I am about to build a ported speaker. The manufacturer of the woofer (Vifa) recomends enclosures 40-100 liters. How do I pick which volume I would preffer? Is bigger best for sound? Is smaller for apearances? Bass vs. transient responce? I have put off building the speaker for some time trying to reach a decission. Have the drivers, x-over, wood and all... They are for music in a 2- channel system.
There are a few approaches to understanding the problem. First, Dickason's 'Loudspeaker Design Cookbook' is a good read and technical reference on the theory behind the grill cloth.
All the math that determines box size and alignment is discussed there. Once you get a handle on that you begin to understand why the wide latitude of volume cited. Box alignment basically determines the frequency response and type of rolloff.
Other design considerations will impact a design. If you can't support a large box, then you can trade size for a different response.
I tend to think of box size as a series of tradeoffs. You have to list what criteria is most important to you and design with those in mind. As you go through that process the design will materialize.
Another advance is CAD (Computer Aided Design). There are many design programs out there that allow you design virtual boxes and see the predicted response on a graph. You can go back and change parameters and see the impact and dial in what you want.
The range of tools for this are programs like Leap, Bass Box Pro, WinSD, and even some Excel spread sheets. Leap is over $1,000. Bass Box Pro under $200, and WinSD is a popular free program. I suggest you start with WinSD and learn the basics along with purchasing the book I cited.
Now, you are probably disappointed because you probably have your hammer and saw in hand and ready to roll up your sleeves. What I am suggesting will send you back to square one for a time. However, you can build your speakers twice or once. Ask me how I know! :-)
What I found was that in designing a cab the volume varies from what Vifa states as a recomended enclosure. Usualy it seems a Mfgr recomends a box which is smaller. So I begin to second guess. The mfgr should know best? I generaly build a sealed enclosure and use a sub.
So I guess my original question was more like; do mfgrs recomend enclosures for sound quality or WAF. It seemed odd to go against their recomendations. eg. A speaker I have models for 13ft and is recomended for 3ft.
13 CF vs 3 CF? Something seems way off there.
Which model Vifa are you looking at?
The 13-3 example is a different driver than the Vifa. The Vifa woofer I am working with now is P21WN-20-08. I bought them in 2000 or 2001. I put off building them becuase I got into tube amps at that time, so am using high SPL units w/ 300B's.
I saw a cell phone comercial which is a limited ed Fender model. Seeing the add reminded me I have some unused veneer. And that bookmatched veneer and a sun burst finish would make for one cool looking set of speakers.
In my notes from 2001 I wrote "2.5 ft^3 tuned to 28hz." I also have the vifa recomended enclosures for 40, 60, 80, 100. I need to read more to remember all I forgot about choosing box volumes. I just don't build that many speakers, so, I forget everything by the time I build a new set.
Considering that I build my sealed boxes for .7 Q which box size do you think I should go with?
And then there's the damping...
I am toasted tonight, but I will see what happens tomorrow and try to plug in that data into Bass Box Pro for both vented and sealed.
That would be greatly appreciated, let me know if you need parameters. I had a link but the page expired...
Jefforson, I would suggest you download a piece of freeware called WinISD from LinearTeam in Denmark, and model various volumes to see what it will show you. You will quickly get a basic understanding of what the differing volumes and tuning frequencies will do to the bottom response.
Aloha,
Poinz
AudioTropic
>That would be greatly appreciated, let me know if you need
>parameters. I had a link but the page expired...
Okay, here is what I did. I ran three different scenarios, all vented design. Curiously, BassBox Pro recommended a sealed design, but the bass rolloff was so severe I did not bother going any further.
What I did was to play around with the box volume and port tuning. All three used two 3" ports and the worst case vent speed was under 14 m/s.
http://www.mdbq.net/audio/vifaP21WN-20-08.jpg
Let's talk about the three plots. The first one (red) has a box volume of 2.7 cubic feet. I choose this after some experimentation. My goal was to get as flat a response as possible. The Fs was set to 42 Hz and the rolloff was less than 1.5 dB below the point where the graph peaks at 400 Hz.
The second plot (yellow) used the same box size and set the Fs at 28 Hz, which is the resonance of the speaker. As you can see, the yellow line falls off at about 30 Hz, but it is also 6 dB down from 400 Hz. Even though the bass Fs extends much lower, the SPL volume is very low. This design will sound as if there is little bass to it.
The last plot (green) was a compromise by adding volume to the cabinet (up to 3.5 cubic feet) and moving the Fs up to 33 Hz. The Fs point is just a tad over 3 dB below the 400 Hz point. Again, my opinion is that this design will sound a little light in the bass, but not nearly as much as the second design.
One thing you could do is build the box as in design 3, but change the box tuning to 42 Hz and add heavy damping material inside. To do this you change the two port lengths to 2.613" long instead of 6.444" long. This will give you almost the same bass response as design #1, but you could easily play with the port length to get the response curve of design #3.
Changing the amount of fill inside the cabinet tends to affect the Fs point by either depressing it slightly (adding more fill) or causing it to raise up like a pimple slightly (less fill).
I would use flared ports on both ends of the port tubes. Internal cabinet volumes are the actual volume after bracing and speaker driver displacement have been subtracted.
Your crossover point for the woofer will impact how high or low the mid peaks at, so there is more to do with this design before it is really done, but I wanted to give you an idea of what cabinet size and port tuning will do as far as frequency response.
Finally, it is a fallacy to try to get more bottom end bas by tuning a cabinet lower such as design #2. While you see a longer extended bass, you also loose a lot of bass volume across the whole bass spectrum, which makes the speaker sound like it has no bass at all, defeating your purpose.
If you want deeper bass you either need to add a sub woofer or select a different driver and design.
Thank - you Loren,
I don't have my own computer at this time. I had design software on my old one. I was curious as to which software you were using...
I will go with the "box 1" scenario. I saw the extended shelf before but never thought about how much lower the spl would be. And that at that F it probably would seem more than a little.
I had avoided using a tuning lower than about 30 because I imagine the cone slapping wildly on a deep note. Decoupling? I'll give it a try and see.
I did notice that heavy fill is in the box properties, is there a definition for that. I was thinking of placing the ports on the rear and 3" below the woofer. That way poly could be added if desired.
Last ported project was my HT sub. 2 10" subs in an enclosure finished to apear as a trunk. I didn't include the port volume in the box volume and was wondering. If I want 2.7 ft net, I need 2.7ft + speaker + bracing + port volume = actual box volume. The sub sounds fine but probably not tuned as I intended?
Again, than you for the assistance. I don't get projects started till I have it all figured out. 02/21/00 is the date on the invoice for the leap designed crossover. Amazing how fast time gets behind us!
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