With a multiple speaker hookup, it is necessary to consider impedance. This is the electrical resistance of the voice coils of the speakers. The lower the impedance, the more electrical energy is applied to the speakers. This is why specifications on power amplifiers will show a given power rating into an 8-ohm load, and a considerably higher power input into a 4-ohm load. If a loudspeaker were 100% efficient, all of the electrical energy delivered to it by the amplifier would be converted into sound energy.

Unfortunately, speakers are not 100% efficient, in fact even the highest quality speakers in the best designed enclosures are not more than 25% efficient.

The electrical energy which is not converted into sound by a speaker is converted onto another form of energy--heat. This heat must be dissipated at two points: the voice coil of the speaker, and at the amplifier. Excessive heat at either point can cause damage to the sound system.

What is impedance? Impedance is how much a device resists the flow of an AC signal, such as audio. Impedance is similar to resistance which is how much a device resists the flow of a DC signal. Both impedance and resistance are measured in ohms. Hold a piece of stiff cardboard out at arm's length and wave it slowly back and forth. Now do it faster. Notice how the air mass is impeding its movement? The same thing happens with cones and even diaphragms to a lesser degree. This is physical impedance - mechanical resistance in technical terms. It gets translated into impedance in speakers by a few other factors including capacitance, inductance, reluctance and reactance. Woofers, for example, tend to increase their impedance at higher frequencies. The higher the frequency goes, the higher the impedance goes and, as a result, the less power the amplifier can deliver. Tweeter and horn diaphragms suffer somewhat less from this process because their surface areas are small and therefore encounter fewer air molecules. Conversely, an 18-inch woofer's impedance may start going up at just a few hundred Hertz. At 1,000 Hz, its impedance could be as high as 12 ohms with the amplifier delivering 60 to 70 per-cent less power to it at that frequency.

If you look at the frequency response graph of a woofer you'll notice considerable "high-frequency rolloff" which reflects the way the line slopes downward thoughout the higher frequencies. That's impedance at work. To verify your findings, look at a woofer's Impedance curve (looks a bit like a frequency response graph in reverse). You will see how the line begins to curve upward as you look from left to right. That is the woofer's impedance climbing as the signal frequency goes higher. You'll also notice something else on the impedance graph - a tall, narrow "spike" over on the left side, down in the low frequencies. This reflects the woofer's natural "resonance". What happens is, speakers, like all things which work by vibrating, have various physical factors that cause them to favour certain frequencies. When a woofer receives an amplifier signal at its resonant frequency, it wants to move farther in and out than at other frequencies. But in the process of doing so, the woofer's voicecoil cuts added lines of force from the magnet and generates extra "counter-EMF".

As mentioned earlier (see Slew Rate & Damping Factor under the Power Amp) counter-electro-magnetic force is the voltage induced in a voicecoil because it is moving back and forth in the magnet's field. This raises the impedance whenever the speaker tries to reproduce that specific frequency. Additionally, the air load of the enclosure (sealed or ported) is the main force on the speaker cone, hence the size of the enclosure and its ports affect impedance. Even applied power affects impedance. Normally the woofer's magnet acts as a heatsink for the voicecoil, but if high power levels are applied for long enough, the voicecoil warms up the magnet and, as a result, the voicecoil gets even hotter. This increases its resistance then up goes the impedance and down goes the applied power. { TIP - If you notice the bass response sounding a little weaker as time passes during a performance, it could be due to this heating effect. Your first reaction might be to boost the low EQ frequencies slightly which is fine, but watch out for signs of amplifier clipping.}

So, what is impedance? Well, it's partly the electrical resistance of the woofer's voicecoil plus the speaker cable, partly the mechanical resistance of the woofer's cone operating within the enclosure and/or horn, partly counter-EMF generated by the woofer's voicecoil moving in the magnet's field, partly the crossover's resistance, inductance, etc. and partly a few other things. One thing impedance is NOT is "fixed". The only time an "8-ohm" enclosure is likely to register exactly 8 ohms on a meter is either going to be when the meter puts a little DC (battery) current through it and the enclosure turns out to have that exact DC resistance (very uncommon) or in those instants when the audio program (music?) produces the frequencies which cause the speaker to operate at exactly 8 ohms.

How do manufacturers determine a speaker enclosure's impedance? First it is designed to operate at that impedance ON AVERAGE, then it is measured while operating to verify the load. { TIP - When shopping for subwoofers, be sure that you find out the manufacturer's recommended crossover frequency. This is important because the impedance of a subwoofer tends to go up quickly when it receives signals higher than those in its recommended range. As a result the average impedance will be higher than you would expect and the amplifier will put out less power. In other cases, the impedance may actually go down within a range of frequencies above the recommended range which would reduce the average impedance and possibly endanger the amplifier. Needless to say, it's wise to make sure that your electronic crossover is working at the right frequency (most of them have Frequency controls which are not very accurate).

Have a technician check it out or you can do it yourself if you have a pink noise generator and a good quality realtime frequency analyzer.} PARALLEL LOADS (MORE THAN ONE SPEAKER PER AMPLIFIER CHANNEL) Calculating parallel loads is an important capablility for two main reasons; first, because dual speaker connections whether on an amplifier, a mixer/amplifier or a speaker enclosure are all wired in parallel. Some people think that if you run separate speaker cables from each speaker output on the amp or mixer/amp to the enclosures you somehow "avoid" putting the speakers in a parallel circuit. Others think that if you run a speaker cable from one cabinet to another you put the cabinets in "series" and that just adds the two loads together (eg., two 4-ohm speakers in series = 8 ohms). But the truth is that everything gets put in parallel. In fact it's quite difficult to put speaker enclosures in series - you need a special wiring harness.

The other reason for needing to know how to calculate parallel loads is because amplifiers don't like running into loads which are too low. As mentioned in the Amplifier section, they will usually shut down if the load is too low and some of them may actually sustain damage.

THE FORMULA 1/R = 1/R1 + 1/R2 + 1/R3 + etc. ("R" = ohms) EXAMPLES Say you have two 4-ohm enclosures, an 8-ohm enclosure and a 16-ohm enclosure all in parallel. Of course you would never do such a thing because the lower-impedance speakers will get more power and be louder than the others (you figured that one out yourself, right?), but this is just an example. The solution goes as follows: 1/R = 1/4 + 1/4 + 1/8 + 1/16 = 4/16 + 4/16 + 2/16 + 1/16 = 11/16. Therefore R/(1) = 16/11 = 1.4545 ohms. If you're not into finding "lowest common denominators", just get out a calculator and turn everything into decimal equivalents as follows: 1/R = .25 + .25 + .125 + .0625 = .6875 Therefore R/(1) = 1/.6875 = 1.4545. To keep life as simple as possible, most people put enclosures of the same impedance in a parallel circuit. If you do this it's all just a matter of dividing that impedance by the number of speakers. Example; four 16-ohm loads in parallel = 16/4 = 4 ohms. Similarly, two 8-ohm loads in parallel = 8/2 = 4 ohms. The following is a quick reference listing of some commonly used parallel loads: ("R" = ohms) 2 x 16R loads = 8R 2 x 8R loads = 4R 2 x 4R loads = 2R 3 x 16R loads = 5.33R 3 x 8R loads = 2.67 R 3 x 4R loads = 1.3R 4 x 16R loads = 4R 4 x 8R loads = 2R 4 x 4R loads = 1R.

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