Hello,
Having watched the following videos on "how to measure small thiele params"
How to measure Thiele Small parameters using your SOUND CARD - YouTube
How to measure T/S parameters PART 2 - YouTube
I have made up the test leads, done the calibrations with much care.
i have been playing around with three different speaker types I have manufacturers spec on to compare with my measurements
And to be frank i can't get my measure Vas anywhere close to what i would expect, some Vas Lt. sizes are match box sizes so i figure i am doing something wrong.
I think this is due to the added mass and wondering are there any guidelines re how much mass to add vs cone diameter/speaker type.
Many thanks imk
PS Happy Christmas
Having watched the following videos on "how to measure small thiele params"
How to measure Thiele Small parameters using your SOUND CARD - YouTube
How to measure T/S parameters PART 2 - YouTube
I have made up the test leads, done the calibrations with much care.
i have been playing around with three different speaker types I have manufacturers spec on to compare with my measurements
And to be frank i can't get my measure Vas anywhere close to what i would expect, some Vas Lt. sizes are match box sizes so i figure i am doing something wrong.
I think this is due to the added mass and wondering are there any guidelines re how much mass to add vs cone diameter/speaker type.
Many thanks imk
PS Happy Christmas
Happy Holidaze and all that jive! 🙂
I've seen various 'specs' to measure Vas over the decades, but the first one I learned is the pioneer's adding enough mass to lower Fs by at least 1.56x to ensure its Mms is more than accounted for and normally heavy enough to keep from vibrating around at the extremely low power T/S measurement requires, so measure Fs, then keep adding mass till it drops to Fs/1.56. Now you can compute Mms, then Cms and finally Vas.
If doing it the old fashioned way of using a sealed box, then it needs to be small enough to raise Fs at least 1.56x.
GM
I've seen various 'specs' to measure Vas over the decades, but the first one I learned is the pioneer's adding enough mass to lower Fs by at least 1.56x to ensure its Mms is more than accounted for and normally heavy enough to keep from vibrating around at the extremely low power T/S measurement requires, so measure Fs, then keep adding mass till it drops to Fs/1.56. Now you can compute Mms, then Cms and finally Vas.
If doing it the old fashioned way of using a sealed box, then it needs to be small enough to raise Fs at least 1.56x.
GM
I use plastic modeling clay(putty) or silly putty (Dow Corning dilitant silicone putty) and a gram scale. The modeling clay is a bit oily so a protective layer of blue masking tape over the cone and then affixing the putty to the tape works. Silly putty is safer in general and does not require blue masking tape. The nice thing about the putties, they adher to the cone a bit so move in and out in unison. I have gotten pretty good measurements using this method. The mass needs to be in a sweet spot per GM's suggestion above. Too heavy and the software will tell you. Too light and it won't give much of an effect.
Good Boxing Day Morning
So I'll apply the 1.56 rule and re-test all.
However i think i have another more worrying problem,
I just put an un-damped driver on my test rig and ran the sweep test on it four times and each test returned and different impedance of:
8.82, 10.4, 9.88, 8.98 Ohms that's about a 15% variance.
So i am now wondering if this simple rig of sound card output power is simply not enough power to run the test reliable and i should be driving the speaker via a power amp at n watts as what i am trying to measure is get colored by low signal level noise.
Which raises the question should the driving power be proportional to the speaker. eg tiny speaker 1w is fine, 15" woofer 10w is required.
Any thoughts please?
So I'll apply the 1.56 rule and re-test all.
However i think i have another more worrying problem,
I just put an un-damped driver on my test rig and ran the sweep test on it four times and each test returned and different impedance of:
8.82, 10.4, 9.88, 8.98 Ohms that's about a 15% variance.
So i am now wondering if this simple rig of sound card output power is simply not enough power to run the test reliable and i should be driving the speaker via a power amp at n watts as what i am trying to measure is get colored by low signal level noise.
Which raises the question should the driving power be proportional to the speaker. eg tiny speaker 1w is fine, 15" woofer 10w is required.
Any thoughts please?
Most manufacturers tightly control coil impedances (it's one of the easier aspects for them to get right) so some investigation is clearly indicated; make sure you calibrate before each measurement, including the resistance of the test leads.
As I've noted elsewhere, Vas measurements are notoriously trickly for many; you need an accurate set of scales, appropriate material carefully fixed (ensuring an even distribution) etc. which isn't always as straightforward as it might seem. Larger masses, which are typically needed to get the reduction the pioneers aimed for and GM described, make life a little more straightforward / improve consistency. The delta compliance method is also a bit of a pain given the need to make a quality sealed box etc. An alternative is the known-mass derivation the late and much-missed Jeff Bagby sometimes advocated. In addition to coil impedance, total mass is one of the easier aspects for most manufacturers to get consistent, with relatively little variation, most of which is in fractions of a gram from the adhesives or pulp cones. So using the supplied Sd (or your own calculated if you prefer) and Mmd, or Mms adjusted to subtract the effective air-load, you can mathematically derive Vas, B*L &c. While this is naturally a bit of a fudge, it usually gets close to the reality, and sometimes ends up being more consistent. I've often used Delta Mass and compared to Known Mass derivation, and assuming you do the rest accurately, they both work pretty well with relatively little spread.
As I've noted elsewhere, Vas measurements are notoriously trickly for many; you need an accurate set of scales, appropriate material carefully fixed (ensuring an even distribution) etc. which isn't always as straightforward as it might seem. Larger masses, which are typically needed to get the reduction the pioneers aimed for and GM described, make life a little more straightforward / improve consistency. The delta compliance method is also a bit of a pain given the need to make a quality sealed box etc. An alternative is the known-mass derivation the late and much-missed Jeff Bagby sometimes advocated. In addition to coil impedance, total mass is one of the easier aspects for most manufacturers to get consistent, with relatively little variation, most of which is in fractions of a gram from the adhesives or pulp cones. So using the supplied Sd (or your own calculated if you prefer) and Mmd, or Mms adjusted to subtract the effective air-load, you can mathematically derive Vas, B*L &c. While this is naturally a bit of a fudge, it usually gets close to the reality, and sometimes ends up being more consistent. I've often used Delta Mass and compared to Known Mass derivation, and assuming you do the rest accurately, they both work pretty well with relatively little spread.
Re power required; 10 mA max to meet the small signal parameter, though IIRC at least one major manufacturer specs 7.5 mA, so power up the mouse wheel generator 😉.
Also, forgot to mention that for heavy added mass, ideally need the driver vertical and clamped to something rigid/massive, though the former isn't practical, so just assume that any DIY added mass measurement won't be as accurate as a manufacturer's acoustical pressure measurement [if still done this way].
Regardless, the Vas spec isn't all that critical since Qts' normally dominates, so ~ +/- 25% is normally close enough unless it's a large Vas relative to ~ Qts'^3.3.
Qts' = Qts + any added series resistance [Rs]: HiFi Loudspeaker Design
GM
Also, forgot to mention that for heavy added mass, ideally need the driver vertical and clamped to something rigid/massive, though the former isn't practical, so just assume that any DIY added mass measurement won't be as accurate as a manufacturer's acoustical pressure measurement [if still done this way].
Regardless, the Vas spec isn't all that critical since Qts' normally dominates, so ~ +/- 25% is normally close enough unless it's a large Vas relative to ~ Qts'^3.3.
Qts' = Qts + any added series resistance [Rs]: HiFi Loudspeaker Design
GM
Thanks all for the input, much appreciated as always on this most helpful forum.
Well after more fiddling i raised the output power to max on my sound card recalibrated etc etc. and start afresh. This seemed to remove a lot of the variance between test runs.
I tested a new Visaton FRS8 and FSR7 and my ST readings seemed close enough to spec.
I pulled the 4" driver out of a pair of old'ish ported Mission i have and it had a very nice spec.
I retested the new Audio Labs drivers i got off Ali Express and not even close to spec. If i was to make a guess they seems to have weak magnets compared size for size with the above other three drivers. Hence i think i'll ebay them and get a pair of MA Pluvia 7 PHD, wish me luck 🙂
So i have learnt a lot; hence i think I'll put a little amp in a box and make up a proper rig for ST measurements.
Thanks again for all the help imk
Well after more fiddling i raised the output power to max on my sound card recalibrated etc etc. and start afresh. This seemed to remove a lot of the variance between test runs.
I tested a new Visaton FRS8 and FSR7 and my ST readings seemed close enough to spec.
I pulled the 4" driver out of a pair of old'ish ported Mission i have and it had a very nice spec.
I retested the new Audio Labs drivers i got off Ali Express and not even close to spec. If i was to make a guess they seems to have weak magnets compared size for size with the above other three drivers. Hence i think i'll ebay them and get a pair of MA Pluvia 7 PHD, wish me luck 🙂
So i have learnt a lot; hence i think I'll put a little amp in a box and make up a proper rig for ST measurements.
Thanks again for all the help imk
Actually VAS specs wil vary widely. Some flat out lie. Many factors effect it . You should do it on a well broken in driver, at room temp, VC allowed to cool off to room temp every time etc. Do not be surprised if you measure more than 20% off. Amazing, real is always higher than published, never lower.
You can also be the victim of the specs were the prototype and the Chinese factory substituted materials. Cheaper rubber, one less turn on the coil, a host of ways they can make a fraction on a cent without the brand name knowing it.
If there is one parameter that is often 100% off, it is Le.
I started out doing sealed box. Had a special box I added blocks of known size into vary. PIA. I then got smart and bought a Woofer Tester. ( Or DATS as Parts Express sells the clone) and use delta mass. I have slugs of nickels glued together I stick on with a bit of blue-tac or self stick tape. I get consistent results. Sealed box method was usually more variable. Some argue more accurate, but I found when done perfectly, I got the same answer.
To really understand what you ae measuring, I recommend Testing Loudspeakers by Joe D'Apolitto.
1.56 is rather nit-picky . Something close to 1.5 to 2. I have to laugh at these measures out 4 decimal places. Room temp, RH, and elevation matter more. Are you measuring you mass to 5 decimal places? RH? ( If so how, as even a sling pycnometer is only good to a few %) A mm or two incorrect guess in SD has a huge impact on TS parameters. The fact is, you are only getting close so you can prototype. If a sealed box, you stuff to measured Qtc. If ported, you trim the port. Rookie mistake to build a nice cabinet first.
You can also be the victim of the specs were the prototype and the Chinese factory substituted materials. Cheaper rubber, one less turn on the coil, a host of ways they can make a fraction on a cent without the brand name knowing it.
If there is one parameter that is often 100% off, it is Le.
I started out doing sealed box. Had a special box I added blocks of known size into vary. PIA. I then got smart and bought a Woofer Tester. ( Or DATS as Parts Express sells the clone) and use delta mass. I have slugs of nickels glued together I stick on with a bit of blue-tac or self stick tape. I get consistent results. Sealed box method was usually more variable. Some argue more accurate, but I found when done perfectly, I got the same answer.
To really understand what you ae measuring, I recommend Testing Loudspeakers by Joe D'Apolitto.
1.56 is rather nit-picky . Something close to 1.5 to 2. I have to laugh at these measures out 4 decimal places. Room temp, RH, and elevation matter more. Are you measuring you mass to 5 decimal places? RH? ( If so how, as even a sling pycnometer is only good to a few %) A mm or two incorrect guess in SD has a huge impact on TS parameters. The fact is, you are only getting close so you can prototype. If a sealed box, you stuff to measured Qtc. If ported, you trim the port. Rookie mistake to build a nice cabinet first.
Hi imk,
I have no idea if your testing method is good or not, but might add a few practical hints:
- The mass to add is about the same as the Mms. A software like DATS is easy to use and will tell you whether your added mass is OK or not.
- The driver needs to be rigidly clamped in vertical position, but as far away from any reflecting surfaces as possible. I built a simple rig from leftovers.
- I use a stuff called plastic-fermit (the green box, easy to find online) and an electronic scale for the added weight. I make 3 blobs from the required mass and attach them to the cone symetrically and as close to the VC as possible. This normally peels off without issues.
- I repeat TSP measurements several times and integrate the values.
All the best
Mattes
I have no idea if your testing method is good or not, but might add a few practical hints:
- The mass to add is about the same as the Mms. A software like DATS is easy to use and will tell you whether your added mass is OK or not.
- The driver needs to be rigidly clamped in vertical position, but as far away from any reflecting surfaces as possible. I built a simple rig from leftovers.
- I use a stuff called plastic-fermit (the green box, easy to find online) and an electronic scale for the added weight. I make 3 blobs from the required mass and attach them to the cone symetrically and as close to the VC as possible. This normally peels off without issues.
- I repeat TSP measurements several times and integrate the values.
All the best
Mattes
Thanks again for the further insights.
As for added mass i made a ring of blue tack and trimmed it to 1.56 fs shift, several testing runs.
All testing was done on my sturdy desk top, i tried holding them down but it made little or no difference. (Some say to hang from ceiling?)
The Audio Labs are new'ish only a few hours of TV sounds so i think i'll run them in more.
I originally made a pair of simple baffle board 150x250mm for them and they sounded terrible hence me wanting to ST them.
So change of plan; i have some bits of 22mm ply so i am going to turn the baffle boards into boxes, stuff, run them in for few weeks then measure again to see if they are keepers 🙂
If keepers make nice box, if not ebay :-(
BTW below are the RAW ver 5.20 results. for further comment if you wish.
Again many thanks to all and merry festive season.
Going to turn the left overs into curry 🙂
RE 5.892 ohm
fS 89.6 Hz
MMS 5.19g
Zmin 6.197 ohm
QMS 6.843
CMS 0.608 mm/N
fmin 308 Hz
QES 1.230
RMS 0.427 kg/s
f3 2,483 Hz
QTS 1.042
VAS 2.38 litres
LE (f3) 0.160 mH
FTS 86.0 Hz
Bℓ 3.741 Tm
Dd 8.25 cm
LP 83.36 dB (1W/1m)
Eta 0.14%
Sd 53.5 cm2
Added mass 13.190g
As for added mass i made a ring of blue tack and trimmed it to 1.56 fs shift, several testing runs.
All testing was done on my sturdy desk top, i tried holding them down but it made little or no difference. (Some say to hang from ceiling?)
The Audio Labs are new'ish only a few hours of TV sounds so i think i'll run them in more.
I originally made a pair of simple baffle board 150x250mm for them and they sounded terrible hence me wanting to ST them.
So change of plan; i have some bits of 22mm ply so i am going to turn the baffle boards into boxes, stuff, run them in for few weeks then measure again to see if they are keepers 🙂
If keepers make nice box, if not ebay :-(
BTW below are the RAW ver 5.20 results. for further comment if you wish.
Again many thanks to all and merry festive season.
Going to turn the left overs into curry 🙂
RE 5.892 ohm
fS 89.6 Hz
MMS 5.19g
Zmin 6.197 ohm
QMS 6.843
CMS 0.608 mm/N
fmin 308 Hz
QES 1.230
RMS 0.427 kg/s
f3 2,483 Hz
QTS 1.042
VAS 2.38 litres
LE (f3) 0.160 mH
FTS 86.0 Hz
Bℓ 3.741 Tm
Dd 8.25 cm
LP 83.36 dB (1W/1m)
Eta 0.14%
Sd 53.5 cm2
Added mass 13.190g
Maybe with big heavy woofers, the upright clamp is required, but for everything I have ever done, just sitting on some blocks so the vent is open has worked fine. As I mentioned, un-controllable variables are larger than the measurement errors and you need to tune the prototype anyway. Horseshoes and hand-grenades. ( or "close enough for government work" )
In the US we have the advantage of our nickel coin being 5 grams. No scale needed.
In the US we have the advantage of our nickel coin being 5 grams. No scale needed.