How big can a port be before its too big?

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Myleso

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Joined 2017
So my parts just arived in the post and the first thing I noticed when I opened up the box is that my 2 x 2.5" ports seem like overkill for the size of my enclosure.

I am building a portable powered bluetooth speaker using 2 FaitalPRO 5FE120 5" woofers and some silk dome tweeters. They will be mounted in a 15L enclosure tuned to 55Hz and using WinISD I worked out that to keep port air velocity below 17m/s with 100W input, I would need 2 x 2.5" ports at 16.3 cm long.

I am assuming it is possible to have a port too large, but how large does a port need to be before it causes problems? I have read something to do with the 1st port resonance getting too low but I am not realy sure what that means.


Thanks!
 
infinia

infinia

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Joined 2005
Myleso said:
So my parts just arived in the post and the first thing I noticed when I opened up the box is that my 2 x 2.5" ports seem like overkill for the size of my enclosure.

<snip>
I am assuming it is possible to have a port too large, but how large does a port need to be before it causes problems? Thanks!
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It's too big when it doesn't fit in your box. Its one reason passive radiators exist but these problem tunings are usually much lower than midbass.
woah 2x5 inchers @100W port noises would be least of worries. Pipe resonance isn't a 1st order effect ( it looks worse on some simulations ) they can tamed with more box stuffing and keep the opening from facing the rear cone/s
I'd settle on one port 2-2.5". and deal with any chuffing by light EQing. IDK review your SD and Xmax ...seems this cant be a real problem
 
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DiscoJones

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Joined 2017
A larger diameter port means a longer port. The longer the port, the lower the port resonance. The lower the port resonance, the more likely it gets into a frequency range of high energy and difficulty to absorb with internal damping materials. This is especially useful information when designing a small subwoofer as you can make choice to push the port resonance above the crossover point. When porting midwoofers, it's hard to push the resonance above the crossover point, so you just try to find a good balance between chuffing and port size and deal with the minimal resonance energy.

With all that said, you seem to be in a good range as is. I don't think 2 x 2.5" ports are necessary for 2 x 5" woofers, but 16cm is also not a length for concern regarding resonance.

Dan
 
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Myleso

Member
Joined 2017
Alright great. Thanks for all the input everyone. So, it can be a problem when the port starts getting too long but it shouldn't be an issue in my design?

Just did a quick simulation with 2 x 5fe120 (8ohm version In parallel) and one of those 2.5" ports In a 10ltr box look about right. With a 15ltr box and 2 x 2.5" 6.5" long ports I get a huge 7db peak centred around 85hz.
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What program did you use to simulate it? Just realised my enclosure is actually 17 not 15L...whoops. Does that fix the simulation?

Cheers!
 
planet10

planet10

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planet10-hifi.com
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DiscoJones said:
A larger diameter port means a longer port. The longer the port, the lower the port resonance. The lower the port resonance, the more likely it gets into a frequency range of high energy and difficulty to absorb with internal damping materials.
Click to expand...

And at some point, it transitions from a BR box to a Daline-like TL.

dave
 
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andy2

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Joined 2004
DiscoJones said:
A larger diameter port means a longer port. The longer the port, the lower the port resonance. The lower the port resonance, the more likely it gets into a frequency range of high energy and difficulty to absorb with internal damping materials. This is especially useful information when designing a small subwoofer as you can make choice to push the port resonance above the crossover point. When porting midwoofers, it's hard to push the resonance above the crossover point, so you just try to find a good balance between chuffing and port size and deal with the minimal resonance energy.

With all that said, you seem to be in a good range as is. I don't think 2 x 2.5" ports are necessary for 2 x 5" woofers, but 16cm is also not a length for concern regarding resonance.

Dan
Click to expand...

I usually prefer two smaller ports vs. one big port. Probably a cosmetic thing since they look better proportioned vs. one big port.
 
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fatmarley

Member
Joined 2003
Myleso said:
Alright great. Thanks for all the input everyone. So, it can be a problem when the port starts getting too long but it shouldn't be an issue in my design?



What program did you use to simulate it? Just realised my enclosure is actually 17 not 15L...whoops. Does that fix the simulation?

Cheers!
Click to expand...

Rather than reply to your pm I thought It would be best to clear up my mistake here :eek:

I was using Woofer Box Model and Circuit Designer by Jeff Bagby. I think I forgot to click the button "Load selected configuration" and my model was for just one FE120. In my defence, It could have been a glitch In the software because I have had that happen before (two identical models with different results).

It looks like 17ltr Is fine, but I found the 2x2.5" ports needed to be 16" long for a flat response. I think a single 2.5" x 6.62" long port would be better.
 
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Myleso

Member
Joined 2017
Phewf, thats a relief since the box is already built haha.

Hmmm I didn't realise that the port length given was EACH port, I had been thinking is was the sum of the port lengths :/ Thank god you pointed that out.
damn, it will have to be just one port then...mgiht have a 17.4 L box now haha

Thanks heaps for your help :)

Myles
 
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