Sometimes we get questions about our recommendation for the LaVoce 8-ohm driver. The story typically goes that the 4-ohm driver will “pull” twice the power from the amp and since “more power is always better”, the 4-ohm driver is clearly better, right?
As a quick primer on amp power, watts are equal to volts squared divided by resistance:
W = V^2 ÷ R
W = watts
V = volts
R = resistance (or impedance)
An example can illustrate how this works. A Behringer NX6000D will be used for the amp and the drivers will be the B&C 21DS115-4 (nominal 4 ohm) and B&C 21DS115-8 (nominal 8-ohm). The actual resistance of each driver is
B&C 21DS115-4: 2.2 ohms
B&C 21DS115-8: 5.1 ohms
Each channel on the amp is capable of 100 volts unloaded (roughly); and each channel on the amp is capable of sustaining 1200 watts of power continuous (roughly). Any more than that and the amp will “power cycle” (i.e. it will shut down so that it doesn’t destroy itself).
And each channel of the NX6000D amp is limited by either:
Max voltage: 100 volts
Max sustained power: 1,200 watts (roughly based on actual measured results)
The GSG 21” Full Marty Roundover cab will be used for the enclosure.
This is the response of the two drivers at precisely 1 watt each in the cab:
B&C 21DS115-4 = black line
B&C 21DS115-8 = red line
The two drivers have essentially the same efficiency, perhaps the 8-ohm has a hair more, but for all practical purposes, the efficiency across the whole bass region is the same.
So let’s turn up the gain and see how many watts will each driver attempt to “pull” from the amp?
Using the equation from above W = V^2 ÷ R
B&C 21DS115-4: W = 100^2 ÷ 2.2 = 10,000 ÷ 2.2 = 4,545 watts.
B&C 21DS115-8: W = 100^2 ÷ 5.1 = 10,000 ÷ 2.2 = 1,961 watts.
Assuming that the amp could survive under those conditions, this is the response that would result:
In the model, the 4 ohm driver is pulling a little more than twice the power than the 8-ohm driver, so it would be expected to have a little more than 3dB advantage across the board.
However, we need to look further into what is going on.
Recall that subwoofers typically have a large change in resistance (impedance) based on frequency). This is how it looks for these subs (the value on the Y axis is ohms). What can be seen here is that where impedance is lowest (around the tuning frequency 17 Hz or so), the amp will be producing the greatest power and where the impedance is highest (around 44 Hz), it will be producing the least power.
[Side note, guys in the car audio world who “burp” their cabs in SPL competitions will typically do so right on the impedance peak frequency in their cabs because it makes the loudest noise while putting the least demands on the amp.
