Liasom,
Remember that just because your handheld DMM has a resolution of 0.1 ohms doesn't mean that is has an accuracy of 0.1 ohms. Most meters with a resolution of 0.1 ohms will have an accuracy of +/-0.1 ohms +/-3% of the full scale range.
A decent handheld meter with a resolution of 0.01 ohms, is a Fluke 76. It is a true RMS meter also. You have to calibrate it for the 0.01 ohm resolution everytime you change functions or power it up, but it will do it. I think you can get it for around $130.
Remember that just because your handheld DMM has a resolution of 0.1 ohms doesn't mean that is has an accuracy of 0.1 ohms. Most meters with a resolution of 0.1 ohms will have an accuracy of +/-0.1 ohms +/-3% of the full scale range.
A decent handheld meter with a resolution of 0.01 ohms, is a Fluke 76. It is a true RMS meter also. You have to calibrate it for the 0.01 ohm resolution everytime you change functions or power it up, but it will do it. I think you can get it for around $130.
The most accurate measurement can be done using the 4-wire method. Drive 100 mA DC current through the speaker. You can connect the multimeter in series and measure the current precisely. Then measure the DC voltage across the speaker terminals with the same or another multimeter, and calculate. This way you don't include the multimeter probe wire resistance in the measurement. Also it is more realistic, because the speaker coil resistance is temperature dependent. Driving 100 mA through the coil approximates normal operating conditions.
Thanks! 
for all the responses. After reading them I went to the Fluke website to see their 2007 products and then to Amazon to get a ballpark for pricing. Fluke has three handheld True RMS models - 179 (~$200), 87v (~$350), and 187 (~$400). As far as I can tell the 179 MIGHT work, but when compared to the other two it seems like a waste of money. Might as well shoot for the 87v or 187 at that point.
That said - I only have one project I want to do for the near future that will be stereo loudspeakers with a mono sub to augment the low frequencies. My budget is ~$1000 for the loudspeaker system. I plan to build the stereo pair from an existing design (Zaph's L18 as sealed version). The T/S measurements I need are for the sub driver (NHT 1259 I purchased used). My feeling is I don't need to spend $350-400 testing a $150 woofer that I bought for $100. I have a some engineer types as friends. Maybe I can talk one of them into helping if I buy the beer...
...and, of course, some more beer when I need to test my crossover components to insure they are within tolerances.
Mike
for all the responses. After reading them I went to the Fluke website to see their 2007 products and then to Amazon to get a ballpark for pricing. Fluke has three handheld True RMS models - 179 (~$200), 87v (~$350), and 187 (~$400). As far as I can tell the 179 MIGHT work, but when compared to the other two it seems like a waste of money. Might as well shoot for the 87v or 187 at that point.That said - I only have one project I want to do for the near future that will be stereo loudspeakers with a mono sub to augment the low frequencies. My budget is ~$1000 for the loudspeaker system. I plan to build the stereo pair from an existing design (Zaph's L18 as sealed version). The T/S measurements I need are for the sub driver (NHT 1259 I purchased used). My feeling is I don't need to spend $350-400 testing a $150 woofer that I bought for $100. I have a some engineer types as friends. Maybe I can talk one of them into helping if I buy the beer...
...and, of course, some more beer when I need to test my crossover components to insure they are within tolerances.Mike
juergenk said:
juergenk, thank you for this and your other advice. I plan to use my existing DMM to perform the testing and then do the calculations. Then try to get one of my friends to assist me in duplicating the tests with their more capable, accurate, and (hopefully) calibrated DMM and see what the difference is. I completed a ~1 cu.ft. sealed test enclosure today and am making a stand for the free space testing this weekend. I'll report back when all that is done so we can see what happens!
Mike
Thanks oshifis and Ron E for the details on the process to measure and calculate resistance accurately and to AndrewT for the caution about measuring frequency and amplitude at low frequencies. BTW I've read and plan to use Rod Elliott's process description as a guide.
Mike
Mike
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