A small update. I am still working on modification of the baffle for the midrange and tweeter part. Sofar a direction is sort of developing.
Also i decided in agreement with my wife to keep the enclosures and replace woofers and midrange with Purifi drivers, and ordered these today at Purifi.
The current drivers are some 17 years old. Funny enough for the basic parameters the Purifi 8" woofers are almost a drop-in replacement for the modified scanspeak 21w8555-01. Even the Thiele-Small parameters are quite close. The efficiency of the Purifi is a bit higher( BL factor, ~ 2dB), its dynamic linearity is way better.
The mod was to make them suited for sealed box, it was adding weight with strips of lead glued with industrial glue to the inside of the voicecoil former. After 17 years of use and occasional abuse still solid. Nowadays the ERB gives an indication and this dropped from 71 to 51 thus suited for sealed box.
The midrange 4" is physically quite different, that will require a redesign of the baffle opening and likely also a bit forward (vituixcad Z direction -10mm) or thereabouts.
Until they arrive i can focus on the baffle shape for midrange and tweeter part.
Slow but steady progress.
Also i decided in agreement with my wife to keep the enclosures and replace woofers and midrange with Purifi drivers, and ordered these today at Purifi.
The current drivers are some 17 years old. Funny enough for the basic parameters the Purifi 8" woofers are almost a drop-in replacement for the modified scanspeak 21w8555-01. Even the Thiele-Small parameters are quite close. The efficiency of the Purifi is a bit higher( BL factor, ~ 2dB), its dynamic linearity is way better.
The mod was to make them suited for sealed box, it was adding weight with strips of lead glued with industrial glue to the inside of the voicecoil former. After 17 years of use and occasional abuse still solid. Nowadays the ERB gives an indication and this dropped from 71 to 51 thus suited for sealed box.
The midrange 4" is physically quite different, that will require a redesign of the baffle opening and likely also a bit forward (vituixcad Z direction -10mm) or thereabouts.
Until they arrive i can focus on the baffle shape for midrange and tweeter part.
Slow but steady progress.
I hope you derive much pleasure from the new 8”. Interesting that T/S are matching.
The EBP rule of thumb regarding which cabinet type is completely misunderstood (actually meaningless). EBP=fs/Qts has the unit Hz and already that gives away that it says something about he attainable bass extension and not the box type. It’s rather the dimensionless Qts that is guiding the box choice. if Qts is above 0.41 we get peaky reflex tunings whilst peaking first happens when Qts is higher for sealed box. So for Qts in the 0.5 to 0.8 range, sealed in a huge box is the only option without peaking. Higher Qts means generally bigger box and lower F3/fs ratio. Therefor fs/Qts aka EBP is good indicator for the bass extension. For reflex F3 is approx. 0.4EBP. I have a pending article for voice coil that explains this
The EBP rule of thumb regarding which cabinet type is completely misunderstood (actually meaningless). EBP=fs/Qts has the unit Hz and already that gives away that it says something about he attainable bass extension and not the box type. It’s rather the dimensionless Qts that is guiding the box choice. if Qts is above 0.41 we get peaky reflex tunings whilst peaking first happens when Qts is higher for sealed box. So for Qts in the 0.5 to 0.8 range, sealed in a huge box is the only option without peaking. Higher Qts means generally bigger box and lower F3/fs ratio. Therefor fs/Qts aka EBP is good indicator for the bass extension. For reflex F3 is approx. 0.4EBP. I have a pending article for voice coil that explains this
Sort of matching, and in vituixcad enclosure sim it is close enough.
Not behind my computer so cannot share screencopies,other than ebp calcs i foundation on a website yesterday.
Fisrt one is puri 8", second one ss 8555 , thirth one ss8555 mod
I am not so sure about the accuracy of this calc as f.i. the sensitivity is a bit off, but then again the woofer mod i figured out was 17 years ago and cannot recall using this ebp. I did use some tools for sim then but records are gone helas.
The vituixcad enclosure sim i will show later, the f3 is little bit higher and with a bit lower q, which is what i prefer. The woofer enclosure volume physically is 27 liters, the type of sheep wool i use acts like a volume increase when you try to sim the result obtained with wool. At least then and if memory is correct i used boxsim ;-)
For sure i will build a testbox and test with current tools. Interesting !
Not behind my computer so cannot share screencopies,other than ebp calcs i foundation on a website yesterday.
Fisrt one is puri 8", second one ss 8555 , thirth one ss8555 mod
I am not so sure about the accuracy of this calc as f.i. the sensitivity is a bit off, but then again the woofer mod i figured out was 17 years ago and cannot recall using this ebp. I did use some tools for sim then but records are gone helas.
The vituixcad enclosure sim i will show later, the f3 is little bit higher and with a bit lower q, which is what i prefer. The woofer enclosure volume physically is 27 liters, the type of sheep wool i use acts like a volume increase when you try to sim the result obtained with wool. At least then and if memory is correct i used boxsim ;-)
For sure i will build a testbox and test with current tools. Interesting !
Attachments
Regarding Qts and sealed box volume relation , that is what we used if memory serves me correctly @lrisbo . Your remark triggered my memory.
It was a search then for a woofer fitting the limited size of the box as that was driven by WAF (wife acceptance factor) (my dear wife).
I knew from my history then that adding a bit of weight would help, and by glueing that on the inside of the vc former where the cone is attached would not give resonance issues. The 12 gram is what i believe is used. Lifting the dustcap was easy then ;-)
It was a search then for a woofer fitting the limited size of the box as that was driven by WAF (wife acceptance factor) (my dear wife).
I knew from my history then that adding a bit of weight would help, and by glueing that on the inside of the vc former where the cone is attached would not give resonance issues. The 12 gram is what i believe is used. Lifting the dustcap was easy then ;-)
yes, adding mass is lowering EBP and thus increases the bass extension possible. In terms of T/S the fs goes down and Qts up. Alternatively, adding extra series resistance is increasing Qts without changing fs Again lowering EBP
you need to consider the very high speed of sound in the voice coil former. the AC is mostly determined by the cone shape. It’s still on my list how we can specify the acoustic center. The most reliable is to measure the difference between eg a tweeter and a woofer using the same mic in the same distance of the baffle. It’s harder measure absolutely.
My Purifi drivers 4x ptt8.0x08 Nac and 2x ptt4.0m04 Nac arrived today. Well packaged, no visible damage, great.
First do some 'burning in', while building testbox for woofer and midrange to establish volume and damping.
This time also with mic in box measurement and distortion measurement during the steps. Just to see how well the wool filling will work.
First do some 'burning in', while building testbox for woofer and midrange to establish volume and damping.
This time also with mic in box measurement and distortion measurement during the steps. Just to see how well the wool filling will work.
After some time(other priorities helas) something to report. I did a breaking in on one woofer, after a couple of hours it is indeed close to the specified t/s Parameters.
The test cabinet for the woofer is almost ready:
Internally coated now and just the wiring of the unit and the inside mounted tweeter to complete.
Expect this weekend to start with measurements.
The test cabinet for the woofer is almost ready:
Internally coated now and just the wiring of the unit and the inside mounted tweeter to complete.
Expect this weekend to start with measurements.
Finally had the opportunity to do some testing with the PTT8.0, with the focus on the special wool filling i have been using for many many years now.
Steps of 100gr increased the wool amount and measured the impedance with Arta-LIMP, including the T/S parameters:
I have not yet extensively studied the outcome, but i was surprised how much wool i could place without signs of too much, which in the past was that Fs no longer dropped in value. In '91 i did a system with 8" Focal woofers, and in a 22liter enclosure the max amount of wool was 300-350gr as i found on a sheet in the remains of my dossier. I knew i did a similar study in 2006 (for my current loudspeakers), so i searched through my dossier again for measurements done in 2006 and after touching each sheet one by one found the one, and that with the current woofers (to be replaced with PTT8.0) i indeed used 700gr wool in 28.5liters netto volume.
I did listen to the woofer with 900gr wool , no filter etc, just one box, and it did sound surprisingly clear and dynamic.
I tried to simulate the result in the enclosure tool of VituixCAD, and came up with this:
So a lot of absorption is going on, also a significant apparent increase of the volume. Note that the enclosure is quite free of air leaks (cone popping back immedeately), but there is always some, so i set it to 70 instead of 100.
I also used a tweeter in the box (TIB), unfortunately with the last wool fillings the wool pressed on the dome distorting the measurement, and i ran out of time to correct it. So those measurments will be shown tomorrow .
I then tested the ARTA and ARTA-STEPS only to understand its working, but the THD and IMD results are quite promising assuming the setting of the tools were ok enough, i hope to hehave some time available the coming days to do a properly calibrated distortion test.
You see what surprised me were the low distiortion numbers, also in the cone-breakup range, as if the wool damping also damps the cone breakup ?
Note, i also tested 2 differnet mic's (Samson MM1 and Beyerdynamic-MM01) to see if that gave different THD and IMD readings, luckily not.
All in all quite promising results.
Steps of 100gr increased the wool amount and measured the impedance with Arta-LIMP, including the T/S parameters:
I have not yet extensively studied the outcome, but i was surprised how much wool i could place without signs of too much, which in the past was that Fs no longer dropped in value. In '91 i did a system with 8" Focal woofers, and in a 22liter enclosure the max amount of wool was 300-350gr as i found on a sheet in the remains of my dossier. I knew i did a similar study in 2006 (for my current loudspeakers), so i searched through my dossier again for measurements done in 2006 and after touching each sheet one by one found the one, and that with the current woofers (to be replaced with PTT8.0) i indeed used 700gr wool in 28.5liters netto volume.
I did listen to the woofer with 900gr wool , no filter etc, just one box, and it did sound surprisingly clear and dynamic.
I tried to simulate the result in the enclosure tool of VituixCAD, and came up with this:
So a lot of absorption is going on, also a significant apparent increase of the volume. Note that the enclosure is quite free of air leaks (cone popping back immedeately), but there is always some, so i set it to 70 instead of 100.
I also used a tweeter in the box (TIB), unfortunately with the last wool fillings the wool pressed on the dome distorting the measurement, and i ran out of time to correct it. So those measurments will be shown tomorrow .
I then tested the ARTA and ARTA-STEPS only to understand its working, but the THD and IMD results are quite promising assuming the setting of the tools were ok enough, i hope to hehave some time available the coming days to do a properly calibrated distortion test.
You see what surprised me were the low distiortion numbers, also in the cone-breakup range, as if the wool damping also damps the cone breakup ?
Note, i also tested 2 differnet mic's (Samson MM1 and Beyerdynamic-MM01) to see if that gave different THD and IMD readings, luckily not.
All in all quite promising results.
For wool stuffing in a sealed box, I generally use 16 grams / liter. Ironically (or amazingly) this is equivalent to 16 oz / ft^3.
I weigh the wool to get the correct amount, then tease it out as loose as possible. When stuffing this fully fluffed-out wool, it is necessary to slightly compress it to get the full 16 g/l pushed into the box. I found this level of stuffing is sufficient to suppress standing wave acoustic resonances.
I weigh the wool to get the correct amount, then tease it out as loose as possible. When stuffing this fully fluffed-out wool, it is necessary to slightly compress it to get the full 16 g/l pushed into the box. I found this level of stuffing is sufficient to suppress standing wave acoustic resonances.
350 g into a 22 l box is 16 g/l, so our build process is similar.
A good question @lrisbo !
The first testrun was to find out how much wool is too much, apparently the PTT8.0 can handle a lot, i did not reach the usual signs of too much.
The wool amount is driven by three things:
Like @hifijim already mentions it needs to fill the space firmly, not just a loose hump of wool.
It needs to dampen the cabinet modals
It needs to get the Qt in a nice "range"
The issue to deal with is the behaviour of the wool fibres, being "excercised" by the pressure variations the fibres eventually curl, thus the volume of the wool filling as a block reduces a certain amount. It can result in a loose lump of wool, and that does impact the sound.
The wool is shipped in compressed blocks of up to hundreds of kilo's, and the fibres are relatively straight.
Once used in boxes it will curl. The wool i use today are some 40 years old and used before, so well curled ;-)
So i had to find the amount that would give me a firm fit , yet "open" enough to let the pressure waves get into the wool.
In the past we did not have the measurement capabilites we have today. So the drive to measure a lot and to listen also for the needed correlations.
The TIB will give a good feedback, on the enclosure modes reduction (and thecimpedance curve ;-)),
Regarding the Qt the PTT8.0 is already close to the "range".
What the PTT8.0 and the scanspeak have in common is the moving mass. And the focals moving mass was considerably less. Apparently the moving mass is a factor in the amount of wool needed, as is the enclosure volume (Qt).
So the next steps :
Make sure the TIB stays clear of the wool
Redo the filling steps , but from the point where it is already filling the cavity.
Correct the speaker response : bafflestep, cone breakup, . So it has sort of flat response.
Include a series resistente to simulate the Rdc of the filter inductors.
Measure the lot per step, listen to music.
Check with the Scanspeak woofers how well the wool filling still is and how much Qt has changed.
All in all still something to do, but very interesting and sofar positive!
In addition to the above, the damping of the woofer resonance at fs is also a result of the fibre structure itself responding to pressure variations, thus absorbing and releasing energy. This is what causes the fibre to curl.
This behaviour requires the fibre to be completely cleansed from fats etc, so results in a bare fibre.
This behaviour requires the fibre to be completely cleansed from fats etc, so results in a bare fibre.