One problem in this industry is most technicians do not even know the difference between impedance, resistance, current, voltage or even ohms law. People wire up speakers in all sorts of way without understanding the effect on amplifiers. Even having some basic knowledge of electrical flow can make a huge difference in the quality of installations. Those who don't know just don't know what they don't know.

I worked for an integrator in '05 and after finishing other work one day, I went to the owner's house, where we were installing a theater. The other installer was doing some work on the front speakers (or whatever) and said I should wire the buttkickers in the second sofa, so I asked "Series or parallel?". He said "Do it the way the others are wired" and seeing that they were parallel, I said "The main wires are parallel and the amp isn't going to work if they're all parallel", to which he barked "Just do it that way!". Next time I was there, the lead installer's first words were "Heads are gonna roll". I asked if he meant because of the way the buttkickers were wired and he said it was, to which I said "I told Geoff that the amp would shut off and he just told me to wire the second set the same as the first- you KNOW damn well that I know how to wire these (he was the car audio installer who worked at a stereo store after I left and we had known each other for years).

The guy who said it was my fault couldn't calculate a series/parallel load if someone had a gun to his head and he needed the whole test period for the CEDIA Installer 1 test. Looked like he was rode hard and put away wet, too. Wouldn't tell me his score, either- I handed my test in 40 minutes after starting, having checked my answers three times- he asked "Give up already?".

Usually instruments that are named VOMs cannot measure low enough AC voltages to give good results.

The circuit I described above had AC outputs in the tens of millivolts. It seems a voltmeter that could read that would have to be a couple hundred bucks. The input to the circuit is the output of an amp with ten volts of output. This would be the voltage level of 12.5 watts into 8 ohms. While that's not a huge wattage, it's way more than the average portable amplifier source would output.

What kind of portable amplifier are you referring to?

How would you measure the free air resonance?

Woofers are tested on a baffle, all the time. No enclosure, just a baffle. That way, an enclosure can't influence the outcome.

Argue with the guy who designed it- John Murphy. He's an EE.

It's a very practical thing to have for troubleshooting.

I used my WT3 and WinISD to design my cabinets with excellent results. I also had some drivers which were old enough that the specs are no longer available or almost impossible to find and was able to use them, rather than let them sit or waste time with trial & error.

About measuring low AC voltages...

This meter ([Link: amazon.com]) measures AC and DC, but note that the lowest AC scale is ten volts full scale, while the lowest DC scale is 0.1 volts full scale. We're so used to using such meters for DC that we're not likely to have spent much time thinking about the AC voltage measurement limitation.

This comes from the fact that all (analog) meters are actually DC meters. To measure AC at high voltages, a diode can be used to simply convert "AC" to "DC." The problem is that diodes have voltage drops around 0.7 volts, so if you're measuring voltages in that range, the meter has to compensate for the non-linear voltage drop introduced by the diode. To accurately measure low AC voltages, a circuit has to be included that compensates for this. This is not cheap!

Why would you use an analog meter?

Check out the one in this link-

[Link: amazon.com]

Good question. That's just the first meter I ran across.

Well, things have definitely improved!
About five years ago I attended a seminar put on by Middle Atlantic, with Bob Whitlock on site to add comments and be sure the MA guys told us everything... about grounding. Grounding troubleshooting can involve very very low AC current measurements. At that time the meter they recommended looked very much like the one you link to, but was about $250.00.

What is the frequency response of the unit?

Actually, for many measurements I prefer an appropriate analog meter.

At one point, in cooperation with a manufacturer, we held a "clinic". The national service manager came to town to participate. Early in the day he criticized me for not providing a DVM for the low level measurements. After he got to know the instrument, he admitted to liking my "old fashioned" analog meter. (1mv full scale, this was not a cheap meter)

Yes, you can easily find a digital meter with four or five digits, but two or three of the digits are usually bumbling around, forcing one to mentally average the bumbling. An analog meter will automatically smooth over a little noise.

This is EXACTLY my point. If one duplicates the impedance measuring circuit I outlined above, a very accurate at low voltage levels meter -- and I mean AC, not DC -- is required.

The parenthetical comment is a very important point.

Yeah, but if your AC source is a power amp that can output 10 volts, the "little noise" is much littler than with a signal generator of some sort.

I'd love to hear from Fins as to why he wants to measure speaker impedance. It'd be a bit funny if all this discussion resulted from him wondering if there's a simple way to figure out if he's holding an 8 ohm or a 4 ohm midrange in his hand.

You mean Bill Whitlock, right?

Wasn't it you who has written that if the ground loop is around a Volt and no hum is audible, that it's really not a problem?

Yes.

Never. A volt is about the same voltage range as a line level signal so there's no way that could even happen.

I have said this: our equipment has ground loops all the time but we don't even notice that they exist unless there is a hum problem. Normal stereo analog audio wiring has two shields and two wire paths, so is, by definjtjon a ground loop. Yet we don't think of that wiring that way because there's no problem.

Totally agree. And coincidentally, I actually used to own a pair of 804Ss before I "upgraded" to the smaller 805Ss. The 804Ss were really barky in the mid. B&W didn't really make any improvement on that until the Diamond range was launched. However, I was driving them with my Gryphon Callisto 2200, so fluctuations in impedance were never an issue.

But, back to testing, I can guarantee that outside of carrying an impedance tester that does a full sweep, the 1kHz tester is going to give a far more accurate result than a multimeter, which is what I've seen countless "techs" use incorrectly over the years.

“Ask them what time it is, and they’ll build you a clock.” I know you guys are right, but damn! I’m happy Fins asked the question in the first place- and I’m assuming he did what the rest of us would have done in the situation (VOM). Most folks would’ve just hooked up all the speakers they could find to some poor little amplifier.

Regardless, this turned into a fascinating conversation. Should I ever explore building my own amplifiers and speaker cabinets this thread will come in mighty handy.