PDA

View Full Version : Discrepancies in box tuning calculations on the web



GrnEydDvl
06-07-2008, 01:18 PM
I was eying some Digital Designs subs this weekend which led to me looking over their box designing page. I've never owned or even heard a DD sub.

For those that might not know, they recommend a certain amount of airspace, port area, and to tune to 40hz, due to the way they design their subs. According to their site they design the subs somehow to specifically benefit from the cabin gain in automobiles and give good response when tuned to 40hz, as opposed to a more common tuning range of 30-36hz. Link to their site for more info (http://www.ddaudio.com/dd/caraudio/enclosureInfo/boxSize_hiEff.asp)

I took their numbers and ran them across at least 6 online port length calculators. None of them matched the port lengths found on the DD site. Most gave answers that did not equal the other calculators as well - a multitude of "correct" port lengths even though all other variables were kept constant.

Looking over threads here and on other sites, the general consensus is the online calculators are just wrong.

Does anyone know why this is?

If they are all wrong, which is shocking (why build a calculator that cannot handle the basic function it is designed to do), then what is right?

Do any of you test the boxes you have built to see how they perform as far as port tuning? I understand a car is not an anechoic chamber and that the common tools we have to measure with are not terribly accurate. But we should be able to get a general idea if the tuning is close enough to what we had intended.

Does winISD get port calculations right? The tutorial here by Noah uses winISD.

I imagine DD has it right, as they would have tons of upset customers who built to their specs with poor performance if they were wrong.

Mr Cabinetry
06-07-2008, 04:06 PM
I was eying some Digital Designs subs this weekend which led to me looking over their box designing page. I've never owned or even heard a DD sub.

For those that might not know, they recommend a certain amount of airspace, port area, and to tune to 40hz, due to the way they design their subs. According to their site they design the subs somehow to specifically benefit from the cabin gain in automobiles and give good response when tuned to 40hz, as opposed to a more common tuning range of 30-36hz. Link to their site for more info (http://www.ddaudio.com/dd/caraudio/enclosureInfo/boxSize_hiEff.asp)

I took their numbers and ran them across at least 6 online port length calculators. None of them matched the port lengths found on the DD site. Most gave answers that did not equal the other calculators as well - a multitude of "correct" port lengths even though all other variables were kept constant.

Looking over threads here and on other sites, the general consensus is the online calculators are just wrong.

Does anyone know why this is?

If they are all wrong, which is shocking (why build a calculator that cannot handle the basic function it is designed to do), then what is right?

Do any of you test the boxes you have built to see how they perform as far as port tuning? I understand a car is not an anechoic chamber and that the common tools we have to measure with are not terribly accurate. But we should be able to get a general idea if the tuning is close enough to what we had intended.

Does winISD get port calculations right? The tutorial here by Noah uses winISD.

I imagine DD has it right, as they would have tons of upset customers who built to their specs with poor performance if they were wrong.

Here we go again.

There been many topic's/threads created about this very issue to which various opinions have been formed and discussed at great length. Though it may appear the consensus leans towards that none of the calculators online accurately provide or fall within what would be consider a tolerable level of error between them as to what is a " recommended enclosure " vs using a calculator to confirm that recommendation as correct.

Unless you are using the same method of calculation or program the subwoofer company used to design the enclosure, you are going to end up with the discrepancies you discovered.

Therefore, you two choices, use the recommended enclosure as prescribed and ignore the minor discrepancies the calculator is presenting or use the calculator as such to the spec's stated which you will arrive at a totally different design from the company.

Keep in mind as well that not all calculators allow for the same input variables which can cause such discrepancies in the design.

Basically, it comes down to faith, which program or method of calculation are willing to accept as the " right one ".

GrnEydDvl
06-07-2008, 06:27 PM
Right.

The speaker companies have the resources and a very compelling reason ($ales) to thoroughly test their products in a very large variety of enclosures and environments.

Logic would dictate that if we as individuals wanted to ensure a higher level of performance, we would do the same thing - build a large number of boxes, and use the one we prefer for whatever reason. Or take the easier route and have faith, as you put it, in the manufacturers recommended enclosure or what has worked well for someone we know.

We can, of course, build as many boxes as we have desire, money, and time to dedicate. SPL competitors do just that and for good reason. Me? I don't want to build 20 boxes to find out which one peaks one tenth of a dB higher, I just want a well performing system attained in a reasonable amount of time and resources.

My point/question was - aren't simple ported boxes, subwoofers, and how these two interact well understood by now? I guess they aren't, or at least not in a way we can apply across every subwooofer and come up with a universal equation.

Your point is well taken, though. I already have my answer. Go on faith or experiment until I decide enough is enough.