming Xmax is the same, then you need the cone areas of the two smaller drivers (a) to sum to the larger one (A). Using some math:

2*pi*(0.5*a)^2=pi*(0.5*A)^2
2*a^2=A^2
2^0.5*a=A
1.41*a=A
a=A/1.41
a=A*0.71

So, as you can see that two driver with about 70% diameter of a larger one, and having the same Xmax will provide the same max SPL at a given frequency. For an 18" diameter driver 0.71*18=12.75 but there is no driver of this nominal diameter, so two 12" would be a bit less, and two 15" quite a bit more.