What if we have a self-powered subwoofer?
In this case, our loudspeaker wire is probably around 14 gauge, and since the amplifier is in the loudspeaker enclosure, it is probably less than a couple feet long.
Assuming the manufacturer is connecting two 8-ohm loudspeakers to the amplifier, and 14 gauge wire has a resistance of .00256 ohms/ft times 21 equals 0.00506 ohms of resistance, and our amplifier has a damping factor spec of 400, what do we get?
Wow! Now that’s a significant difference! Kind of supports the idea of using self-powered subwoofers, or at least putting the subwoofer amps as dose as possible to the subs.
So we’ve looked at the differences in the size and length of our wire and the differences in hanging one loudspeaker on the line versus two to change the impedance of the line.
What if we choose an amplifier with a higher damping factor spec, say 3,000? That’s a big difference, so we should see a much higher damping factor in our circuit, right? Assuming this amplifier can drive a minimum 2-ohm load, we find the output impedance would be 0.001 ohms.
Plugging the numbers into our single loudspeaker with 12 gauge wire system, we get:
Hmm, not such a big deal. That higher amplifier damping factor only improved our system damping factor by 0.31 over the amplifier with a DF spec of only 400.
What if we use the amplifier with the 3,000 DF spec in our self-powered sub with 2 feet of 14 gauge wire?
Remember our calculation using the 400 DF amplifier was 264.55, so now we start to see when the amplifier spec becomes significant.
Essentially, in sound reinforcement systems where we have some significant length of wire between the amplifier and the loudspeaker, the amplifier DF spec has little affect on the performance of the system.
So what have we learned? In live sound reinforcement systems, damping factor is really driven by the length and size of our wire and the impedance of the loudspeakers we connect at the other end.
Since damping factor mostly affects low frequency, we should endeavor to keep our subwoofer loudspeaker lines as short as possible and/or use larger gauge wire. We should keep the impedance of the connected load as high as possible by connecting only one transducer per wire instead of two.
So is more amplifier damping factor better? As one of my colleagues recently said, “Sure! If the loudspeaker terminals are welded to the amplifier output terminals!”
Well, maybe he overstated it a little bit, but yes, as long as the loudspeaker wire is really short, then by all means!
Jerrold Stevens serves as director of the Application Support Group (ASG) at EAW. Previously, he was principal consultant with PMK Consultants in Dallas. He has more than 25 years of pro audio experience, including contracting, independent sales, live sound and studio engineering, and audio system consulting and design.