Please 'Boom' Responsibly As most of you have noticed, the noise ordinances have become much tougher lately. Most of this is due to idiots, yes IDIOTS, who drive through residential areas with their windows down while their system is playing at full power. To make things worse, the music they listen to has all sorts of foul language that's not suitable for small children, (who may be playing outside). There are even a few people, who are even beyond idiot status, that play their systems at full power through residential areas after 10:00 PM (when many people go to bed). I don't believe that this type of behavior is good for the industry. If the fines get too stiff, people will stop buying large systems. If this happens, more people will get out of car audio (who wants a mediocre system). People get interested in things because they're exciting. A deck and four 6.5" speakers are not going to interest many of the younger car audio enthusiasts. If car audio enthusiasts keep annoying more and more people, the fines will keep getting tougher. All of this will only reduce interest in the equipment that fuels the industry. If you want to listen to your system at full volume, get out on the highway where there's little chance of bothering anyone. When you get to a red light, turn it down. If the only thing attractive about you is your 'system', you have some work to do. Bottom line... Think about what you're doing. Think about other people. It's not the end of the world if you have to turn the volume down for a little while.

Resistance vs Impedance:
Until this time, we only discussed resistance. I said that resistance is the opposition to the flow of current. While this is true, it would be more precise to say that resistance is the opposition to the flow of direct current. Impedance is the opposition to the flow of alternating current. AC causes some devices to act differently than they do with DC. This is true with speakers. If you use a voltmeter to measure the resistance of a speaker's voice coil, you may read something like 3.2 ohms (resistance) even if the speaker has a rated impedance of 4 ohms. The impedance of a speaker is not a constant. It's actual impedance changes with frequency and can vary greatly. Manufacturers give you a nominal impedance, which is generally the average impedance that the speaker will present to the amplifier when driven within the part of the audio spectrum that its designed to reproduce.
The diagram below may give you some idea of the difference between the current flow through a resistor and the changing current flow through a speaker. Keep in mind that the resistor's resistance will not change (significantly) as the frequency changes (as indicated by the lower yellow line). The current through the resistor will also remain constant (as shown by the upper yellow line). You will notice that as frequency changes, the impedance of the speaker changes. As the speaker's impedance increases the current flow decreases. The voltage applied to the resistor and speaker remains constant across the audio spectrum. I will explain speaker impedance in more detail on upcoming pages.

Plotting a Speaker's Impedance:
  • To measure a speaker's impedance you can use a device called an impedance bridge which compares the impedance of the speaker to a reference resistor. Some impedance bridges only work at 1 or 2 frequencies and wouldn't be good for plotting impedance across the bandwidth of the driver (speaker).
  • You could use a tone generator, an amplifier, digital voltmeter and a resistor to calculate the impedance of the speaker but that is time consuming.
  • There are also computer programs that allow you to measure a speaker's impedance across the audio spectrum with your computer. 'Liberty Audio Suite' and Leap/LMS from Linear-X are 2 of the top packages.
  • Parts Express has a device that's supposed to work well (I haven't used one yet). It costs about $US200. This is a link to it.
The 'crossover slope II' page has more impedance plots if you're interested.

You should remember:
1.Resistance refers to the opposition of D.C. current flow.
2.Impedance refers to the opposition of A.C. current flow.

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