Well it is actually, really and truly NOT a good thing to do. It is the old impedance gotch'a that makes things get ugly in a hurry. IMPEDANCE? What's that? Impedance is AC resistance and differs from DC resistance in that impedance changes with frequency. DC resistance is only valid at DC, and since DC is a bad thing to have in your audio circuit path DC resistance is not particularly useful in looking at loudspeakers. Say What!!! ....................... DON'T PANIC.
I would like to toss out a couple of tips on how a loudspeaker's impedance behaves and what that means to us as sound system operators. I've done it, you have probably done it and so have most of the old timers out there too. That's right! I connected too many speakers to an amplifier and blew it up or caused it to protect itself. Of course the amplifier left the scene of the party at the most opportune time - right!?!
Here are some points on loudspeaker impedance which may be of interest to you.
First off , a loudspeakers impedance is everything but consistent across its frequency range. That is why when you look at a speaker specifications they say something like "NOMINAL" impedance 8 ohms. Nominal means, somewhere around, about, average, could be, that's what we want you to believe, we think it is or just about anything. In fact a loudspeaker impedance may have very different impedance values at various frequencies. Take a look at the low frequency speaker impedance curve in Figure-A.
Note that the high point on the curve is 17.5 ohms while the minimum impedance is 4.95 ohms. It is the minimum impedance that is the problem child. This loudspeaker was listed as an 8 ohm speaker. You will note that the impedance value is below 8 ohms for all frequencies between 100 to 3500 Hz. The amplifier will see less than 6 ohms between 125 and 1000 Hz which is certainly within the mid band of the audio program. What is important to note is that the actual load at a particular frequency represented to the amplifier is dependent on the impedance at that frequency. So if an entire band of frequencies is, say 4.8 ohms, the load to the amplifier is 4.8 ohms or less. What this means is if we paralleled two of our alleged 8 ohm speakers and expected to have a 4 ohm resultant load we would be real wrong. In actuality the load would be closer to 3 ohms for the frequencies between 125 and 1000 Hz. That would be 1.5 ohms for four speakers in parallel. Just for reference Figure-B is a low frequency ported speaker's impedance response.
Note the double hump and small dip in between them. This is typical of any ported loudspeaker. Band pass woofers have three humps.
Figure-C is a 1" high frequency driver on a small horn. The intended bandpass of this driver is 1.2 kHz minimum on the low end, out to 20 kHz on the high end. It is rated at 8 ohms but the majority of the impedance response is below 6 ohms.
In Figure-D you see an impedance curve of the Mid/High section of a touring loudspeaker whose low frequency crossover point is at 250 Hz, active, and the high frequency is passive.
Note the curve is reasonably close to 8 ohms with the exception of a dip around 1050 Hz. This dip, however, caused an amplifier to clip before the low frequency side when four loudspeakers were parallel wired. It doesn't look like much but it is in the middle of the vocal range where there is a high energy concentration. This information has changed the way I look at load impedance in regards to amplifiers. You can not take the "8 ohm" thing for granted.
When you are parallel wiring loudspeakers, calculating the resultant load can be easily accomplished. If each of the loudspeakers is rated at the same impedance, for instance 8 ohms, you can simply divide the impedance by the number of devices. Two 8 ohm speakers: 8 divided by 2 equals 4, 4 ohms. Three 8 ohm speakers: 8/3=2.6666 or 2.67 ohms. It is when the loudspeakers are of different values that things get a bit more complicated. To calculate the resultant impedance: take the invert of the individual values, sum them together and invert the sum. EASY....right? If you need to do more than 20 speakers in parallel and are not using a 70 volt system, go find a large hammer and whack your foot with it.
Try to avoid loading amplifiers too low. If the amplifier is rated for 4 ohms minimum, don't give it a 2 ohm load - bad things usually happen. Even when the amplifier is rated down to 2 ohms remember that in order to keep up with the power the circuit will have much higher current than before and the wiring will have to handle it. Not only will the wiring losses grow but the damping factor of the system will be degraded. It might be better to run separate cables from the amp to the speakers or divide the load across two amplifier channels.
Do not series-wire speakers in order to bring up the impedance. Yes it works, kinda. You will usually buy more trouble than it is worth and that doesn't even begin to cover the performance degradation with series wired speakers. I'll save the wiring discussion for another time.
So what does it all mean? It means 8 ohms ain't. You should check out the impedance versus frequency response of the loudspeakers you are using to be sure that there are not any surprises. Watch out with stage monitors especially, it is really easy to loose track of how many speakers are on a particular amplifier channel. So try to make good choices when it comes to loudspeaker wiring and watch out for the impedance gotch'a.Copyright 1997. Taipale Media Systems, Inc. All Rights Reserved
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