difraction compensation

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I am planning to build 2 speakers that are thin tall deep towers looking physically like NHt's 3.3 or 2.5.

The Side facing woofer is a 12" Audio Concepts DV12. The Front drivers are a 6" 18W8546 ScanSpeak and a 1" 9900 ScanSpeak tweeter. Before I start I must warn that my post is long and since I do not have regular email access I would appreciate it if I am emailed at navin@vsnl.com.

I have the following requirements:
1. As high efficiency as possible
2. Use for Home Theater as well as Stereo
3. Resonable Power handling (100-150W rms)

I hope to use the 12" to compensate for the difraction loss of the 18W8546. Difraction loss is a loss (claimed to be 6db) of low frequencies based on the width of the cabinet (in this case 8" or 20cm). Hence to compensate for Difraction loss most 6" 2 way spekaers employ either a 2nd 6" covering only the lower frequencies or reduce the
upper frequencies to make the speaker sound better balanced.

Not having a 2nd 6" I hope to use the output of the 12" but dont know if this make sense as the 12" will be quite some distance from the 1st 6" and the frequencies that are compensated for are from 100-1800Hz. The 12" might not be able to reproduce 1800 Hz accurately and in any case 2 drivers reprducing the midrange but being far apart might create problems (lobing, delay, etc.). Any advise would be welcome.

Also to increase power handling it makes sense to roll of the low frequencies to the 6". Power handling for this driver is not limited by it's thermal limits but by it's physical excursion limits. Hence the same power being supplied
to the 12" and 6" operating in parallel at low frequencies (below 100Hz) can stress the 6" while not even exciting the 12". However to employ a low frequency high pass crossover using passive crossovers is a challenge. Again I welcome any advise on this front.

Lastly given the physical constructon of a 8" wide, 44" high, 30" deep tower will be difraction loss be 4db since the only edges are 2 sides (4" from center of woofer) and the top (10" from center of woofer) or more? I assume this
because it commonly held that difraction compensation for a 6" 2 way is about 6db-8db. However 6" 2way monitors have 3 edges close the the woofer (4" from center of woofer) and a 4th edge about 10" from center of woofer.

So there you have it, can one use a side facing 12" for difraction compensation?, Can one increase the power handling of a 6" using passive crossovers?, and what it the difraction compensation for a 6" on a thin, deep, tower cabinet?.

Regards
Navin
 

kelticwizard

diyAudio Moderator Emeritus
2001-09-18 2:33 am
Connecticut, The Nutmeg State
Even side-facing, the 12" will have output well above 100 Hz coming to the front of a speaker.

If you are worried about diffraction loss, try crossing the 12" half an octave above 100 Hz and see if that fills it in.

There are a lot of 8" wide speaker cabinets on the market and most of them go below 100 Hz. Are you certain this phenomenon is going to be a problem?

The best thing to do is take some frequency measurements in the room. The web has free programs for that, if you are interested. An inexpensive Radio Shack sound level meter and a free program is probably all you need. With some programs, yo might be able to substitute an inexpensive microphone for the Sound Level Meter.
 
Thanks for your reply.

What I am worried about are:

1. How much difraction do I need for a tower cabinet assume I round edges. I think I will still need 6db as the driver will still be radiating into half space. Am I right?

2. Assuming I need 6db difraction compensation from about 100Hz to about 1500Hz the 12" woofer will have to produce frequencies upto 1500hz which might pose 2 problems: a) the 12" woofer might not produce these frequencies (above 300Hz) accurately and b) with 2 drivers producing teh same midrange frequencies quite fasr apaart I might run in to problems of imaging, lobing etc... Right?

3. The 6" woofer (like any small woofer) is lim9ited as to how much bass it can produce. Hence it makes sense to roll it off at the low frequencie (below 100 Hz). However designing a accurate PASSIVE HIGH PASS crossover at 100Hz is difficult and expensive as big capacitors are needed.

If you have any suggestions let me know.

The speaker i am hoping ot build will be about 8" wide, 44" tall and 28" deep.

Regards
navin
 
Hi Navin,

I had analyzed a bit the NHT 3.3 project, and maybe my considerations can help you in the design of your 'clone'

The 3.3 are carefully optimized for a strongly defined project philosophy: obtain a full range system with optimal image and soundstage recreation. The system deals with room interactions by defining a 'controlled' placement of the speakers in the room and of the uniots in the cabinet.

Bass section
NHT reccommends to place the system close to the room's rear wall, and at some distance from the room's side walls.

The whole design of the speaker is optimized for this placement:
1 - the side placed 12" works in about 70 lt sealed box, and is placed as close as possible to the floor and to the rear of the speaker.
This means that the 12" alignement is designed to work using the reinforcement of the 2 walls near him
I don't have the unit TS parameters, but probably the way chosen by Ken Kantor (who is surely not a newcomer in design of sealed box systems...) is to obtain a very low Qtc (0.6 ?) for fast transient response and control, and to 'compensate' the roll-off at low frequencies with the contribution of floor and wall.

2 - This unit is more a built-in sub than a true woofer. The x-over freq is around 100 Hz. This means that the unit will not substantially participate to the recreation of the ambience and soundstage.
To filter with a passive x-over, in my experience this is the best way: a) use a woofer with an early 'natural' roll-off. Units designed for sub use (high mass, low Q) often shows this behavior. b) compensate the impedance peak with a RLC net; a sealed box with low Q is not difficult to compensate, also if you use a big qty of filler the imp. peak will be already dumped. c) stay, if you can, on a 1st order x-over cell.

Mid-High section

The 3.3 has a sort of 3-way box, with a 6.5" driver in about 10 lt, a 4" (probably a custom version of the Seas 11 unit) in about 3 lt and a dome tweeter (still Seas, I guess).
The cabinet design brings the mid-high section 'far enough' from the room rear wall, resolving the possible interaction problems with the wall itself. This, added to the minimized front section, makes me guess that KK tried to obtain a kind of 'free standing' optimization for the mid high section (remember the good results achieved in ambience, soundstage and 'air' by projects like the ProAC tablette?)

The problem for a replica is that this section is optimized for a well defined dispersion behavior, helped also by the inclination of the front panel and by the block of felt placed in the outer side of the panel itself.
If you want to replicate the project (and not only the look of it) it takes to know something more about the original design, choose the appropriate drivers and design the x-over accordingly.
You also need a good simulation software, unfortunately it is almost impossible to design such a complex x-over only applying the book formulas: if you succeed to obtain good results you are either very lucky or a real wizard!

Not knowing the 3.3 project details, I would stay with a 2 way system with a 6" and a 1" dome driver, trying to design it as if it is a stand alone mini-speaker. I personally like very much the Seas P17 RCY for this job (or his son, if the driver is not anymore in production). If you're afraid by the 6" dispersion in the x-over with the tweeter try a 5", trading some bass for upper mid accuracy.

I hope I have not confused you, anyway let us know if you need some help in calculations or other aspects: there is plenty of very experienced people in this list, and NHT 3.3 are well worth the work for a replica!

bye
sandro
 
Thanks. I am sorry I misled you by using NHT 3.3 as an example. My speaker will inly look physically like the 3.3.

It will effectively be a 2 way (6" woofer and 1" tweeter) with a side firing 12" subwoofer.

What I hope to do is the use the higher midrange output of the subwoofer (above 100Hz till 1000Hz or so) to compensate for the difraction loss of the 6" on the 8" baffle. This will allow for a more efficient speaker.

Also I hope to roll of the 6" below 80 or 60Hz so that the speaker's capacity is not limited by the excursion limitation of the 6". One way to do this is to use a 500uf cap in series with the 6". This can be done by using 2 1000uf caps in series bypassed with a 10uf polyester cap or 2uf Wonder cap/Multi Cap/ or other exotic cap.

Regards
Navin
 
Hi Navin,

I don't understand your estimate of the cutoff
frequency for your diffraction compensation. For
a 20cm baffle, John Murphy's formula (see his
article at http://www.trueaudio.com/st_diff1.htm )
gives a centre frequency for the step at 575Hz,
which is a lot less than your figure of 1800Hz,
and which is certainly possible with a 12" driver.
At 1800Hz a driver this size is anyway going to be
beaming quite severely and a side-mounted woofer
is pointed in the wrong direction.

My feeling is that you would be better off using
the mid-bass driver to do the baffle step
compensation, and to leave the subwoofer to do
what it does best - in other words the low bass.

Alex
 
ok here goes ... 6dB is the theoretical value.... it will be less than this in real life.... how much less? it depends on your drivers, any stuffing in the enclosure, ported or sealed, what material the enclosure is made of and how thick it is.... best is just to listen and see where it sounds best. The centre frequency for the baffle step will be around 575Hz but it starts rolling off @ around 9KHz and finishes @ around 80Hz this is usually corrected for in the crossover.
 
Hi Navin,

my fault I had not read carefully your mail, I try to be more on topic this time.
I have run a quick simulation with the Scan 8546, and at first sight it seems that you should not be so concerned about it.

In a 10 lt box (filled) and an additional resistence of 1 ohm (the filter's coil that you must use anyway) you have a Qtc of 0.5 and a F-3 of about 90 hz, both very good for bass-midrange use.
Also with 200W he reaches 6.5 mm exc. at 120 Hz and 10mm at 80: given the normal energy distribution of music you are on the safe side.

I need to go back to work now, but later I will try a simulation of your drivers on a panel with dimension and shape that you want to use, my feeling is that you don't need to play tricks with the woofer to compensate the Scan. btw, if you can post the tS parameters and the freq. and impedance curve of the woofer I can try a simulation of the complete system.

Sorry, I really have to run
bye
sandro
 
Thanks or all these replies.

As was said eariler the difraction loss starts at quite high a frequency 9k by one estimate) and continues to 80Hz.

I am not to worried about the 2 extremes as they are easily compensated with output from the tweeter (2k-9k) and sub (80-200Hz). It is the range between 200Hz and 2kHz that needs to be addressed.

This loss is about 4db from what I understand. To put some more clairity to this my speakers will be 44" high, 8" wide and 28" deep and will sit as close to wall behind them as physically possible.

The speakers will be 3 feet from the side walls and about 7 feet between them. The room is 14.5 feet wide.

So now from what I understand is the sub can operate till about 200Hz. The 6" will operate from 100Hz to about 2500Hz and the tweeter from 2500Hz on up.

The sub (90db 4 ohms nominal 3 ohms DC) can easliy be rolled of using an inductor. The 6" (88db 8ohms nominal 5.5 ohms DC) will need a large cap for high pass and a crossover using some difraction compensation. The 1" (91db 8 ohms nominal 7.1 ohms DC) will have a crossover as well as some damping circuit to normalise the 500Hz impedance peak.

Thanks again. Any more advise is well come.

Regards
navin
 
difraction

Thanks for teh help.

What I think I will need to do is to build the box and then install the drivers.

The 12" can initally be full range with any crossover just to see how much response I get and how it's response affects the
midrange (imaging, etc...). It can be rolled of later.

I downloaded LSPCAD. Has anyone used this? Is it accurate for basic simulation?

Using LSPCAD I built a very simple crossover which by no mena is a final crossover. It is just so that one can understand what
the pros and cons of crossing over at different points is and how the upper response of the 12" affects the senstivity of the
system.

I ran the 12" with a series inductor L1.
The 6" with a 500uf cap to stop low frequencies and a LC circuit for the low pass crossover the LC circuit is called L2 C2.
The 1" was first damped with a series resistor to reduce it's level and then had a CL circuit using a 6.8uf cap and 1 mh inductor.
Finally I added a paralell 10 ohm ressitor with the tweeter to damp the 500Hz impedance peak.

Hence having only 4 variables L1, L2, C2, and R1 one can understand the behaviour of the drivers better.

I estimated various responses of teh 12" by making L1 2.5mh, 3.5mH, 4.5mh and 7mh. The increase L1 simulated a quicker
rollof of the upper frequencies of the 12".

These are my results.

L1=2.5mH, L2= 1.1mH, C2=4.7uf R1=2ohms db=87.5db
L1=3.5mh, L2=1.2mH, C2=6.8uf R1=2.5ohms db=87db
L1=4.5mh, L2=1.4mh, C2=6.8uf R1=3ohms db=86db
L1=7mH, L2=1.7mH, C2=6.8uf R1=4ohms db=85db

I hope I am working the right direction.

Regards
 

kelticwizard

diyAudio Moderator Emeritus
2001-09-18 2:33 am
Connecticut, The Nutmeg State
Navin:

I think you have a good idea in building the thing first and fine-tuning it later.

The only reason people say not to cross a subwoofer over above 150 Hz or 100 Hz is that they figure it will be in a separate unit several feet from the main speakers. If the main box contains the subwoofer, that is obviously not the case here.

There seems to be only octave in question-100 Hz to 200 Hz-and surely you can hit on the right combination of crossover point and slope to fill it in correctly.

Good luck.
 
Pre-scriptum (!) I've realized that the attachment is too big for this list, so I've sent it to the e-mail address you have provided.



Hi Navin,

as promised (..or threatened..) I did some calculations on your project. I guess the outcome
can help you in the setup of your system. All graphs mentioned are in the attached 'zipped' word file.

Some assumptions

1 - Panels and box volumes are simulated according to the dimensions you have supplied (8x44x30).
2 - Woofer: no parameters available, I've estimated a 'generically good 4-Ohm woofer'. If you want to send me the data of your unit we'll complete the simulation.
Anyway, since it is used only as a sub, we should be reasonably accurate.
3 - Drivers are placed on panel with tweeter on the top edge and mid as close as possible.
This assumption is important for all response simulations.
4 - As general rule, the purpose of any Simulation is just to put the project on the right path. All fine tuning will always be up to your ear!


Now, in Pic 1 and 2 we have the simulation of the Scan 8546 in free air and in 10 lt sealed box.
First note: having a lin X-max of 6.5 and a X-max of 10, with 200W you should not fear any damage.

Pic 3 is the free air/infinite baffle simulation of the tweeter

In pic 4 we see the response of the drivers on infinite baffle and on the actual panel that you use.
I think this graph answers your first question about panel diffraction, this being the difference between continuous and dotted lines.
Note that the woofer is simulated as mounted in the lateral panel, that is much bigger than the front, hence the smaller effect on his curve.
Note also that at certain freq. we have the effect of the reflection of the emission on front panel.
The bigger the panel, the more appreciable are the reflections, that's why many hi-end designs tends to have a small front
panel or put a lot of attention on dumping the reflections

With pic 5 and 6 we have the system behaviour with the 'best combination' of the filters you
want to test.
This is : 7mh coil on woofer, 500microF + 1,4 mH/6.8micro on mid and 2Ohm/6,8micro/1 mH on tweeter.

The output really looks not bad, but I think there's a couple of things that is possible to improve.

A - the big 500 cap on the mid. For sure a big electrolitic before the mid is not a reccommended practice for
a good sound. And anyway this big cap is doing very little to protect your speaker, see Pic 9. It reduces the output of
a couple of db in the 60-250 Hz range, but brings 2 resonances of 3 db at 40Hz and of 0.5 db in the 400-800 range.
I would simply get rid of it, it's giving more troubles than it's worth. The Scan in 10 lt has a really smooth roll-off, why
change something that is already very good?
Maybe you could put a smaller cap with a switch and activate it when in HT mode, for normal music you don't need any
protection on the Scan.

B - Look at the response in pic 6 : at 0 degrees is good, at -5 is almost perfect but at +5 there is a dip of 6-7 db, most
important the depression is about 1-1/2 octaves wide (see the 85 db spl line as ref.)
At 10 degrees the behaviour is nearly horrible.
This means that the energy (emission in a solid angle) delivered by the system is not optimized.
(or, in other words, there is a destructive interaction between the 2 Scan drivers)
Normally the effect on the musical reproduction is a non good 'reconstruction' of the ambience.

Pic 7 and 8 shows the system with the x-over changes I would suggest.
The woofer is cut with a bigger coil, the mid is free on the high pass side, while his low-pass and the tw high-pass are
modified (see draft in the attached, note the position of the 1.8 resistor that must be between cap and coil to properly equalize
the tweeter response)

In pic 8 you see that the emission at +/- 5 is within +/- 2 db, and the response @ 10 degr. is well acceptable,
showing only a narrow dip in the 3.5-4.5 Khz range (less than half octave wide, and so not critical)

One last tip: in both systems the woofer must be connected out of phase, see pic 10 if you don't believe me!


bye
sandro
 
Hi Sandro,

How can I ever thank you? You have spent so much time energy and intellect over this problem.

I know the 8546 has a Xmax of 6.5mm but the 12" I am using (Audio Concepts dual voice coil version of the SV12 (www.audioc.com) has a linear one way Xmax of about 11mm.

Instinctively I guess quantity of bass is a function of volume of air moved. This means that Volume displace of te 6846 is 150cm2 x 0.65cm = 97.5cm3. The volume displacemetn of the DV12 is 420cm2 x 1.1cm = 462cm3. That means that the DV12 can move about 5 times the air of a 8546.

Hence I was worried that when the 12" is really moving the 8546 will have reach it's bass limits. That is what I am worried about.

Also the reason I was loooking to cross the woofers a little higher (9mh is about 80Hz I think) is that it would have afforded me a bit more senitivity. WIth a 9mh Inductor the system sensitivity will be about 85db which awful low. even 2db more would be much more reasonable.

Do the simulations you have done include difraction loss. From fig 4 it seems the 6" (solid black line) has a 10db loss from about 400Hz to 40Hz.

Lastly the 12" is about 90db at 4 ohms (3.1 ohms DC with both coils in parallel), the 6" is 88db at 8ohms (5.5 ohms DC). Given that we are compensating for difraction loss and further reducing the level of the 6" (and also padding down the tweeter to match) how does one match the 12" which is more sensitive to he 6" or (padded down) 1".

I'll see if I can find specs of the 12" and send them to you.

Regards
Navin
 
navin said:
Lastly the 12" is about 90db at 4 ohms (3.1 ohms DC with both coils in parallel), the 6" is 88db at 8ohms (5.5 ohms DC). Given that we are compensating for difraction loss and further reducing the level of the 6" (and also padding down the tweeter to match) how does one match the 12" which is more sensitive to he 6" or (padded down) 1".

Well the best way to do this is to biamp driving the 12" from a seperate amp to that which drives the mid/tweeter. You just use an amp with less gain for the 12" than for the mid/tweeter ... problem solved and you didnt have to dissipate a whole heap of energy as heat.... if you can only use a single amp then you can use an active line level circuit to provide 2dB of gain above where the 12" driver plays its part.
 
Navin,
first you don't need to thank me at all: I'm playing with this hobby for fun. If this means that my brain is deranged, well, this is another problem!
Ok, let me try to answer your questions

1 & 2 - You're right about you calculations on qty of air moved, but again don't worry too much about it.

Excluding the effects of reflections in the room, the SPL that you listen is the emission of DV12 plus 8546 minus (destructive interaction between the two).
In simple words, it is exactly the yellow curve in the graph. This curve is simulated with the mic at 1mt from center of tweeter @ 2.83V (= 1W/8Ohm) input, and you see that is in the range of 87-88 db.
All theoretical 'point calculations' of the sensitivity are superseeded by this global SPL curve, that is a (still approximated but more accurate) picture of your system.

So, in reality, the limiting factor is the mechanical and termical handling of the 'weakest' unit. This is the Scan 8546 and, in my opinion, you will never reach this limits with a 200w amp and a musical program. Of course if you plan to play a 32 Hz tone at 0db for 5 minutes, there is more than a chance to kill the driver, but first you'll have some other interesting side effects, like cabinets opening, glasses broken and your wife/fiancée or neighbour that will try to set fire to your house or directly to your pants.

The point is that you can pull more SPL on the bass side by the DV12, but the linearity of the system will be affected (emphasized on the bass), unless you don't filter the 8546 in order to match the higher level delivered by the DV12.

9mH filters the woofer nominally at 55 Hz, that is it also equalizes his level. The idea is to use the DV12 to complete the 8546 emission at lowest frequencies, providing the 'punch' but leaving to the 8546 the responsability of the sound.
Again, this is a simulation: you may start this way and try with different coils until you reach the best sound balance.

3 - Yes, all global simulations are on actual panel. Normally these simulations are 'conservative' at low freq, that is you should expect a low freq level a bit higher in the real world.

4 - as said above, by equalizing the emission level of the units in order to equilibrate the global response of the system.

ciao
sandro
 
Thanks again Sandro,

one more question came tome.

Why are the DCR of the inductors so high.

I have a 1mh indusctor that is only 0.2 ohms and a 5 mh Indctor that is only 1 ohm. And these are both air core. Using a lathe I can easily build a 9mh air core inductoer that is less than 1 ohm. It will big for sure but is not air core better?

One reason I find to use 9mh is if I find too little resonse in the 60-100Hz range I can wind off a few outer turns of the 9mh inductor and get a smaller inductoer. It is more difficult to make a small inductor larger.

Regards
Navin
 
DC Resistance: just because I assumed a 'conservative' value!

With low DC coils you'll see some benefit for those in series (the wf and Mid), almost nothing for the tweeter one.
The simulation for 9mH/0.5Ohm, 1mH/0.2 Ohm gives an increase of about 0.5 to 1 db in the range 40Hz-4000Hz

If you can build a low res air core inductor of 9mH it is better than a ferrite core, I just estimated a ferrite because normally the commercial coils of that value are solid core.

bye
sandro
 
difraction control

Sandro! You have spent so much time energy and intellect over this problem. You are blown!

I know the 8546 has a Xmax of 6.5mm but the 12" I am using (Audio Concepts dual voice coil version of the SV12 (www.audioc.com) has a linear one way Xmax of about 10.5mm. Instinctively I guess quantity of bass is a function of volume of air moved. This means that Volume displacement of the 8546 is 150cm2 x 0.65cm = 97.5cm3. The volume displacement of the DV12 is 420cm2 x 1.05cm = 442cm3. That means that the DV12 can move about 4.5 times the air of a 8546. Hence I was worried that when the 12" is really moving the 8546 will have reach it's bass limits. That is what I am worried about.

Also the reason I was loooking to cross the woofers a little higher (9mh is about 60Hz I think) is that it would have afforded me a bit more senitivity. WIth a 9mh inductor the the 12" would not be able to help the 6" in the 150-200Hz range and the 6" would require more difraction compensation hence the system sensitivity will be lower. Right?

Do the simulations you have done include difraction loss. From fig 4 it seems the 6" (solid black line) has a 10db loss from about 500Hz to 70Hz. The simple circuit that you have sent does not have any visible difraction copensation circuit. Are you assuming tha the 12" will be able to compensate for difraction loss even after having a 9mh inductor in series? Also from fig. 4 I notice that the loss between 200hz and 500Hz is 6db. This is the worrisome range. I read your graphs wrong initally as I assumed the marking on x axis wer 10, 100, 100, 10,000Hz not 20, 200, 2000, 20,000Hz. Hence I got the post on the site wrong too. Please forgive me.

I think the 1ohm resistance of the 9mh inductor will reduce the level of the 12" to the level of the 6" and 1" 2 way. Right? However wont this affect the damping of the 12"? I am told that any resistance between a driver and a amp reduces the damping drastically. This damping is not noticeable for tweeters as it only affects low frequency performance. Right?

Also I wonder why you put the 1.8ohm resistor between the cap and inductor in the tweeter circuit. Is there a reason for this? I have never seen this topology before. Also the Q of the tweeter crossover will be rather high with a 5uf cap and 0.25mH inductor. Given the 9900 has no ferro fluid damping and hence a high resonance peak at 500Hz would this not make the tweeter suseptable to damage. The 10ohm resistor in parallel with the tweeter was a simple way to damp this peak. As the tweeter impedance went up (with lowering frequency) more current and hence power would be absorbed by the 10 ohm resistor.

Lastly the 12" is about 89db at 4 ohms (3.1 ohms DC with both coils in parallel), the 6" is 88db at 8ohms (5.5 ohms DC). Given that we are compensating for difraction loss and further reducing the level of the 6" (and also padding down the tweeter to match) how does one match the 12" which is more sensitive to he 6" or (padded down) 1". What I am trying to say is if the 6" is 88db then we add 3db of difraction compensation and the sens. is 85db. 12" is 89db nand hence there is a 4db gap in sensitivity.

I went to the audio concepts site. It seems that the do noy sell drivers anymore. Hence I will give you all the specs I have. Hope this helps.

Imp: 4 ohms (2 coils in parallel)
Sens: 89db
Magnet: 1.7kg
Bl: 7.8Tm
Fs: 17Hz
Qms: 5.101
Qes: 0.481
Qts: 0.44
Vas: 380liters
Cms: 902.0436
Mms: 80.340gms
Mmd: 73.024gms
Xmax (one way linear): 10.54mm
Voice coil dia: 2"
Voce coil Height: 30.61mm
Air gap Height: 9.525mm
Voicel coil inductance: 2mH
Voice coil resistance: 3.11ohms
ACI (www.audioc.com) recomended sealed boxes of 2-3 cu. ft.

The specs of the 8546 and 9900 can be got from http://www.scan-speak.dk/

With reference to my above posts and all your posts and help I would like to suggest the following:
L1 = 8 mh (1 ohms) air core (this is the inductor for the 12" woofer)

C1 = 500uf (2 x 1000uf caps in series, 1 10uf polyester cap in parallel and 1 2uf wonder cap in parallel) the parallel caps will act like bypass caps. Maybe we can remove this cap and see. I only worry about excursion.
L2 = 1 mh (0.2 ohms) air core
C2 = 8.2 uf

R1 = 2 ohms (v/s 1.8 omhhs in your circuit) although I feel I would need 4ohms instead
C3 = 4.7 uf
L3 = 0.27mh
R2 = 10ohms (only to protect the tweeter).

Invert polarity on the tweeter since it's effective slope is 2nd order.

Does this make any sense?

Regards
navin
 
Navin,

just a quick reply, I have just little time today

C1 : if you want the cap, use a 5x100micro good non-polaryzed electrolitic (ROE, Monacor or Intertechnik the brands I know, maybe your 'pusher' has a better alternative) + the poly & wonder cap.
It sounds better than the series, and easier to modify later if needed

R1 : I put a 1.8 because it is a standard value, if you have a 2 it's ok . Just stay on 10W dissipation.

C3 & L3 : Ok, nothing changes

R2 : don't use it, it is not necessary and brings some troubles. If you want to linearize the imp of the tweeter use a RLC vs ground with L = 1.2 mH, C = 86micro, R = 6.2 Ohm instead of the simple R2

With the current filter, the TW is already attenuated by 12 db at 2kHz, it should not burn.

DO NOT invert the polarity of the tweeter, theory and real world are a bit different! Or, if you can measure the output, try both connections and see the plot!

Will be back for the 'theory' and the bass calculations when less stressed, I am in year closing period now!

bye
sandro
 
I thougth the R2 (parallel cap with tweeter) would be a cheap way to avoid using 3 components. I know the notch filter is better. Just trying to save money!

Ok will build notch filter. Since you have helped so far the least I can do is try out what you recommend.

I blew out 2 Morel MDT 33 tweeters before. How I dont know. I had a MTM using 2 Morel MDT33 tweeters and blew out both tweeters. I guess the SPLs I use are a bit more than normal.

Thanks again.

Regards
navin
 
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