Design of Rectangular ports for a BassReflex Box |
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endre_k
Registered User Joined: 23 September 2010 Status: Offline Points: 201 |
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Posted: 31 March 2011 at 8:09pm |
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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) ================================================== 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 :)) |
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Steve_B
Old Croc Joined: 29 September 2007 Status: Offline Points: 1587 |
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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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audiomik
Old Croc Joined: 06 April 2010 Location: Bath, UK Status: Offline Points: 2962 |
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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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bass traffic
Registered User Joined: 11 May 2010 Status: Offline Points: 1283 |
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Always think bigger because your dreams might be the only thing holding you back.
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endre_k
Registered User Joined: 23 September 2010 Status: Offline Points: 201 |
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hmmm Any more information please? Or where to find more?? :) thx cheerz from Spain! |
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Steve_B
Old Croc Joined: 29 September 2007 Status: Offline Points: 1587 |
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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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greeef
Registered User Joined: 25 June 2009 Status: Offline Points: 128 |
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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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Steve_B
Old Croc Joined: 29 September 2007 Status: Offline Points: 1587 |
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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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Lee In Montreal
Registered User Joined: 09 March 2011 Location: Montreal Status: Offline Points: 345 |
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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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endre_k
Registered User Joined: 23 September 2010 Status: Offline Points: 201 |
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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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endre_k
Registered User Joined: 23 September 2010 Status: Offline Points: 201 |
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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 all
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endre_k
Registered User Joined: 23 September 2010 Status: Offline Points: 201 |
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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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