speaker system impedance measurements

[jcaut]

Given all the discussion here lately about whether or not certain speakers are "difficult" for an amp to drive, I thought I'd post this here.

I like to build speakers (or at least, try to), and I was using my impedance test setup to check out the box tuning on my latest project this evening. I decided to round up some of the speakers I have here in the house and test them. The results were not surprising to me, as there's nothing unusual about them, but I thought some of you might find it interesting.

First I checked my RT55i's and RT800i's. Very similar speakers, with the exception of enclosure configuration, and, as one would expect, their impedance curves are very similar. The box tuning frequency is at the minimum impedance between the two hills. The little "blip" at about 175Hz on the 800i, that's not there on the 55i is probably due to a cabinet resonance in the tall, slender enclosures.

The impedance curves for these speakers dip to around 4 Ohms in several spots. These don't look like anything a decent receiver should have trouble with (my opinion), but they might make one work a bit harder than a "typical" 8 Ohm speaker.

[jcaut]

Now here's the curve I measured for a Polk RT3, 5.25" two-way bookshelf, and an Advent Legacy 3 floorstander. The Advent, for some reason, has a dual voice coil woofer, and a switch that allows operation with one or both coils for 6 or 8 Ohm impedance. I only ever used them in 6 Ohm configuration, and that's the way I tested them.

Again, the RT3 dips to a little less that 4 Ohms. The Advent dips to around 5 Ohms, in "6 Ohm mode". Note the single impedance peak for the Advent: It's a sealed speaker. The box tuning frequency is at the "peak", about 50Hz.

[jcaut]

Finally, here's a pic of the project I'm working on, and it's measurement. This is a "test" enclosure, so it's not particularly pretty. It's NOT as crooked as it looks in this picture! There's a 2" port on the rear.

The drivers are 5.25" cast frame drivers, supposedly from Definitive Tech. They are 4 Ohm drivers, and I've got two of them wired in series, for an 8 Ohm load. The tweeter is a D25AG35 Vifa aluminum dome. It's nominal impedance is 6 Ohms.

I've not really gotten started on the real crossover design for these. For this test they had a simple first order network: a coil on the woofers and a cap and a series resistor on the tweeter. I've been listening to them like this and they sound pretty good.

You can see on the graph that the impedance on these barely drops below 8 Ohms. Also the "noise" above 2Khz shows up on this graph because I failed to smooth it on my computer, like I did the others, before I copied it.

[jcaut]

If anyone wants to send me theirs, I will glady give them a test.

Same goes for any LSi's, SDA's, even Carver Amazings. :)

Seriously though, I hope the stuff I posted above helps to show the way that impedance varies in a speaker system. I think most everyone on here knows that, but often there is a tendency to think of an "8 Ohm", "4 Ohm" etc. speaker as a resistive load. In addition, it's not just low impedance that might make a speaker "difficult" to drive: Wild phase swings (the dotted yellow lines) also present a challenge to an amp. I don't have the knowledge to give a good explanation on that topic, however.

Note that I'm not really saying anything about whether or not a given speaker "needs" more power to sound better, just whether or not the speakers are a difficult load, relative to other comparable speakers. I firmly believe that ALL speakers can benefit from better, more powerful amps.

[Charles Rose]

Very interesting information. So, how difficult is it to measure a speakers impedance? What sort of equipment is needed? Id love to make some measurements of my speakers. Granted impedances are listed as nominal values (which I take to mean average). Also, a speakers effciency is important . :cool:

CR