Delta mass - Vas too small

[Dave Bullet]

I am using SpeakerWorkshop to measure my drivers via the delta mass method. Problem is estimated Qes and Qms (therefore Qts) are way too low. Vas is also too low for my larger L26 driver.

The two drivers I've measured are a Seas L15 and Seas L26. Both drivers don't appear to have any mechanical issues (ie. no rubbing or scraping noises when driven to excursion) and have been broken in for about 4 hours each using 1/3rd octave warble tone centered on 30Hz with a +/- 10Hz warble offset.

The L15 measurements look reasonable, although Q is too low.

The graph below is the L26. Here's the stats
manufacturer T/S params
Qms = 2.15
Qes = 0.39
Qts = 0.33
Vas = 171 litres

my measured T/S params
Qms = 2.615
Qes = 0.279
Qts = 0.252
Vas = 108 litres

The bottom two impedance curves are Free Air (left) with added mass (right) from the after breakin measurements.

At first I thought the mass I calculated was wrong. Of course a smaller calculated mass than actual used will result in a larger Vas, however I'm using coins with (I assume) reasonably accurate weights. Even if they are off, to get the Vas into the approximate region (ie. 171 Litres for the L26 instead of 108 shown after breakin below), I need to put in about 2/3rd of the actual weight used in the calculation (ie,. I added 36gms of coins so put in about 22 gms into SW to get Vas = 168 litres. The coins simply cannot be that much out).

Any ideas?

Is my breakin period not long enough?
Am I not driving the woofer hard enough to properly breakin? (I doubt it as I believe it is being driven close to its 7mm xmax).
Is it possible to have a Q lower (in my case I believe significantly lower) than the manufacturers after breakin?

You might notice the weight used for before and after breakin differs. I tried doubling the weight after breakin to see if that made a difference, Vas however was still in the low 100s.

Thanks,
David.

[Dave Bullet]

Tried again - this time using a new clamping technique. Driver was not moved / reclamped between measurements. First was free air, then weight was added. This time I added 49 gms to ensure the Fs was lowered more at least 25%.

Another possibility is my sound card is having problems measuring frequencies below 20Hz accurately enough for T/S calcs. I might have to use the delta compliance method.

The graphs look cleaner (no humps / bumps around Fs indicating vibrating weight on the cone, reflections or other movement). So I think these are the cleanest plots I am going to be able to make with the delta mass method.

[Dave Bullet]

Here's the L15 measurement.

Much closer to manufacturers specs and what Zaph measured for his L15 samples (albeit a few years ago) here (L15 T/S table at bottom of web page):

http://www.zaphaudio.com/audio-speaker12.html

[Dave Bullet]

Ok, maybe I am being paranoid, or my Vas accuracy goes down with increasing driver Vas.

Here for example is an 8" woofer. First set of T/S params are delta compliance measured in a 14.5 litre box. The second are delta mass with 7.6 grams added.

Vas in both cases is pretty similar

[Svante]

I have seen several cases where the Cms value is pretty far off the datasheet value. This typically results in a higher fs and smaller Vas.

Several manufacturers of drivers are reducing the cost of their drivers, and that can result in eg stiffer Cms and also different cone break-up behaviuor. I recently saw this in a Peerless driver. I don't know about Seas, though.

Anyway, the value of Cms is usually not as critical as one might think. Cms has the greatest effect below the system resonances, and this range is usually the least interesting in loudspeaker design. This is not always easy to understand when looking at "high level" parameters, such as Vas, Qts and fs. A variation in Cms affects them all, and the co-variation leads to rather small differences above fs. In this case it might be easier to understand the effects through Mms, Cms and Rms/Rme instead.

[Dave Bullet]

Thanks Svante,

Well - I did a few more hours breaking in, and different mass and results are very similar - infact Vas hasn't moved at all.

Previously after 3 hours of breakin:
Fs = 24.34Hz
Qts = 0.375
Qms = 3.06
Qes = 0.42
Vas = 115.2 litres

with 49gms mass added

After another 4 hours breakin
Fs = 24.39Hz
Qts = 0.389
Qms = 3.166
Qes = 0.44
Vas = 115.2 litres

with 38gms mass added

One more test... I'll get out the other L26 and see how that compares.

Otherwise, I'll conclude my L26's have a much smaller Vas than published and will design accordingly.

Cheers,
David.

[oldbar]

I am in the middle of a seas project using the L26 and the vas is around about 142 Litres from memory.
Would you like me to post my T/S Parameters when I arrive home later on ?
Also my delta mass was 76grams.

[Dave Bullet]

Hi oldbar,

Yes thanks to posting your T/S params. That would be helpful.

I did get a 144 litre Vas at one point, but that was a one off, while hand holding the driver. I threw the measurements away thinking it was an erroneous measurement.

Out of interest, could you please post a little about your project with the L26, especially your enclosure type and xo point and low pass slope for the L26.

Cheers,
David.

[oldbar]

Here are my measurements

[Dave Bullet]

Hi Oldbar,

I also notice your Q and Fs are in the same region as mine.

I think your Vas is artificially high. Your effective area is 397cm2 - when the L26 is actually 330cm2 (you have to remove the phase plug area from the overall cone area measurement). This would result in an inflated Vas calculation.

For example - if I set my Sd to 397cm2 - I get Vas = 166 litres (not 115).

Cheers,
David.