Your fuel pump analogy is a perfect illustration of this. When you turn on the signal lights, the overall current demand on the alternator/battery combination jumps and the voltage it produces drops slightly. You notice the sound of the fuel pump changing to a lower pitch as it receives less energy. How do you prove this? Simply connect a digital multimeter set to the DC voltage setting across the battery posts and watch what happens as you turn on your signals-with every pulse, the voltage swings between two voltage levels due to the load change. Not only is your fuel pump sounding different, but your audio system is also adjusting for the shifting voltages.
But here's the difference: Your amplifiers have internal capacitors that store extra energy for just that situation. I probably shouldn't have made it look like I think adding capacitors to headlights is a must, since my lights blink and I don't mind-it just means my audio system is taking the power it needs, and that is what I'd prefer. However, for someone that can't live without perfectly stable lighting, the capacitor is the most common "quick" method of compensating for the alternator and battery variations.
In reference to the risk of shock, that's also not possible. A capacitor will only charge up to its supplied voltage and 14 to 15 volts, which is well below a level that would cause a shock. However, using an inductor-based "noise suppressor" or "choke" is a different situation. In the case of an inductor, the application of 12 volts can cause a spike of several thousand volts to be produced. I've been jolted more than once by connecting or disconnecting an inductor on a circuit that wasn't purged. It's not much worse than a bad shock from static electricity buildup when you touch a doorknob-although it always startles me on those occasions where I get tricked.
So you can rest assured, there are no safety concerns to worry about, and thanks for another great illustration to use with my car audio students.
Q Thank you for the great magazine. The only complaint I could make is that I read it cover to cover in a week and have to wait three weeks for another issue. My question pertains to the Alpine SWR-1222D subwoofer. The company recommends a sealed enclosure volume of 0.7-1.0 cubic feet. Where in this margin should I shoot for when building my enclosure?
I plan to build the enclosure using the Perfect Match how-to article in the May '08 issue. I know that MDF is used for its density but plywood is stronger. Would building the enclosure out of 1/2-inch MDF and then covering the outside with 3/4-inch plywood have any benefit or just be a waste of time and cash?Thanks,Sam DeVaney
A When you start talking about sealed enclosures, in most cases the margin is pretty wide. One of the main advantages of a sealed design is that it often will allow variations of as much as 50 to 100 percent and still sound great. If you have a manufacturer-recommended cabinet volume of 0.7 cubic foot, you could likely stretch it to nearly 1.4 cubic feet without major problems, so anywhere within the Alpine recommended range is fine.
What changes when the volume of the enclosure is varied? The first characteristic is the system's Q-factor, which is a dimensionless calculation that loosely describes the characteristic of the resonance of the box and speaker combination. Typically, reducing the size of the box will cause the Q to rise, and the sound will begin to get a bit "boomy." As this increases, the accuracy of the bass gets looser and the point where the boom reaches a peak rises in frequency. A maximum Q of 1.5 is still considered acceptable for car audio.