Port location on box / Multiple Ports

I have this current situation:

Enclosure (4,1 Liters)
224mm (h)
142mm (w)
142mm (d)

Driver 1
102mm or 4"

Driver 2 Piezo Tweeter
62mm or 2.5"

After calculating the right parameters for Driver 1, I found out that its EBP is around 155Hz.

WinISD suggests that I should have a port below Driver 1:

102mm wide tube (same size of the Driver 1) being 98mm long OR
a 102 x 102mm squared port with about the same length.

The issue is that I don't have any space for that on the front.
What are my options here?

a) Don't mess with it anymore and leave it as sealed design.

b) Make the same suggested dimensions port on the back of the enclosure
(if that is the case, up or downwards the enclosure).

c) Make the same suggested dimensions port on the side on symmetric sides of enclosure (outwards).
(again, upwards or downwards the enclosure)?

c) Make multiple ports with PVC on the front until the port area is satisfied.
(if that is the case, how many, say 2" tubes, should I have on the front and how long should them all be to match the suggested WinISD dimension).
 
No, I am not joking...
It seems then that the software has a bug.
Anyways I feel more relieved that I just need to make a front 35mm port 10cm long.

Please take a look at the WinISD screens.

[IMGDEAD]http://img803.imageshack.us/img803/3199/driverparameters.jpg[/IMGDEAD]
[IMGDEAD]http://img838.imageshack.us/img838/8080/driverresults.jpg[/IMGDEAD]
[IMGDEAD]http://img855.imageshack.us/img855/2189/drivergraphic.jpg[/IMGDEAD]
 

dumptruck

Member
2010-05-04 5:02 pm
MN
WinISD does not attempt to optimize the port dimensions, it just sets 10.2cm every time and calculates the length for that diameter. It doesn't try very hard to optimize much of anything automatically, really. You're intended to increase the signal level setting while watching the excursion plot, see what level you can reach for the tuning you chose, check the airspeed at your max level, adjust the port diameter, check the length, tweak the tuning or cabinet size, do it all over, etc.
 
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dumptruck

Member
2010-05-04 5:02 pm
MN
That's kind of like loading a bunch of things into a crossover simulator and then saying "so how do I make a crossover?" You need to read a book or a tutorial or something. Sorry I can't think of anything to suggest at the moment. Experiment with what happens to the excursion and transfer function plots as you change the cabinet volume and tuning frequency. Add some power to the signal tab. You'll start to get the idea. As Pete was getting at, the port dimensions should be checked after everything else is how you want it, and then if you find you can't get your tuning with a port that fits in your box, OR a port that has extremely high airspeed at the levels you intend to use, then you have a problem to solve.
 
dumptruck....

I understand. However I ain't picky. The software helped me out and looks like I am good.

The port was designed for the speaker in question, it is 4,1 L on the nail.
I passed the T/S parameters into WinISD and it suggested the box I have.
As for the port, yes, it was a mind change at last minute. I read more about it and finally went on with the port way (i was leaning to the sealed way as some misinformed folks told me to go). So now I can fix this but just adding this port.... how come you say you don`t know what to suggest.

The way you put sounds like I shouldn`t come here for help in the first place. Well... there are people and people. Some are helpful, some are not. I will stick with the more practical folks. Next project will be more meticulous.
 
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re:'It seems then that the software has a bug' no- it's just that it calculates for the worst case scenario, e.g. a huge PA system, where port chuffing may be an issue. For most home systems it is not a problem. Like any tool, you need to learn how to use it...

The "accepted tolerance" for port velocity has dropped over the years. Where it used to be about 25 f/s, it is now more like 16, which WinISL uses. The Alpha version actually gives you this in a plot, not a number.

Ports are not linear with power. They are modeled as a spring-mass, but this is actually a fluid dynamics problem. Way above my education. ( hard stuff so I am told) So, the suggestion above is the most reasonable engineering approach. Prototype!

My experience, 4" was not enough for a 10" sub I built. (now sealed) Chuffing at barely background levels was an issue.
Constant dimension ports are not optimal.
Surface texture is irrelevant at the velocities in play
One larger is easier to deal with than many small
Basically, I don't like the sound of ported boxes at all, so I build few.
 

dumptruck

Member
2010-05-04 5:02 pm
MN
how come you say you don`t know what to suggest.
I meant that I was sorry I did not have a particular link or book to suggest for you to read.

The way you put sounds like I shouldn`t come here for help in the first place. Well... there are people and people. Some are helpful, some are not. I will stick with the more practical folks. Next project will be more meticulous.
I didn't mean to sound like that. I already gave you practical advice - use WinISD as a simulator, not a suggest-er. Things are simplified if 4.1 liters is already the box size, though. If you use the suggested 75Hz tuning, estimated level at xmax is 92.5dB/5W. With an 80Hz highpass, you could get another 6dB. I guess there's not really a more appealing option, so maybe WinISD wins this time ;). As for port size, 35mm looks good, unless you want to go for more volume with something like the 80Hz highpass I mentioned, in which case more like 50mm diameter would be nice.

The port location in this small box is unlikely to matter outside of how easy/difficult it is for midrange from the back of the woofer to get into it. However, if you put it close to cabinet wall, it will act longer than it is. You can verify Fb frequency after you build it, and shorten if needed.
 
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I didn't mean to sound like that.

Ok, thanks. It was just my impression then.

I already gave you practical advice - use WinISD as a simulator, not a suggest-er. Things are simplified if 4.1 liters is already the box size, though. If you use the suggested 75Hz tuning, estimated level at xmax is 92.5dB/5W. With an 80Hz highpass, you could get another 6dB. I guess there's not really a more appealing option, so maybe WinISD wins this time . As for port size, 35mm looks good, unless you want to go for more volume with something like the 80Hz highpass I mentioned, in which case more like 50mm diameter would be nice.

Ok, the part I don't get. Why would I want to set it to 80Hz and gain 6dB. You mean where will this gain be transferred to, the highs? Or are you speaking about volume level?

I have PVCs that are as good as 38mm, so WinISD adjusted the length to 11,5 something. (longer).

I also have 50mm but I need to place it in the very middle/side of the box. What is the safest bet, I can still do either.

The port location in this small box is unlikely to matter outside of how easy/difficult it is for midrange from the back of the woofer to get into it. However, if you put it close to cabinet wall, it will act longer than it is. You can verify Fb frequency after you build it, and shorten if needed.

Did you mean that the ideal was for the port to be below the midrange/woofer? (in this box the midrange is the woofer per se).

I will post some CAD pictures of 38,1mm openings and 50mm so you guys can advice which is best.
 

dumptruck

Member
2010-05-04 5:02 pm
MN
Ok, the part I don't get. Why would I want to set it to 80Hz and gain 6dB. You mean where will this gain be transferred to, the highs? Or are you speaking about volume level?
I was talking about a high pass filter, as in an active crossover or a setting in a home theatre receiver. Then you could get around 6dB higher volume before distortion, because you're not pushing as much energy below Fs. This is all with the same port.

Did you mean that the ideal was for the port to be below the midrange/woofer? (in this box the midrange is the woofer per se).
No. Any place where the midrange from the back of the woofer cone can't shoot directly through the port will probably work fine. If you place it right up against a cabinet wall, you will probably have to cut it shorter than the simulation, but it will still work fine.
 
[IMGDEAD]http://img443.imageshack.us/img443/4189/cadport.jpg[/IMGDEAD]

I was talking about a high pass filter, as in an active crossover or a setting in a home theatre receiver. Then you could get around 6dB higher volume before distortion, because you're not pushing as much energy below Fs. This is all with the same port.

I will have a crossover. It will be the 1st Order Butteworth 6000Hz/12db Cut. Piezo will have an attenuator too. I am still not sure if I should go for that 1st Order though. Would I benefit from other orders?

At first thought I didn't consider a highpass filter, but is it too hard to bring it into this?


No. Any place where the midrange from the back of the woofer cone can't shoot directly through the port will probably work fine. If you place it right up against a cabinet wall, you will probably have to cut it shorter than the simulation, but it will still work fine.

As you can see, from the CAD drawing, the space on the front is very limited. I can make 2 holes of 25mm on each side, middle level. A 35mm hole will just make the whole thing ugly.

So I thought about this port on the back. Does it matter? Isn't it better to do it?

With 50mm diameter, WinISD is asking now a lenght of 210mm (21cm) and that goes beyond the enclosure wich is maximum 160mm.