# Port dimensions vs speaker size



## Pancakes (Jan 16, 2008)

What are the "rules of thumb" people here normally use and why/what results or variables can be expected from various port designs? I normally try to make the ports as large as possible withouy having the length be much more than ~4x the diameter. Have built a few speakers but never with the same drivers / box twice and haven't ever made a test jig to play with these variables in a controlled way.

I'm keen to hear any thoughts especially regarding large volume ports (by-product of large diameter therefore long ports) and any pros or cons of restricting my diameter to length ratio?

Would any of you have thoughts on using an 8" woofer with a 4.5" port?

Thanks.


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## Lucky7! (Jan 7, 2008)

*Re: port dimensions vs speaker size*

Port size depends upon the tuning of the box, and it's size. It also depends upon the volume of air that it needs to move to keep the speed low enough (<0.05 Mach is my preference) not to chuff or make discernible noise.
Download Unibox, throw the T/S parameters in for your driver and start modelling. Remember to update graphs after each mod and look at the tabs on the bottom line for more.


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## Pancakes (Jan 16, 2008)

*Re: port dimensions vs speaker size*

Yeah I'm ok with tuning etc, just haven't played with ports longer than 4-5 x the diameter, not modelled but in the real world. From my experience somethings that aren't noticable in speakers and amps make a difference to the human ear and sometimes tiny changes in equiptment sounds way different.


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## Lucky7! (Jan 7, 2008)

*Re: port dimensions vs speaker size*



Pancakes said:


> Yeah I'm ok with tuning etc, just haven't played with ports longer than 4-5 x the diameter, not modelled but in the real world. From my experience somethings that aren't noticable in speakers and amps make a difference to the human ear and sometimes tiny changes in equiptment sounds way different.


My experience is that the modelling of tuned bass enclosures is very accurate to real world (measured) results. The maths for how to tune enclosure has been known for 30 years and is now easy to change it quickly and graph it easily. 

Port length to diameter ratio is meaningless without knowing what sort of mass of air you are capable of moving and the speed of it through the selected port. Port noise and chuffing can be very audible and the only way to get this info is to model or measure.

If you've been tuning enclosures in the part, have you been measuring the impedance curve to se the tune?


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## SteveCallas (Apr 29, 2006)

*Re: port dimensions vs speaker size*

One thing to keep in mind is the first port resonance frequency which is dictated solely by port length. I would aim to keep this resonant frequency above the crossover frequency to that of the driver(s) being ported.

For example, if building a ported three way, and you cross to the woofers at 500hz, it would be wise to keep the first port resonance above 500hz. If building a two way, and crossing over the midwoofers at 1.5khz, it would be wise to keep the first port resonance above 1.5khz. You don't want this frequency to be excited within the operating range of the related driver if at all possible. 

Hope that helps.


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