I'm making a new pair of cabinets for an older speaker from MDF. The speakers were manufactured in Europe and the bass port measures a perfect 80mm ID x 165mm in length.
It's easy to find 3" ID bass port tubing here in NA...but 80mm, not so much. Being that the 3" ID tube is roughly .125" ID smaller, will the low frequency performance suffer or sound much different?
It's easy to find 3" ID bass port tubing here in NA...but 80mm, not so much. Being that the 3" ID tube is roughly .125" ID smaller, will the low frequency performance suffer or sound much different?
audio39,
My understanding is that to get the same tuning of a 80mm x 165mm port, a 3" (76.2mm) port needs to be slightly shorter. But since this is a matter of a few mm, keeping it 165mm should be ok.
If you want to find out the new port length required for the change diameter, you can enter the info into the cells and see what you get:
Port Length Calculator
My understanding is that to get the same tuning of a 80mm x 165mm port, a 3" (76.2mm) port needs to be slightly shorter. But since this is a matter of a few mm, keeping it 165mm should be ok.
If you want to find out the new port length required for the change diameter, you can enter the info into the cells and see what you get:
Port Length Calculator
I entered the data into the calculator and got numbers VERY different than those of the present port length and diameter.
For the original 3.125" diameter the calculator says the length should be 15.4", entering the 3" diameter renders a port length of 14.1". The original port is 6.5" in length!
For the original 3.125" diameter the calculator says the length should be 15.4", entering the 3" diameter renders a port length of 14.1". The original port is 6.5" in length!
audio39,
From the measurements you provided, the original cabinet seems to have an internal volume of ~1.99 cu. ft. A box tuned to 34.5 Hz gives us an approx port size of 6.44" (163.5mm). By reducing the port diameter from 3.125" to 3" and keeping the length same tunes the box to approx 33.3 Hz. So keeping the port length the same should not cause any major issues. To keep tuning at 34.5 Hz with a 3" port, the length needs to be 5.85".
Hope this helps. 🙂
From the measurements you provided, the original cabinet seems to have an internal volume of ~1.99 cu. ft. A box tuned to 34.5 Hz gives us an approx port size of 6.44" (163.5mm). By reducing the port diameter from 3.125" to 3" and keeping the length same tunes the box to approx 33.3 Hz. So keeping the port length the same should not cause any major issues. To keep tuning at 34.5 Hz with a 3" port, the length needs to be 5.85".
Hope this helps. 🙂
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Thanks very much!
The label on the rear of the cabinet states 24hz - 34khz...I had been punching 24hz into the calculator. I'm fabricating the new cabinets deeper with a narrower front, while the height will remain the same.
I found this on your site earlier but was unclear as to whether 1/8" would realize any "real" difference.
Thanks very much!
After more then 40 years in this hobby this is the first time I'm attempting this kind of project...a change to a slot vs a port I can buy off the shelf is beyond my present comprehension!!! 😉
Is the new front baffle significantly narrower? Are these multi-way speakers? Some speakers have BSC (Baffle Step Compensation) in the XO and the the front baffle width needs to be considered for BSC calculation. Speakers with wide baffles might not need any BSC circuit. Again, a small change might not matter much.
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The change is 2" in width on the outside, with 1.5" side walls the internal width is changed by 3.5". The speakers are 3 way.
This BSC circuit...why would they do this to me? 😱
Make an adjustable length port that overlaps the 'correct' length by a few centimetres +/-
this way you can fine tune as you like.
this way you can fine tune as you like.
Well it's a simple matter of using area of circle calculator, then dividing the result into whatever dimensions are convenient, and makes it easy enough to adjust for net port volume.
I've become a fan of higher aspect ratio rectangular slots ( i.e. wide, short and deep).
Just so I'm clear...it's the area of an 80mm circle x the original length of the bass port?
If it's actually that simple then adding that to my design is easy. The difference being that the "slot" would be roughly 3" under the low frequency driver vs the port which was to be located on the rear of the cabinet at the top (this to keep it as far away from the woofer as possible).
See any problems here?
For future reference, this calculator is accurate to 0.01" lengthwise for yours and several other combinations according to Hornresp, a common reference around here.
GM
Down below ~80 Hz, few folks would be able to discern a few Hz difference even if doing an A/B comparison since room modes dominate. About the only time it might make a difference was if one was tuning to exact driver Fs to maximize bass response when driven by a high output impedance such as a SET amp.
GM
Been one since the beginning due to starting with the simple baffle thickness reflex vents of the day [Altec VOTs with up to 12:1 aspect ratio], but get too high [> 9:1 if an old heat/AC duct design routine is accurate] and the added friction lowers tuning/bass output Vs what a simple calculator computes, so may be too long, or if baffle thickness, too small in area [Av].
GM
Greg - I guess you're probably aware, but perhaps the wider audience isn't - most of my builds are with either smaller (2-4") wideband drivers, midwoofers in the 6-8" range, or combinations thereof, in enclosures designed by Dave or Scott Lindgren. The ratio of slots is generally less than 6:1,except perhaps on the larger MLTLs, and while I can't address how they factor the variable of the air flow friction into their tuning calculus, the results speak for themselves.
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