For estimating the volume of cabinet, fs, Qts and VAS are very important.
I use 'Woofer Tester 2' for testing the woofer's T/S parameter.
However, there are huge discrepancies between manufacturer's specs and Woofer tester 2 results.
Some woofers are similar. But some woofers are very different.
For examples (Manufacturer vs Woofer Tester 2)
- Delta mass method used for VAS in Woofer tester 2
Audiotechnology C-Quenze 18H52
Fs : 40Hz vs 36Hz
Qts : 0.32 vs 0.286
VAS : 28.5L vs 23.9L
Vb (Bassbox pro for ported) : 10.3L vs 6.4L
Morel CAW638
Fs : 43Hz vs 48.5Hz
Qts : 0.46 vs 0.66
VAS : 15.7L vs 13.5L
Vb (Bassbox pro for ported) : 19.2L vs 35.8L
Eton 5-880
Fs : 58Hz vs 56Hz
Qts : 0.4 vs 0.41
VAS : 7.5L vs 6.1L
Vb (Bassbox pro for ported) : 5.7L vs 5.1L
SB15MFC30-8 (using DATS ; Dayton Audio Testing System)
Fs : 39Hz vs 49.8Hz
Qts : 0.36 vs 0.542
VAS : 17L vs 9.1L
Vb (Bassbox pro for ported) : 9.1L vs 16.4L
Eton is similar with each other.
That means the Woofer Tester 2 somehow works well.
I know the condition and break-in can affect the parameters.
Nevertheless, the above discrepancies are beyoud it.
What do you think about the reason of above results ?
Is it because the test condition is different ?
Or, are the manufacturer's liars ?
Then, what parameter shall I believe ?
I use 'Woofer Tester 2' for testing the woofer's T/S parameter.
However, there are huge discrepancies between manufacturer's specs and Woofer tester 2 results.
Some woofers are similar. But some woofers are very different.
For examples (Manufacturer vs Woofer Tester 2)
- Delta mass method used for VAS in Woofer tester 2
Audiotechnology C-Quenze 18H52
Fs : 40Hz vs 36Hz
Qts : 0.32 vs 0.286
VAS : 28.5L vs 23.9L
Vb (Bassbox pro for ported) : 10.3L vs 6.4L
Morel CAW638
Fs : 43Hz vs 48.5Hz
Qts : 0.46 vs 0.66
VAS : 15.7L vs 13.5L
Vb (Bassbox pro for ported) : 19.2L vs 35.8L
Eton 5-880
Fs : 58Hz vs 56Hz
Qts : 0.4 vs 0.41
VAS : 7.5L vs 6.1L
Vb (Bassbox pro for ported) : 5.7L vs 5.1L
SB15MFC30-8 (using DATS ; Dayton Audio Testing System)
Fs : 39Hz vs 49.8Hz
Qts : 0.36 vs 0.542
VAS : 17L vs 9.1L
Vb (Bassbox pro for ported) : 9.1L vs 16.4L
Eton is similar with each other.
That means the Woofer Tester 2 somehow works well.
I know the condition and break-in can affect the parameters.
Nevertheless, the above discrepancies are beyoud it.
What do you think about the reason of above results ?
Is it because the test condition is different ?
Or, are the manufacturer's liars ?
Then, what parameter shall I believe ?
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I'm in the same boat as you I noticed discrepancies as well. I use the Delta mass method. My scale that I use to measure the mass goes down into hundredths of a gram. Sorry I can't offer anything helpful. but it would be good to investigate this further. Some people here say that the Delta mass method is not as accurate as the box method.
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The differences are as normal as anything. That's how it is.
Because of that it is a good practice to measure yourself.
Those people, saying delta mass method would be wrong,
are wrong. If one is more comfortable building a test box
for TS parameters, so be it. Delta mass method can be
troublesome if excitation force makes the weights dance
on the cone creating a small spike on the impedance plot
which may or may not alter the calculated values.
Anyway, a bit of a blue tack helps and its mass should
be accounted for in total added mass for that last bit
of accuracy.
I like SBaccoustics site as they tell you exactly how they have measured the T/S params. I found that with my 4" midrange driver that I got very similar results (to published specs) using REW to do the measurements/calculations. Speaker Workshop with this particular driver gave me quite different results (mainly because the FS was off by around 20Hz!!)
Another driver when measured with Speaker Workshop and REW gave almost identical results for each program.
REW uses swept sine, speaker workshop uses MLS. I don't know if the test signal can make such a big difference with certain drivers or not.
Do you have the option of changing the level with the WT2? You might want to try varying levels if you do. You might (or may not) find it makes quite a difference.
Also keep the driver well out into the room away from other objects, and keep ambient noise to a minimum (though this will mainly influence how clean your measurement is rather than affect the accuracy of the T/S calculations.
Tony.
Another driver when measured with Speaker Workshop and REW gave almost identical results for each program.
REW uses swept sine, speaker workshop uses MLS. I don't know if the test signal can make such a big difference with certain drivers or not.
Do you have the option of changing the level with the WT2? You might want to try varying levels if you do. You might (or may not) find it makes quite a difference.
Also keep the driver well out into the room away from other objects, and keep ambient noise to a minimum (though this will mainly influence how clean your measurement is rather than affect the accuracy of the T/S calculations.
Tony.
Blue tack is good idea.
But, even though using blue tack, the difference of VAS value is within 5%.
And though delta mass method is wrong, it is just for calculating VAS.
It can not explain the huge difference of Qts.
For Qts value, you use neither delta mass nor delta compliance.
Q is calculated just on one impedance measurement of a raw driver.
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A few general pointers:
-T/S parameters are not fixed per se; they can and often do vary with measurement methodology / equipment & point. For example, most manufacturers measure under higher (more representative) voltage levels than the majority of home gear, which typically uses a low voltage driver.
-They can (can) vary with driver break in. Lots of myth over this one, but since it's a mechanical component, the suspension stiffness does affect behaviour for obvious reasons, so make sure it's within the intended operating window.
-Published data is often a batch average, so the individual averages may not always agree with each other.
-Production tolerances come into play. Industry average is about +/- 10% tolerance for a given component (Scandinavian built units & a handful of others are usually narrower). But there is no standard.
-T/S parameters are not fixed per se; they can and often do vary with measurement methodology / equipment & point. For example, most manufacturers measure under higher (more representative) voltage levels than the majority of home gear, which typically uses a low voltage driver.
-They can (can) vary with driver break in. Lots of myth over this one, but since it's a mechanical component, the suspension stiffness does affect behaviour for obvious reasons, so make sure it's within the intended operating window.
-Published data is often a batch average, so the individual averages may not always agree with each other.
-Production tolerances come into play. Industry average is about +/- 10% tolerance for a given component (Scandinavian built units & a handful of others are usually narrower). But there is no standard.
High quality drivers are +/- 10% or maybe a bit less (this is where batches of Mark Audio usually falls). Closer to +/- 20% for more mainstream (ie Fostex), and even higher with cheap stuff.
dave
High voltage!!
I think it is the major variable factor.
Then, with more expensive facility, for example LMS or LX500, do you think I can get more credible data which is more similar with the manufacturer's spec ?
Smith & Larson WT2 comes with its own hardware which includes a small current amplifier. It is unique AFAIK in that it doesn't estimate parameters from a sweep, but actyually hunts down the important parameters just as if you were doing it manually.
dave
You get manufacturers data if you use the same test kit the manufacturer does (and the weather is the same, and you have avaerafe units), or if the T/S curves are close to horizontal.
Remember T/S are not scalar numbers, they are curves.
dave
Hi Tony,
I wonder how the impedance of your midrange driver would look like.
Does that mean you are thinking of building a new toy ?
Good observation! Most DIY-ers measure DC voice coil resistance with a DMM, that can have significant errors in the lowest (200 ohm) measuring range. If you short the measuring cables, you will se something like 0.2-0.5 ohms on the display instead of 0, because most of them do not have auto-zeroing function. In case of a 4 ohm driver, where DC v.c. resistance is around 3-3.6 ohms, this can cause an almost 10% error in all of the successive measurements, and in the end result.....
And one more thing.... Unless your driver is not totally decoupled phisically (i.e.: hangs in the air), or completely, stiffly coupled to a rather large mass, your measurements will be false due to mutual resonances induced by the measuring signal. So measuring your driver just sitting on a table will give you false results.
Hi Tony,
I wonder how the impedance of your midrange driver would look like.
Does that mean you are thinking of building a new toy ?
Hi Lojzek, here is the thread. http://www.diyaudio.com/forums/full...ics-sb12mnrx25-4-full-range-build-thread.html 2nd Post has the measurements I referred to above (re difference between SW and REW) No plans at this stage for a three way, but it is always there in the back of my mind, and I will do one one of these days. Have to get my existing project finished (+ a few others) before I tackle that!
Tony.
So the remedy would be to subtract the value shown by the
multimeter shorted. Big deal.
I have seen Voltcraft multimeters for 20€ measuring zero ohms
with probes shorted.
Strange thing. I wonder why do I get almost the same results just like
manufacturer, measuring Focal's TS parameters with the driver just put
on the table or floor?
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