W Dipole (Linkwitz alignment) Sizing Q

arc2v

Member
2007-05-09 7:15 pm
Okay, I have searched, read, searched more, and still can't find a good resource on how to size a W frame dipole sub.

Starting from these pages:
http://www.linkwitzlab.com/woofer.htm
http://www.linkwitzlab.com/woofer3.htm

I see he has a couple of designs for different drivers, but they are mostly the same principal. Drivers force air out of the front in a combined slot and out the back in two individual slots.

It seems he sized the height just to clear the drivers and I never read anything about why its depth was set.

The real confusion was how to size those individual slots. There seems to be no rhyme or reason. Originally I thought he was trying to get the output slot near the same size as the Sd of one driver (double that for the middle), but all the slots are different widths.

Also, he mentions that you can make one of the cabinets narrower by shortening one of the slots by 1.5"!!! But he never mentions trade-offs other than it will be a pain to mount the drivers.


I have 2 15" drivers that have been playing true Open Baffle (no baffle :) ) on my floor for few months. They sound pretty good just like that, but the wife wants a bit more aesthetics, so I need to box them in :( The above designs intrigued me, but I can't seem to find a design guideline.

All help appreciated!
AC
 

arc2v

Member
2007-05-09 7:15 pm
Sorry, I should have mentioned this:

The sub is going under a screen, so it can't be too tall, and it *may* end up being the stand for my DIY center speaker, if I ever get around to finishing it.

That's a nice looking sub, but too tall for my application. Thanks though.
 
arc2v said:
Also, he mentions that you can make one of the cabinets narrower by shortening one of the slots by 1.5"!!! But he never mentions trade-offs other than it will be a pain to mount the drivers.
You may search the forum for the "ripole" variant of the W-frame to get the lowest bass at the lowest box volume. By shortening the slot (mainly the slot in front of the cone) you lower the Fs of the driver and raise the resonance peak of the cabinet. For a front slot area of 1/4 Sd of the driver you can expect Fs lowered about 20 %. But that depends much on the driver too.
You pay with a lowered efficiency. I measured about 5 dB less SPL compared to a H-frame of equivalent dimensions.

You mention that you will position the sub under the screen. Are you aware that dipoles normally need at least 3 feet distance to the front wall?
 

footloose

Member
2007-10-02 9:11 am
Do you think the following drivers would be ok for use in a open baffle subwoofer, (4 per side)?

# Nominal Diameter 380 mm
# Power Capacity 500 watts
# Sensitivity 96 dB/watt/m
# Frequency Range 30-500 Hz
# Impedance 8 ohms
# Bl Factor 19 N/A
# Voice Coil Diameter 75 mm
# Voice Coil Material Copper
# Maximum Excursion 28 mm (before damage)
# Magnet Assembly Weight 8.1 Kg
# Effective Moving Mass 0.098 Kg
# Compliance 0.00017 M/N
# Volume displaced by unit 6 litres
# Fs 39 Hz
# Qt 0.32
# Xmax 4.5 mm
 
Hi,

I´d say they work, but do they work fine?????
Two of the most important parameters are missing
a) does the driver work noisefree under heavy excursion condition and
b) the quality factor Qm

The excursion capabilities of the drivers are quite low, though a quad each side will lessen the stress a bit. Still though You´d build a rather big subwoofer with rather low SPL capabilities.
The Qt is ok, but since it isn´t mentioned which part of it relates to Qm resp. Qe the Qt-figure is of lower usefulness.
The Fs is quite high. You can expect a serious reduction of Fs when built into a small ripol-cabinet (because of the low excursion capabilities the dimensions can indeed be very small) but as long as You don´t know Qm You can´t estimate the degree of the resonance-lowering. It might be (quite often to find with lossy drivers) that the fs-reduction is negligible. On the other hand can a lowloss driver show reduction values of 10Hz and more in the same cabinet.
So I wouldn´t say it´s a bad drivere for this application, but I wouldn´t say its a good driver for this application either.

jauu
Calvin

(ps: this comment relates only to the ripol-topic in this thread!)
 
Hi,

with the given values You can expect a serious reduction of Fs in a small ripol-cabinet (Fb~25Hz, W: 39cm, H: 90cm, D: 41cm, front opening 10cm, back opening 10.1cm, height 2x 40.6cm). The passive filtering (Notch) that belongs to this special concept of dipole-sub sinks the Fb further a bit (~23Hz). Excursion limit will be reached already around 10V signal voltage at 20Hz. This translates into ~100db@1m distance for a series-parallel connected quad of drivers.
It might be that bass level and extension is sufficient for Your room already, i.e. passive mode drive without active equing. If not a Subsonic-filter fb ~25Hz and +3 dB gain should be allright.
Since the drivers excurb much at low voltages I recommend a subsonic filter in any case.

jauu
Calvin
 

Empee

Member
2006-09-04 7:39 pm
unbranded 8" woofers....

Hi Calvin !

what would be your verdict on 8" drivers, rubber surround,
inverted dome, stamped steel basket, with the following data:

overall diameter 218mm
Re = 3.2 ohm
Fs = 45 Hz
Qms = 4.92
Qes = 1.15
Qts = 0.93
Cms = 0,00041 m/n
Vas = 17,87 ltrs.

(data measured by myself, averaged of 4 pcs.)

I'd like to make two ripoles, so two drivers each....
What dimentions would you recommend ?

Thanks for your help !!

Cheers,

Empee
 
Hi,

it just came to my mind, that maybe I should open a business doing sims :rolleyes:
Please allow me some personal words, regarding inquiries like the above.
Well, I like helping and think that I´ve given lots of valuable infos to many of you -and got a lot of precious help in return. So far so very good ;) The profit is on both sides----a good deal after my taste.
But every answer takes its time -of which I spent too much here already. So please don´t be upset when I won´t answer each and every Q at once. I do try to answer to my best knowledge, but understand that I feel responsible about every answer and what may follow out of that. I had cases where a hint or recommendation backfired (it can always look perfect on paper and still be a desaster in praxis). Especially frustrating were cases where the builder didn´t follow the guidelines and after beeing not successful claimed the working principle to be useless or even blamed me as giving wrong advice! If I recommend something, than because I feel confident to do so and I´m interested in the results. I don´t feel confident at all when I´m asked to evaluate the quality or useability of drivers the only thing I know from is that I don´t know them at all.
There are many incarnations of drivers/subs to find with prooven useability and correct functioning. I can only say: " If You want a success with Your project, than build the successful things".

Sorry Empee to not supply for the answer You hoped to get.
Just so much. The stated driver parameters don´t disclude the useabilty in a dipole, in fact it might be the only way to get clean bass out of this driver (apart from active motional feedback), but neither the ´noise´ is mentioned nor the excursion capabilities. Definite is that just 4 of the drivers will lead to a seriously reduced SPLmax. For sufficient dynamics I´d use at minimum(!) 6 8"-drivers per side.

jauu
Calvin
 

Empee

Member
2006-09-04 7:39 pm
Hi Calvin !


I really respect your comments and appreciate it,
that IF you supply an answer, you want it to be the best possible.
However, I cannot understand people who, if they don't like the outcome,
blame the person who was kind enough to help !
(and invested time and effort etc.)

But still, and please forgive me for being so persistant,
I cannot see the relation in all the W-frame dipole / Ripole
dimentions I´ve seen on the internet with regard to the drivers used,
and I´d like to come up with just something.
Not only for me, but for everybody who, in the future, would like to build a Ripole.

The two most important questions are:
-How does one determine the front opening and the volume of the shared cavity in front
-How does one determine the back openings and the volume of the cavity behind each driver

I always thought the openings were merely a function of the drivers Fs, Qts and Sd,
until you asked for the Qm on Footloose's drivers


I really appreciate all the help I can get,
and will NEVER "sue" anyone who did their best to help !

Cheers,

Empee
 
Hi,

Qm is one of the decicive factors regarding the Fs-reduction
The ranking is:
1st: the size of the smaller chamber opening -with ripols the front chamber
2nd: the size of the larger chamber opening -usually the back chamber/chambers
3rd: Qm

As rules of thumb:
back chamber opening area ~1/2 of diaphragm area
front chamber opening area ~1/4 to 1/3 of diaphragm area (the larger value for longstroke drivers, the lower value for shorter stroke drivers.
The rest is measurement, equing and amplification ;)

jauu
Calvin
 

Empee

Member
2006-09-04 7:39 pm
Gracias !!


I will try the 1/2 Sd & 1/3 Sd figures,
since the stroke is fairly big (did 8mm on the test bench without a problem :smash: )
unfortunatly, the movement is not soooo quiet when it's doing that (free air, freq. generator @ 15Hz )
On the other hand, bass is **** loud at that setting (original ported box / freq. generator sweep 25 - 100Hz....)

Well,
I guess I'll just have to give it a try, don't I ? :cheerful:


....Will post any results as soon as I get a test box together....


:drink: Cheers ! :drink:

Empee