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More mud
A few days ago I wrote a blog post about impedance. It seems to have stirred more mud than cleared the waters. Might as well go for broke. This post will focus on 4Ω vs. 8Ω speakers and varying impedances with frequency of those speakers. First a bit of a refresher. It is a misnomer to think that 4Ω speakers take more power than 8Ω speakers. They do not. Loudspeakers are rated according to their sensitivity: how loudly they play with a 1-watt input. A 4Ω speaker with a sensitivity rating of 90dB takes the exact same amount of power as an 8Ω speaker rated at 90dB/watt/meter to generate 90dB of sound in a room. Second bit of refresher. Most loudspeaker impedance ratings are nominal (existing in name only). This means that your typical 8Ω speaker is mostly 8Ω but not always. Scrounging around the internet I found the following graph from Stereophile. Take a look at the solid line and ignore the dotted line. The vertical row of numbers is the impedance. The horizontal row of numbers is the frequency. Though the graph shows the speaker's impedance varying between 12Ω and 4Ω, we would say the nominal (average) impedance is 6Ω. Now. let's imagine we have a power amplifier that is capable of producing its rated wattage at 6Ω and above. That would mean that from 6Ω up to anything higher (to the limits of its output voltage), the amplifier could output its full rated power. Call it 10 watts maximum. Third bit of refresher: amps x volts=watts. This means the wattage is a combination of voltage and amperage. The less current (amps) needed (by a rising impedance) the more voltage is required to keep the watts the same and vice versa (just look at the formula to see that). Upon close examination of the amp's spec sheet, we see that at 3Ω our amp still only outputs 10 watts. It doesn't double in power as we would hope. This means the amp is running out of current (amps) and its output voltage is dropping with lowered impedance of the speaker. So what? Here's what. In order to maintain a constant loudness, we must maintain the amplifier's output voltage into the speaker. Since amps x volts = watts, then we have to conclude that as the impedance of the speaker decreases beyond the amplifier's ability to produce more watts, the voltage must be going down and hence the loudness at the output of the speaker. See? More mud.
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Paul McGowan

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