Narrowing the port on a prebuilt box?

Nixxuz

Nixxuz

Super Member
So I have this sub;
https://www.skaraudio.com/collections/evl-subwoofer-series/products/evl-18-inch-car-subwoofer
And I bought this box after getting tired of the one I (badly) built that was sealed;
https://www.skaraudio.com/collectio...ts/skenc18vented-blk-18-inch-vented-enclosure

I thought about building another, but I'm short on time for involved projects right now due to work. I figured for $100 I could modify the prebuilt and give it maybe some bracing, internal padding, etc.

What I'm wondering is; can I narrow the vented port to lower the tuning? I have plenty of amplification for it, so a reduction in efficiency wouldn't be an issue. But I assume there are other considerations, or else everyone would do it.

And as an appeal to all the purist engineers out there; I only want to lower it's usable FR. I'm not looking to chase perfection from a million different parameters. I've seen a few videos that seem to claim that a simple narrowing of the port could get me from the 31.5hz tuning of the prebuilt box, down to something like 25hz. This won't be strictly necessary for almost all music, but I'd like to know it could get that low if desired.
 
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Nixxuz said:
I've seen a few videos that seem to claim that a simple narrowing of the port could get me from the 31.5hz tuning of the prebuilt box, down to something like 25hz. This won't be strictly necessary for almost all music, but I'd like to know it could get that low if desired.
Click to expand...
Why exactly do you want to lower the tuning frequency from 31.5Hz to 25Hz? Lowering the tuning frequency doesn't extend the frequency range.
Yes, the port tuning is dependant on port cross sectional area and port length. A larger cross section gives a higher tuning and a smaller one gives a lower tuning. A shorter port gives a higher tuning and a longer port gives a lower tuning. It might be easier to lower the tuning by extending the length. A flat piece of wood glued inside the box would do that.
However, I repeat, why do you want to lower the tuning? I haven't gone through all the specs and the maths but a quick glance at them shows a high Q that suggests the box shouldn't be ported anyway.
There are some calculators you could use to check things out here:
http://www.mh-audio.nl/
 
The frequency range of the driver is published at 20hz-300hz. From what I know, if the driver is capable, it's the enclosure that determines the actual FR output.
Again, while I appreciate the suggestion, I'm looking to modify the box tuning specified lower than stock. Not build a new box. The reasons for wanting to lower the tuning are; I have a very few recordings that manage to put out frequencies lower than 31.5hz.
 
Nixxuz said:
The frequency range of the driver is published at 20hz-300hz. From what I know, if the driver is capable, it's the enclosure that determines the actual FR output.
Again, while I appreciate the suggestion, I'm looking to modify the box tuning specified lower than stock. Not build a new box. The reasons for wanting to lower the tuning are; I have a very few recordings that manage to put out frequencies lower than 31.5hz.
Click to expand...
Just because that's it's tuned frequency doesn't mean it has zero output below that.

Could always try plugging the port, what size is it? I've found Styrofoam cups work great for this purpose. Tried rubber stoppers, the multiple drinking straw method, etc, found the cups work best. Due to their shape they wedge into the port very snuggly.

61jU8kW92JL._SY550_.jpg
 
It's a 4.5"x20" slot port. I'm not sure I wanna see the styrofoam cup that could plug that...
But again, I don't really want a sealed box. I got a good deal on a vented box. The current box is sealed, but looks pretty crappy. $100 for a vented box, made by the same company as the driver, is worth more, to me, than the time it would take me to build another right now.
If it can't be done, that's fine. I just wanted to cut some lengths of wood equal to the port length and hopefully drop the tuning. It was a method I had seen on a couple forums but can't seem to find again.
 
Nixxuz said:
I'm looking to modify the box tuning specified lower than stock. Not build a new box.
Click to expand...
I wasn't suggesting you build a new box. Looking at the picture in your link I see the slot extends part way up the back of the box by about 6 inches. Gluing an 8 inch piece of wood to it would extend it by two inches. Or just a two inch strip glued to its edge would do the same.

I've just plugged the T/S parameters into this calculator ...
http://www.mh-audio.nl/Calculators/ReflexBoxCalculator.html
... and it suggests the optimum box volume is 762 litres! It also suggests the port should be tuned to 18Hz.
A 150L box requires a port tuned to 31Hz but gives an output 10dB down at 20Hz and has a hump at about 51Hz.
 
rothwellaudio said:
I wasn't suggesting you build a new box. Looking at the picture in your link I see the slot extends part way up the back of the box by about 6 inches. Gluing an 8 inch piece of wood to it would extend it by two inches. Or just a two inch strip glued to its edge would do the same.

I've just plugged the T/S parameters into this calculator ...
http://www.mh-audio.nl/Calculators/ReflexBoxCalculator.html
... and it suggests the optimum box volume is 762 litres! It also suggests the port should be tuned to 18Hz.
A 150L box requires a port tuned to 31Hz but gives an output 10dB down at 20Hz and has a hump at about 51Hz.
Click to expand...
Checked the same thing myself and it's 27hz at -3db... I guess I'll have to check it out later today or tomorrow after I get it mounted. According to a different calculator, if I block off about half the slot it would be close to tuned for around 25hz. I'll play with it.

And yeah, that "optimal" thing was weird. A couple people helped me with it over a year ago and I ended up with a 13cuF box that I eventually cut down and sealed. I went about things wrong on that one.
 
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