Measured woofer response is way off from calculated response.

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ByronInPortland

ByronInPortland

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Joined 2010
#1
Greetings. I just carefully designed and built a beautiful set of speakers, but when I measure the woofer low-end response, it's very far from the calculated response. Here are the details.
  1. I'm using pairs of CSS WR125 drivers. I got their T/S parameters from the pdf file included in the documents. (see CSS-FR125S-wr125-dimensions)
  2. I enter these parameters into VituixCAD. and modeled a closed box with Qtc of 0.8. It said 18.6 liters for 2 drivers in parallel. (see VituixCAD showing 18.6 liters)
  3. So I build a cabinet with 18.1 liters and stuff it with the recommended amount of Polyfill - 1lb/ft^3 I think. I mount the 2 drivers in parallel.
  4. I made close-mic (nearfield) measurements with the Arta software. I chose one of the two woofers, but kept them both in parallel. I've got a calibrated microphone from Dayton Audio, a nice external power supply with synchronized clocks for the input and output, a nice LM3886 amplifier etc . . Not sure of where to put the microphone, I made several measurements of one woofer from 0.5cm to 5.0 cm from the phase plug, as well as an off-axis measurement. They were all very similar after normalizing absolute SPL levels, so microphone placement is not an issue.
  5. The FRD data shows a big peak above 100Hz and a 14dB/octave slope!:yikes: (see VituixCAD FRD)
  6. I also measure the impedance (of both drivers in parallel) with the Dayton DATS V3. The result says my Qtc=1.18! (see snip of DATS data)
So I'm kinda' stumped, and hoping there's something wrong with my measurements. One thing I'm considering is that my cabinet is too tall and skinny. the cross-section is a trapezoid with a 5" baffle, an 11.75" back and 5.75" deep for a cross section area of 48in^2. Then it's 21.25" tall. Did I accidentally make a transmission line without an opening?
I'm confused and frustrated. Any advice would be helpful.
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  • CSS-FR125S-wr125-dimensions-1 (1).pdf
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  • snip of DATS data.jpg
    snip of DATS data.jpg
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  • VituixCAD showing 18.6 liters.jpg
    VituixCAD showing 18.6 liters.jpg
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#9

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#14
Unless you have something very sophisticated, I think that most software uses a math model of a non-resonant box with one driver in it. You made a very long box with several drivers sharing the same volume. You can calculate the resonance of that chamber using Hornresp software. The drivers resonance, fs, is often higher than what is in the data sheet. I don't know why, but that has been the case for every woofer I have ever purchased. With use the resonance will drop. Studies have shown that when woofers are not used for a few months the resonance tends to go back up. What to do? Well it looks like you could try to segment the drivers into individual enclosures. It would also help to just stuff the segments with dense rockwool, lambswool or fiberglass batting to absorb the sound.
 
#15
ByronInPortland said:
Greetings. I just carefully designed and built a beautiful set of speakers, but when I measure the woofer low-end response, it's very far from the calculated response. Here are the details.
  1. I'm using pairs of CSS WR125 drivers. I got their T/S parameters from the pdf file included in the documents. (see CSS-FR125S-wr125-dimensions)
  2. I enter these parameters into VituixCAD. and modeled a closed box with Qtc of 0.8. It said 18.6 liters for 2 drivers in parallel. (see VituixCAD showing 18.6 liters)
  3. So I build a cabinet with 18.1 liters and stuff it with the recommended amount of Polyfill - 1lb/ft^3 I think. I mount the 2 drivers in parallel.
  4. I made close-mic (nearfield) measurements with the Arta software. I chose one of the two woofers, but kept them both in parallel. I've got a calibrated microphone from Dayton Audio, a nice external power supply with synchronized clocks for the input and output, a nice LM3886 amplifier etc . . Not sure of where to put the microphone, I made several measurements of one woofer from 0.5cm to 5.0 cm from the phase plug, as well as an off-axis measurement. They were all very similar after normalizing absolute SPL levels, so microphone placement is not an issue.
  5. The FRD data shows a big peak above 100Hz and a 14dB/octave slope!:yikes: (see VituixCAD FRD)
  6. I also measure the impedance (of both drivers in parallel) with the Dayton DATS V3. The result says my Qtc=1.18! (see snip of DATS data)
So I'm kinda' stumped, and hoping there's something wrong with my measurements. One thing I'm considering is that my cabinet is too tall and skinny. the cross-section is a trapezoid with a 5" baffle, an 11.75" back and 5.75" deep for a cross section area of 48in^2. Then it's 21.25" tall. Did I accidentally make a transmission line without an opening?
I'm confused and frustrated. Any advice would be helpful.
View attachment 1358224
Click to expand...
#1 says you got T/s from spec sheet. #6 says you measured impedance with DATS.

If you have a DATS and a measurement mic, first step should be to throw spec PDF away. Use real measurements of your drivers for design. Measure complete T/S parameters with DATS. Best method would me to use added mass method, and run T/S calculations right in VituixCAD. For a speaker of this size, about 5g added mass should be all you need, but check that the measurement with added mass has a Fs decrease by at least 25%.

In VituixCAD T/S calculator, solve for "Extended Z" parameters for accurate impedance characterization. Within simulation of the cabinet with your measured T/S parameters, right click on the chart and select "show effect of impedance". Next, adjust Qa and Ql to some value other than 100 to simulate some fill and some leakage.

Example:
1726791153738.png


Now compare results again and see what it looks like.
 
#16
DcibeL said:
#1 says you got T/s from spec sheet. #6 says you measured impedance with DATS.
Click to expand...
Yes. I designed be box using the Manufacturer's specs. Once built, I used DATS to measure 2 drivers in parallel in the cabinet. I measured both top pair and bottom pair. They were identical, as they should be.
olsond3 said:
You made a very long box with several drivers sharing the same volume.
Click to expand...
You can't see from the photos, but the box has 2 18.1 liter chambers, each with 2 woofers in it. that middle "brace" in the photo is a bulkhead separating the upper and lower chambers. The other 4 are just braces.
 
Last edited:
#17
Well, I have good news everyone. Last night I hooked up both speakers to my stereo system (without tweeters), and they sound fantastic! According to my measurements, I should get a boomy sound around 100Hz with nothing below that. NOPE! I didn't hear any boomy-ness, and the bass was surprisingly deep. So my current theory is that I did something wrong in my measurement. And my best guess is that it has something to do with:
  • measuring 2 drivers in parallel when the software is expecting 1 driver, for both the frequency response and impedance.
  • measuring drivers which had not been broken in. I know Cal (the gentleman who sold me these beauties) broke them in for 24 hrs, but that was a "while ago".
I appreciate the advice from olsond3 and DcibeL, but it's a bit late to re-measure the drivers and design a whole new cabinet- especially considering that I like the sound so much. I will save that advice for my next pair of speakers.

My only challenge now is designing the low pass filter for the lower pair. It's going to be a 2.5-way system. I'll break them in for a few days and re-measure impedance and nearfield and see where I am.

Thank you everyone for your advice!
 
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