Measuring Drivers TB-871S - Help!

[daatkins]

The big difference between this automated method and the two manual methods is the actual impedance at Fs which was around 120 ohms in contrast to the 43 and 72 ohms of the other two methods.

Hmmm... maybe replacing the driver with a 120 ohm resistor might be a good idea to check that SW's calibration is okay. Do a impedance sweep and a straight line around 120ohms would tell you if the cal is acceptable.

Nice one,
David.

 

[gregorx]

Passive Component

Hi David,
I did use the Measure Passive Component using my calibration resistors 10.0 and 14.9 ohms and they were right on. I think SpeakerWorkshop is correct and the other methods or my implimentation of them were not correct for impedance.

It was nice to see the Fs match though with at least one method.

Thanks again for the SpeakerWorkshop tutorial sites. It really helped to get the calibration and levels.

Gregory

 

[ScottRHinson]

Re: Hi All,

Thanks for all the help on this. I have successfully measured the free air impedance of all six drivers using SpeakerWorkshop see attached picture. The numbers on the drivers are the serial numbers.

This method, in comparison to the two manual methods that I used before was of course much easier and interestingly provided Fs results very similiar to the Impedometer. The average Fs for the six drivers (all from the same production batch) was 140hz. The big difference between this automated method and the two manual methods is the actual impedance at Fs which was around 120 ohms in contrast to the 43 and 72 ohms of the other two methods.

Still with this method the Qts are very high ~1.

I will build the Sample Test box today. According to the Weem's Book the standard box should be 8"x8"x5". Does this sound correct?

I've got to get somewhere, but I wanted to make some comments, so these are going to be quick...sorry! 🙁

I find the added mass method more accurate. Stick U.S. nickels to the cone with blue-tak. They weigh 5 grams each.

The impedance plots look way to noisy for a decent curve fit...are you sure it's set up right?????

Scott

 

[gregorx]

Nickel Bag

So.. I did both a sealed box and added mass measurements today and confirmed the calibration as a power outage wrecked the sound card settings. The calibration showed correct values for anything I put in the measure passive component option. I also tried David's suggestion of running the impedance for a 120 ohm resistor and it did show 120 ohm but it was not a straight line.

I am using 65K samples at 48khz and this does make the graphs more bumpy. Is this what others are doing? I also tried smoothing these graphs to 1/16 an octave and this seemed to have an effect but nothing dramatic.

For the sealed box I used a 1.23 liter box with the driver mounted on the outside facing into the box. It calculated 1.275 liter and then calculated, using Scott's suggestion, with a single nickle of added mass and it calculated Vas as 1.134 liters. Remeber these are 3" full range drivers.

So... as expected Fs is higher but Vas is lower than the published specs per David's link to another thread above. My problem as I see it is still Qms which is currently over 250% of the TB published values and CSS published values both around 6.5.

I manually calculated it using the graphs in Speaker Workshop and it is still 16.67 or so which is killing my Qts >.88

Qms seems to be related to the FWHM of the impedance curve and I am not really sure how to more accurately measure this.

So here is the question, I have three sets of parameters. I have the TB published specs, the CSS published specs and my own measured specs. Which should be used to design the box?

 

[kelticwizard]

Hi All,
I have begun measuring TS parameters on my 6 TB-871S and I am getting strange results from just the first pair.

First I used the 1Kohm resistor Impedometer and got very distinct impedance plots with Fs = 139! and Zmax = 72 and 61. F1 = 118 and F2 = 164 . I used 8mv and an 8ohm calibration resistor. More Info Here

Next I tried the method at Rod Elliot's Site with the same drivers and got Fs = 121hz ,Zmax=43 ohms and F1 = 105 and F2 = 140 Here I used an 8ohm series resistor and used 600mv as the Vs.

A) How much did you break the drivers in? That makes a difference.

B) The difference between 139 Hz and 121 Hz is only 20% of an octave. Believe it or not, that is fairly close. And I will bet that the TangBand with the lower Fs also has a larger Vas. Also, the midband SPL comes into it.