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Design of Rectangular ports for a BassReflex Box

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endre_k View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote endre_k Quote  Post ReplyReply Direct Link To This Post Topic: Design of Rectangular ports for a BassReflex Box
    Posted: 31 March 2011 at 8:09pm
Hi,
i've been looking for some information of how to calculate, and desing a bass reflex speaker. But all books i look, they only talk about diameter, only of cilindrical ports.

But what about if i want to design a speaker with a rectangular port? wich formulas do i have to use?? Or is it a cut & try solution only?

how can i calculate, or estimate if you want to put one, two, four or how many ports you want? Or i can put as many as i want aslong as they verify they formulas???

i found this, can anyone verify this is correct? haven't seen it in any of the 3 books i looked :)

Port Length
The port length required to tune a volume of air to a specific frequency can be calculated by using the following equation:

   Lv = (23562.5*Dv^2*Np/(Fb^2*Vb))-(k*Dv)

where,

    Dv = port diameter (cm)
    Fb = tuning frequency (Hz)
    Vb = net volume (litres)
    Lv = length of each port (cm)
    Np = number of ports
     k = end correction (normally 0.732)

port.gif (1467 bytes)

==================================================
i have this formula from a good book, but for circular ports:

Lv=2340*(dv^2/fb^2*Vab) - 0'73dv

looks like it....but..... still not sure :S :S :S



thanks for your help in advance!!
keep up this cool forum :))Clap

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Steve_B View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Steve_B Quote  Post ReplyReply Direct Link To This Post Posted: 01 April 2011 at 10:07am
If you use a program like winisd the port tab (I think it is called vent in winisd) lets you choose between round and rectangular. Otherwise you need to match the area of the port.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote audiomik Quote  Post ReplyReply Direct Link To This Post Posted: 01 April 2011 at 10:18am
isn't there something with rectangular ports that as the diagonal increases from it's minimum (square section) then the Q value changes?

Mik
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bass traffic Quote  Post ReplyReply Direct Link To This Post Posted: 01 April 2011 at 3:05pm
Originally posted by audiomik audiomik wrote:

isn't there something with rectangular ports that as the diagonal increases from it's minimum (square section) then the Q value changes?

Mik
I'd be interested to hear about the effect of that.
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endre_k View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote endre_k Quote  Post ReplyReply Direct Link To This Post Posted: 01 April 2011 at 6:00pm
Originally posted by bass traffic bass traffic wrote:

Originally posted by audiomik audiomik wrote:

isn't there something with rectangular ports that as the diagonal increases from it's minimum (square section) then the Q value changes?

Mik
I'd be interested to hear about the effect of that.


hmmm

Any more information please? Or where to find more?? :)

thx

cheerz from Spain!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Steve_B Quote  Post ReplyReply Direct Link To This Post Posted: 01 April 2011 at 8:42pm

Mathematical models of loudspeaker designs all make assumptions or are based on some ideal situation. Some parameters like box losses (Ql) can only be determined after construction. Whether free or expensive the software programme or person using it has to estimate (guess) the value. If you read through enough posts here or on other forum you will come across people advising making allowances or tweaks based on past experience. Sometimes the best looking response doesn’t sound the best. If the theory gets you close enough then don’t sweat the details. One advantage of round ports, of the plastic type, is that you can make them a tight fit and longer than you think and then measure and adjust before finally fixing them in position.

Back to your question. All ports or vents have some resistance caused by the air in contact with the port wall. Multiple ports have a bigger surface area than a single port with an equivalent area. The resistance of air flowing over a surface varies depending on the speed so in a port it will vary depending on the audio level as well. The same thing applies to a narrow slot type port as with multiple ports. The more you move from square the bigger the ratio of surface area to volume. The larger surface area increases the resistance and lowers the Q. 

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Post Options Post Options   Thanks (0) Thanks(0)   Quote greeef Quote  Post ReplyReply Direct Link To This Post Posted: 02 April 2011 at 1:04pm
Interesting. Will a lower q port have a lower peak velocity, but output across a wider bandwidth? What is the effect of q in this case?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Steve_B Quote  Post ReplyReply Direct Link To This Post Posted: 02 April 2011 at 2:17pm

endre_K, can you keep similar questions to one or two threads at the most. For old codgers like me it gets a bit confusing with all the similar threads that you have started. So the following is in response to your multiple drivers question.

Using more than one drive unit changes the Vas. If the enclosure size is constant then yes fs does change. It is normal to put multiple drive units in a proportionally larger enclosure to keep the response similar to the individual drive units. The isobaric technique uses two drive units in the same cabinet to improve bass performance for a given cabinet size, but that is at the expense of efficiency. I've only seen that technique used a few times for PA work.

If you want to learn the underlying theory you need to read some books on the subject, which will give you a more complete picture of what is going on.

The two standard texts which cover a lot of theory are:

 ‘Acoustics’ by Beranek and ‘Acoustical Engineering’ by Olson.

 ‘Dynamical analogies to engineering problems’ also by Olson is useful if you have any electrical engineering knowledge. (pop quiz question 1: what's the rotational mechanical analogy of two resisters in parallel?)

‘Bullock on boxes’ goes through some of the original alignments.

Google should bring up a lot of information about the Thiele Small papers.

More tuned pipes and horns Martin King’s website has a lot of information.

Re: the question about lowering the Q of the port. This is considered a bad idea because it increases cone excursion. There was a trend in the 1980s of adding straws to ports to improve the laminar flow through the port to reduce noise.  This too had the side effect of adding resistance and therefore reducing the Q. For multiple ports, or slot shaped ports I wouldn’t think that it makes that much difference.

I’ve tended to keep to relatively conventional designs so not looked into the problem at great depth, nor have I come across much theoretical work which discusses it (that’s not to say it doesn’t exist). Most papers on port noise tend to concentrate on flaring the ends of the port.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Lee In Montreal Quote  Post ReplyReply Direct Link To This Post Posted: 02 April 2011 at 5:07pm
Hi endre

I don't know if have found your answer yet. Most ports are round simply because it is cheaper and easier to stick a piece of round ABS pipe on your baffle, than cut more wood to make a square ports. But basically as long as the area is respected, square or round doesn't really matter.

The surface of a diameter is 3.1415 / 4 x D^2
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Post Options Post Options   Thanks (0) Thanks(0)   Quote endre_k Quote  Post ReplyReply Direct Link To This Post Posted: 02 April 2011 at 5:37pm
Steve_B, sorry about it, but i need to finish this school project for  june, and i'm a bit in a hurry, and nobody answered...i got stressed. Sorry again won't happend anymore.  :(


Another question about it,
 But if i'm trying to desing a bass reflex with 2 drivers. How do i start? I start doing the calculation with one driver and the number of ports i want, and once i have it calculated for one.
How do i change the values for two drivers? How do you associate 2 Vas drivers? Or associate 2 Qts,Qms,Qes?etc... Like electronic resistance?!?!:S

Thanks a lot for your help.

Really appreciate it :)


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Post Options Post Options   Thanks (0) Thanks(0)   Quote endre_k Quote  Post ReplyReply Direct Link To This Post Posted: 02 April 2011 at 5:57pm
just one silly question, if i do something like this, but changing the 10Ohm R for a 4 Ohm R (because the 2 drivers in paralel equal to 4Ohms), and after, calculating the T/S parameters with the two drivers wired in paralel, will i be able to know how the T/S parameters change? and then be able to calculate the box with those values?

Could it be possible? Any considerations to do it? Or i'm just dreaming too much?
Thanks again for allClapClap

Set up diagram


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endre_k View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote endre_k Quote  Post ReplyReply Direct Link To This Post Posted: 02 April 2011 at 7:46pm
i found this if someone is interested, or can verify it is correct :

================================================
If
"a" is the number of elements that are connected in series, electrically,
"b" is 1 for normal configuration, 2 for isobaric configuration and
"n" is the total number of elements, the new values are calculated as:

Vas=Vas1*n/b^2
Bl=Bl1*a
Sd=Sd1*n/b
Re=Re1*a^2/n
Le=Le1*a^2/n

So two drivers in series, side by side has a=2, b=1, n=2 and
Vas=2*Vas1
Bl=2*Bl1
Sd=2*Sd1
Re=2*Re1
Le=2*Le1

====================================
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