This post is a result of some musings on the PE board about subwoofer excursion. I was convinced that the RS315HF could be overdriven in a 1.8 cu ft box and the Dayton SA240 plate amp. Others weren’t so sure. Turns out Pete was probably more right than I was, as the amp “theoretically” can drive the sub to its max excursion, but not much more.
When thinking of amps and subs, you have to keep track of a couple of things. “Watts” gets thrown around all the time, but it’s also important to keep track of the maximum voltage and current that an amp can deliver, the listed xmax as well as xmech, and the box size. As the box size has effects on current and power driven, as well as excursion.
So let’s take the SA240. Taken from the spec sheet–
Rated Power Output: 150 watts RMS into 8 ohms @ 0.1% THD
240 watts RMS into 4 ohms @ < 1.0% THD
*Based on one-third power duty cycle
The amp is claiming to deliver 240 watts across 4 ohms. This comes out to a peak voltage of 44 volts and 11 amps. Note also the duty cycle is 33%. Meaning it can’t deliver this voltage and load continuously. This means the amp will engage in some sort of thermal shutdown if driven to it’s maximum for any length of time. I’d be curious to know what the maximum continuous power rating is, but it’s not given.
Anyway, lets but the RS315hf into a sealed 1.8 cu ft box and see what happens. Below you’ll see the excursion plot. Yes, Pete’s right, we’re not driving the unit beyond xmax. These graphs are courtesy of Unibox.
So we’re done. Everything’s cool right. Pete’s right. Well, yes, but it’s still helpful to look at some other graphs. Below, you’ll see the voltage and current draw graphs for 240 watts of power.
Yes, I know the voltage plot is a boring, constant line. I put it there to make the point that amplifiers are constant voltage sources. When 240 watts of nominal power is placed across the drivers leads, the current varies with the impedance of the driver. So the actual power dissapated varies akin to the current graph.
In a sub application, the driver would be rolled off above 80, so the current, i.e. power peak would be limited. But look what happens below 20 hz. The current is rising to the maximum limites of the plate amp, which can only deliver this 1/3 of the time anyway.
So technically we are not exceeding xmax. However, the amp and driver are at their limits when pushed hard and there may be some distortion, more from the amp than the driver. Most drivers can exceed xmax by a fair margin, this “xmech” which is when the excursion exceeds the mechanical limits of the driver, not just an arbitrary distortion figure. Usually xmax is defined as the displacement that causes 10% THD, but there are a number of definitions. Exceeding xmax just causes some extra distortion. Exceeding xmech may permanently damage the driver.
It’s informative to look at some other scenarios. Let’s increase the box volume to 3 cubic feet. The air spring is more compliant, so it’s much easier to drive the unit into over excursion.
Also, the current required goes down a little at the low end. It doesn’t take as much current to push against the springier air, so to speak.It’s not much, an amp or so, but it does reduce the amp load.
Now let’s try the opposite, and put it it a tiny 30 liter box.
The air spring is too stiff to allow 240 watts to drive the unit to xmax. The problem is this-
The amp might very well engage in a thermal shutdown if there is any sub 20 hz content even though it’s not putting out much bass in this region–Now granted, there may not be much sub 20 hz content, but if there is, it will be exceedingly hard on the amp. Remember, the amp has a 1/3 duty cycle. so it can’t sustain these 10 amp loads. Just by looking at the excursion curves you’d think everything was fine, but it may not be.
