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.
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.
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.
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
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.
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.
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?
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?
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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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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.
dave
Turk, yes I do know what F3 represents. I have built closed boxes, and understand the principals, but I found that I did not grasp why there was more than one option for box size, and choices of port tuning for any box size. I think that I am starting to grasp it. I particularly want to use this particular driver, and closed box is simply too bass light.
Have you considered an Linkwitz Transform circuit ? Active Filters
Basically the ported enclosure lowers the F3. It has much less effect on F10.
But, a ported box acts as a third order High Pass filter , verses a second order High pass for a sealed box. The Third order box adds additional group delay.
The ported box has lower excursion and lower distortion near port tuning, but offers no excursion protection below port tuning frequency.
Bottom line is that the ported alignments can be useful, but you have to be aware of the tradeoffs.
HTH
Doug
Basically the ported enclosure lowers the F3. It has much less effect on F10.
But, a ported box acts as a third order High Pass filter , verses a second order High pass for a sealed box. The Third order box adds additional group delay.
The ported box has lower excursion and lower distortion near port tuning, but offers no excursion protection below port tuning frequency.
Bottom line is that the ported alignments can be useful, but you have to be aware of the tradeoffs.
HTH
Doug
I didn't see anyone asking the money question: where did you get the Qts and Vas parameters?
Datasheet?
Small signal measurement?
Cold or Warm?
Etc.
You would be surprised how much the Thiele-Small parameters can be all over the place. The driver manufacturer has about a 20% tolerance IIRC on Fs, Qts, and Vas. If you go off of the datasheet data and aren't just doing a dumb sealed box, things can get off target quickly. Also, the numbers they quote are for the small signal performance, e.g. at vanishingly small cone excursions. In the real world, as you demand more from the driver and cone excursion increases the parameters are often MUCH different than the small signal numbers. This has become more apparent in recent years as drivers undergo more large signal testing, Klippel testing, etc.
Datasheet?
Small signal measurement?
Cold or Warm?
Etc.
You would be surprised how much the Thiele-Small parameters can be all over the place. The driver manufacturer has about a 20% tolerance IIRC on Fs, Qts, and Vas. If you go off of the datasheet data and aren't just doing a dumb sealed box, things can get off target quickly. Also, the numbers they quote are for the small signal performance, e.g. at vanishingly small cone excursions. In the real world, as you demand more from the driver and cone excursion increases the parameters are often MUCH different than the small signal numbers. This has become more apparent in recent years as drivers undergo more large signal testing, Klippel testing, etc.
Of course they are. T/S are curves, not scalars, so what you get depends on where you measure them. That on top of the tolerances.
dave
There are choices due to tradeoffs. You need to make a compromise of the -6db point ( as Dave pointed above, the -3db should be of less importance ), overall acoustic energy in the lower spectrum ( a higher -6db may yield you flatter response to some point above the -6db point ), group delay and its center frequency ( with a small box it would usually peak above driver`s free air Fs ) and the port noise. A small box would force you to use a small diameter port where the flare radius will be limited due to internal height ( h of front baffle would be higher by at least the amount of the sum of the material thickness ( particle board, mdf, plywood or whatever oyu intend to use ) where the use of an internal flare may become limited. As the size is compact, port lenght would usually be insufficient to allow the use of a larger diameter port, thus delivering very audible port noise levels starting as low as 5-6W fed to the speaker. One workaround is to use larger diameter ports, a 46-50mm usually delivers good results, with a 90 degree bend inside, connected to a second port of the same size ( and sealed ). The problem here is that box effective volume drops as the port air volume is removed from the equation, so it must grow in size accordingly. You have to decide where to allow the compromise and trade one benefit for a drawback.
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Thank you for all your replies. If I may, a few final questions, mainly to Streten.
HP? I understood everything else.
Whilst I am sticking with 5L for the moment, I would like to know (assuming>90dB levels) are there any disadvantages in using the 7L tuned to 50Hz alignment? E.g. is the bass of an equal quality?
Dave,
A "classic" TS ported alignment shows a "4th Order" (24 dB per octave) roll off below Fb, but actual acoustical ported box response can range from a bit steeper than that to around 12 dB per octave. Still have yet to build a ported box yet that actually has a measurable roll off at 24 dB per octave.
Here is a ported box (39 Fb) just finished that measures around a 14 dB per octave rolloff below 58 Hz, with EQ -3dB at 30 Hz.
Art
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