What OB size?

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Hi, I want to use a 6.5" Vifa P17 woofer and tweeter in an OB.Under this baffle will be a bass reflex box of 1.5 cu.ft.Now I will have an L frame.I want the c/o to be at 300hz. second order.What size should the OB be in order to ensure the Vifa will go down to intergrate well with the 10" woofer in the box below? Thanks,Leo.Crikey,this reads like a riddle!
 
Leo... May I suggest you use some heavy card board and experiment. I think you will find a baffle width in the 14" - 16" range, assuming no rear wings will give you the roll off (c/o) in the 300 hz area. A useful tool to help with your Vifa baffle design is here:
Electro-acoustic models

Click on design models and scroll down to spl max1.xls for an excellent Exel spread sheet.

Hope you let us know what ends up working best.
 
If baffle width "b" becomes large compared to cone diameter "a", a hump with a following dip will develop in the response:
circular baffle b1.gif
(Fig.5 from Mellow/Kärkkäinen "On the soundfield of an oscillating disk ...")
http://www.mellowacoustics.com/articles/Disk_in_a_circular_baffle.pdf
So just be prepared to equalize that dipole hump.

Rudolf
 
If baffle width "b" becomes large compared to cone diameter "a", a hump with a following dip will develop in the response:
View attachment 332712
(Fig.5 from Mellow/Kärkkäinen "On the soundfield of an oscillating disk ...")
http://www.mellowacoustics.com/articles/Disk_in_a_circular_baffle.pdf
So just be prepared to equalize that dipole hump.

Rudolf

This is the main reason for the narrow baffle. You will do fine with an 8" baffle. For the ported enclosure, you could turn the driver sideways and keep the narrow speaker profile throughout the entire length.
 

guangui

Member
2013-02-12 12:21 pm
Hi, I want to use a 6.5" Vifa P17 woofer and tweeter in an OB.Under this baffle will be a bass reflex box of 1.5 cu.ft.Now I will have an L frame.I want the c/o to be at 300hz. second order.What size should the OB be in order to ensure the Vifa will go down to integrate well with the 10" woofer in the box below? Thanks,Leo.Crikey,this reads like a riddle!

What are your calculated or desired dimensions for the BR enclosure?

Leo... May I suggest you use some heavy card board and experiment. I think you will find a baffle width in the 14" - 16" range, assuming no rear wings will give you the roll off (c/o) in the 300 hz area. A useful tool to help with your Vifa baffle design is here:
Electro-acoustic models

Click on design models and scroll down to spl max1.xls for an excellent Exel spread sheet.

Hope you let us know what ends up working best.

Yes, the baffle should be 14" (326 Hz), 15" (302 Hz), or 16" (285 Hz). If any of those baffle sizes do not match (larger than) the width of your BR enclosure you can always fold the baffle.

I used, mh-audio.nl - Home, to adjsut/calculate baffle width.
 
Leo

I assume, if using a passive crossover, you plan to use a capacitor in series with the Vifa to keep lower frequencies from not reaching the Vifa. If you do get a bit of a bump, and much experience has taught me there will be very little if any bump at 300 Hz with 12"-14" of baffle, (theoretical math withstanding) your capacitor choice will allow you to adjust for any bump that may occur and protect the unloaded Vifa from over excursion.

John
 
Thanks to all you guys.Using cardboard extensions of various sizes,I am convinced 12-16" baffle size will be necessary.My original 8" had more apparant clarity but also sounded "airy-fairy" not getting a grip and giving substance to the music,due to poor intergration between the lower mids and the upper bass.Now for the second part of the riddle! As you know,the P17 is rated at 88db.I believe the second woofer,that in the B/R enclosure should be louder but I don`t know by how much? I am thinking a 10" Altronics rated at 93db will be correct,but perhaps I need a 91-92db unit.This is a critical decision as I am using a passive c/o.Thanks for solving my first problem. Leo
 
Leo

I assume, if using a passive crossover, you plan to use a capacitor in series with the Vifa to keep lower frequencies from not reaching the Vifa. If you do get a bit of a bump, and much experience has taught me there will be very little if any bump at 300 Hz with 12"-14" of baffle, (theoretical math withstanding) your capacitor choice will allow you to adjust for any bump that may occur and protect the unloaded Vifa from over excursion.

John
It`s a 2.5 way system,the Vifa going all the way down .The cap in the 10' will be changed to meet the Vifa.Thanks for that baffle size info_Over excursion is only on occasional frequencies and caused no damage even when very severe,
 

skorpion

Member
2004-10-26 7:12 pm
To illustrate the above discussion I simulated the Vifa P17WJ woofer with Martin Kings wellknown MathCad model.
First picture is of the Vifa with no crossover applied on a 13" wide baffle centered 32" up on a 40" high baffle to make room also for a tweeter. This is the 13" wide baffle:

An externally hosted image should be here but it was not working when we last tested it.


Next is an 8" wide baffle, same otherwise:

An externally hosted image should be here but it was not working when we last tested it.


If going with a Bassreflex (must be very well build and damped) I would certainly pair the Vifa with the same unit in the bassreflex. And make it 3-way on the narrower baffle.

/Erling
 
Thanks for doing that work.My plan is for a 2.5 way.The o/b section crossing to the tweeter at 2.3k second order,the lower b/reflex also second order will go to 300hz.I think the boxed woofer should be louder in order to blend in with the high efficiency of the mids and highs,but by how much? Your simulation doesen`t show it but my cardboard experiments giving a wider baffle definately improve the the bass in the lower midranges. Leo
 
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skorpion

Member
2004-10-26 7:12 pm
The bassnotes will merely make the OB Vifa's cone travel and distort more than contributing to the volume without crossover. I see a crossover around 300 Hz.
Now effeciency of the bassreflex with a Vifa would essentially be the same as that of the OB unit, may be the OB would have to be damped by a couple of dBs to match perfectly flat at crossover.

/Erling
 
Rear radiation

Leo..

I would not worry too much about your woofer efficiency. You should realize about 3-4 db of added output from the REAR of the Vifa. Not too many boxed woofers can deliver above 90 dbw unless you get into a very large box, like with pro 15". And even then, they usually cut off just below 50 Hz.

Continue to experiment. Also remember, you may also reallize a couple more db out of the Vifa because of the band pass gain throughout the midrange caused by the overlap from the woofer below and tweeter above Vifa.

In fact I will be surprised if you don't have actually pad the vifa by about 3 db.

Great to see you making progress.


John
 
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