# SPKR Impedance - Vas measurement issue

**URL:** <https://forum.quantasylum.com/t/spkr-impedance-vas-measurement-issue/2240>\
**Category:** Uncategorized\
**Created:** [March 7, 2026, 8:55pm UTC](https://forum.quantasylum.com/t/spkr-impedance-vas-measurement-issue/2240 "2026-03-07T20:55:05Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![AVO](https://avatars.discourse-cdn.com/v4/letter/a/b2d939/32.png) [@AVO](https://forum.quantasylum.com/u/AVO)\
**Post date:** [March 7, 2026, 8:55pm UTC](https://forum.quantasylum.com/t/spkr-impedance-vas-measurement-issue/2240/1 "2026-03-07T20:55:05Z")

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Hi

today I was using the QA403 (SW release 1.222) and the QA461 transducer driver to measure the Thiele Small Parameters of an Audax HM210G6 woofer chassis (new chassis, no burn-in). I was using the SPKR Impedance automated test.

According to the datasheet (see [https://www.audax-speaker.de](https://www.audax-speaker.de)) the Vas value of the chassis is 85 liters. With the SPKR Impedance automated test (added mass method), I measured the Vas at 10.279 liters. Although TSP measurements of loudspeaker chassis usually show quite some deviations from the datasheet, this result seems wrong. I have used a spreadsheet to verify the Vas calculation.

Formulas used are:

| Cms = (fs^2 – fsm^2) / (4 \* Pi^2 \* fs^2 \* fsm^2 \* m) |
| --- |

| Vas = Rho \* c^2 \* Sd^2 \*Cms |
| --- |

(fs and fsm: resonance frequency w/o and with added mass, Sd: effective area of cone, c: speed of sound, Rho: density of air)

The result of my Vas calculation is 97.08 liters. This seems within the typical error range of such measurements. I re-verfied the Vas calculation using the ls\_param spreadsheet from the Elliott Sound Products website ( [Measuring Loudspeaker Driver Parameters](https://www.sound-au.com/tsp.htm) ) - Vas calculated as 98.09 liters.

It seems that the SPKR Impedance automated test has a bug in the Vas calculation. Does anyone else have experience with this automated test?

Best regards

Andy

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**Author:** ![matt](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@matt](https://forum.quantasylum.com/u/matt)\
**Post date:** [March 12, 2026, 2:46pm UTC](https://forum.quantasylum.com/t/spkr-impedance-vas-measurement-issue/2240/2 "2026-03-12T14:46:39Z")

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Hi @AVO, you should be able to back up and find the parameter that is offending, yes? That is, if VAS is wrong, is it because Rho or Cms. can you share the HTML report maybe as a PDF?

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**Author:** ![AVO](https://avatars.discourse-cdn.com/v4/letter/a/b2d939/32.png) [@AVO](https://forum.quantasylum.com/u/AVO)\
**Post date:** [March 13, 2026, 12:06pm UTC](https://forum.quantasylum.com/t/spkr-impedance-vas-measurement-issue/2240/3 "2026-03-13T12:06:31Z")

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Hi Matt

thank you for your response. My spreadsheet results are the following:

 ![image](https://canada1.discourse-cdn.com/flex032/uploads/quantasylum/original/2X/c/c53fbf9b737f1f1132d43153531a17b9832f0bc0.jpeg)

The spreadsheet is using SI units for calculation.

The input values for fs and fsm were taken from the attached measurement report.

 ![image](https://canada1.discourse-cdn.com/flex032/uploads/quantasylum/original/2X/6/6aeb58517866266be3d6763d23faefa685ff9409.png)

 ![image](https://canada1.discourse-cdn.com/flex032/uploads/quantasylum/original/2X/9/969a8765244b28b033f33da1f691db1378e3d961.png)

It seems that the Cms calculation in SW 1.222 has a bug. If I use the HTML report to calculate the Cms value manually, the result is 1.3718 m/N (1371.8 um/N) - very close to my spreadsheet. I guess this is easy to fix.

Thank you an best regards

Andy