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Old 16th April 2014, 05:02 PM   #1
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Default Different ported alignments

I am making very compact speakers, and need to make them ported. The Qts is .43, and the Vas is 4 litre. I am ignorant on these matters, and would like to ask 2 questions. On the calculator I am using there are 2 possible sizes of box. One is just referred to as "ported box", and the other is Butterworth B4. The latter is about 20% smaller, and the Fb is very slightly higher.

Is there any disadvantage in choosing the Butterworth alignment? (20% is a useful saving)

There is a choice of port frequency. If this is set above Fb, the box size stays the same and the F3 is virtually identical. What are the advantages or disadvantages of choosing a different port frequency to the box frequency?

Thanks in advance.
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Old 16th April 2014, 07:59 PM   #2
Ron E is offline Ron E  United States
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Quote:
Originally Posted by midrange View Post
I am making very compact speakers, and need to make them ported. The Qts is .43, and the Vas is 4 litre.
Smaller boxes tuned higher will often have less extension and poorer transient response. I have found that most simulators that claim to simulate B4 Butterworth are not actually B4, usually they are actually QB3 - quasi butterworth 3rd order.

Speaker building is not deterministic, there are tradeoffs whichever way you go. It is better to give more information rather than less. If you want help deciding between the two alignments, why not share your full woofer parameters, and also the box size and tuning frequency for the two alignments?

If you just want information on how to choose between the two alignments - just graph the response of both and decide from there.
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Old 16th April 2014, 08:15 PM   #3
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Quote:
Originally Posted by midrange View Post
I am making very compact speakers, and need to make them ported. The Qts is .43, and the Vas is 4 litre. I am ignorant on these matters, and would like to ask 2 questions. On the calculator I am using there are 2 possible sizes of box. One is just referred to as "ported box", and the other is Butterworth B4. The latter is about 20% smaller, and the Fb is very slightly higher.
Download and run WinISD pro. Load the driver parameters into the DB and simulate in "vented" QB3 alignment. This will give you a good idea how the driver will respond to different tuning.

The Mark Audio A6p driver has similar VAS and Qts. It's very easy to model in a vented box.

jeff
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Old 16th April 2014, 08:26 PM   #4
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I am hoping for some principals so that I can understand what I am doing. Simply choosing between 2 alignments is not what I am seeking.

Anyway the driver parameters are
Qts .43 Vas 4l Fr 85Hz diameter of cone 8cm

The "ported box" came out at 5litres tuned to 79Hz F3 77.5Hz
The "Butterworth" came out at 4.25litrest tuned to 80Hz F3 80 Hz
The "ported box" also came out as 5litres tuned to 110Hz* F3 79Hz

*a completely arbitary choice on my part. I would try others if I knew why I was trying them.
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Old 16th April 2014, 08:41 PM   #5
sreten is offline sreten  United Kingdom
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Hi,

If you want a guiding principle full range just go sealed
in about 3L or just go vented about 50Hz in about 7L.

For AV the ballpark changes a lot.

rgds, sreten.
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Old 16th April 2014, 10:20 PM   #6
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that's one of the caveats of sim/modelling/box design programs a great many assume prior knowledge of the design process,all too many people(myself included) think "oh boy the 'puter program is magically going to give me a speaker design" (or at least that's me trying to learn Horn Response) without learning some basics first.
please don't take offense to this but a little background reading might be in order to understand what the program is telling you with those magic values!
just a quick query to help evaluate your current knowledge do you know what f3 represents?

when you started out by stating: "I am making very compact speakers, and need to make them ported." is bordering on breaking design/alignment rules.
so a 3 inch driver in as compact a box as possible while still providing deep bass is what your after mayhaps?

Last edited by turk 182; 16th April 2014 at 10:42 PM.
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Old 16th April 2014, 10:39 PM   #7
Ron E is offline Ron E  United States
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Quote:
Originally Posted by midrange View Post
The "ported box" came out at 5litres tuned to 79Hz F3 77.5Hz
The "Butterworth" came out at 4.25litrest tuned to 80Hz F3 80 Hz
The "ported box" also came out as 5litres tuned to 110Hz* F3 79Hz
*a completely arbitary choice
Your actual F3 numbers are at about 72, 74 and 90, respectively, for those boxes. Here is a graph that gives response, power handling, excursion and max SPL for those three boxes, assuming a power handling of 25Watts. Excursion is on the right axis and everything else is on the left. As you can see, the smaller box is not a major change in anything.

If you start with a flat example and tune lower in the same size box, the response will droop, if you tune higher it will peak (as in your third example).

If you change box size and nothing else, you will gain (or lose) 6dB of port output at the port tuning frequency for every doubling (halving) of box size.

Those two last statements are your two main degrees of freedom in the ported system. They explain (almost) all frequency response behavior. Get a program with graphing capability and try them out.
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Last edited by Ron E; 16th April 2014 at 10:54 PM.
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Old 16th April 2014, 11:06 PM   #8
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Thank you Ron E for your answers. I now "see"what is going on. The 3rd result (110Hz tuning) from the website I was using is obviously wrong with respect to F3.
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Old 17th April 2014, 12:17 AM   #9
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Quote:
Originally Posted by turk 182 View Post
...do you know what f3 represents?
F3 is the -3dB point of the filter and oft used for comparison.

As far as what you hear it is not very useful, F6 and better yet F10 have audiable meaning.

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Old 17th April 2014, 01:09 AM   #10
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thanks for that o frugal-phile but my question was for midrange ...if i'm being obtuse sorry...
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