I've had a few speakers kicking about and found a nice box that they fit in. I've also found a couple of D class amps that can power the 2x3.5" speakers (140w) and the 6" sub (250w). The only problem is when running through a port calculator I'm getting a port length of 442.4cm @35hz which isn't gonna fit in the box! I'm not sure if I've messed up on my calculation and should have used a larger box. Can anyone advise if it's worth carrying on with the project either with a short port or as a sealed sub? I'm trying to build a system to run off of 12v for days out during the summer.
The box is 22x20x24.5cm and was thinking of sealing off the 2x3.5" speakers so would loose a bit more space.
Apologies if newbie questions it's my first build and struggling to get my head around it. 🤔
The box is 22x20x24.5cm and was thinking of sealing off the 2x3.5" speakers so would loose a bit more space.
Apologies if newbie questions it's my first build and struggling to get my head around it. 🤔
Welcome to the forum!
It would help if you were to tell us the make and model of the 6" woofer, or supply a link to its specifications.
If it is the outside dimensions of your box you have given, then its internal volume is in the order of 10 litres.
P.S. I'd be interested to know which "port calculator" you used.
It would help if you were to tell us the make and model of the 6" woofer, or supply a link to its specifications.
If it is the outside dimensions of your box you have given, then its internal volume is in the order of 10 litres.
P.S. I'd be interested to know which "port calculator" you used.
Thanks for the replies and for the heads up JMFahey. I will try and find some more details and take another look.
Cheers Galu. The sub is a qtx qwx6 250w model. The dimensions are for the internal space but obv will loose a bit when I box off the two 3.5" speakers. I will try and minimize this as much as poss and can recalculate the internal space. The specs from the website are below and the link to the port length calculator I used was: https://www.omnicalculator.com/other/port-length
Thanks again,
Dan
Cheers Galu. The sub is a qtx qwx6 250w model. The dimensions are for the internal space but obv will loose a bit when I box off the two 3.5" speakers. I will try and minimize this as much as poss and can recalculate the internal space. The specs from the website are below and the link to the port length calculator I used was: https://www.omnicalculator.com/other/port-length
Thanks again,
Dan
| Power max. | 200W | 250W | 500W |
|---|---|---|---|
| Power rms | 100W | 125W | 250W |
| Overall depth | 80mm | 90mm | 95mm |
| Magnet diameter | 100mmØ | 120mmØ | 140mmØ |
| Mounting centres | 122mmØ | 157mmØ | 194mmØ |
| Diameter | 130mm (5.25") | 165mm (6.5") | 200mm (8") |
| Frequency response | 40Hz - 10kHz | 35Hz - 8kHz | 27Hz - 6kHz |
| Magnet weight | 600g (20oz) | 900g (30oz) | 1.1kg (40oz) |
| SPL @ 1W/1m | 84.8dB | 88dB | 85.5dB |
| Weight | 1.27kg | 1.92kg | 2.76kg |
Working in imperial units:
If you were to tune that 0.353 cu ft box to the 35 Hz resonant frequency of the woofer, the round port length calculator in the link that follows suggests that a 1.25" diameter port should have a length of 6.7 inches (17 cm): https://www.the12volt.com/caraudio/boxcalcs.asp#por
To calculate the optimal box tuning frequency, we would require to know the Thiele/Small parameters of your woofer which, unfortunately, are not quoted in the specifications you gave.
Please note, however, that 1.25" is rather a small port diameter and could generate wind (chuffing) noises. Taking this and what JMFahey had to say, I'd be inclined to try tuning the box to, say, 70 Hz. This would require a 2.0" inside diameter tube of length 3.43" (8.7 cm).
P.S.The above calculated port dimensions seem to be in the correct ballpark to me. My David Weems "Building Speaker Enclosures" book gives an example of a 0.5 cu ft box tuned to 70 Hz by a 2" diameter port of length 2". A smaller box volume would require a longer port.
P.P.S. Under the circumstances, the dimensions would not be critical.
If you were to tune that 0.353 cu ft box to the 35 Hz resonant frequency of the woofer, the round port length calculator in the link that follows suggests that a 1.25" diameter port should have a length of 6.7 inches (17 cm): https://www.the12volt.com/caraudio/boxcalcs.asp#por
To calculate the optimal box tuning frequency, we would require to know the Thiele/Small parameters of your woofer which, unfortunately, are not quoted in the specifications you gave.
Please note, however, that 1.25" is rather a small port diameter and could generate wind (chuffing) noises. Taking this and what JMFahey had to say, I'd be inclined to try tuning the box to, say, 70 Hz. This would require a 2.0" inside diameter tube of length 3.43" (8.7 cm).
P.S.The above calculated port dimensions seem to be in the correct ballpark to me. My David Weems "Building Speaker Enclosures" book gives an example of a 0.5 cu ft box tuned to 70 Hz by a 2" diameter port of length 2". A smaller box volume would require a longer port.
P.P.S. Under the circumstances, the dimensions would not be critical.
I did think that seamed a bit small when I adjusted the port diameter parameter on the calculator. Thanks for taking the time to have a look. I have a 2" port here from a damaged speaker at home. So tuning to 70hz could be the best solution. I'll check the book mentioned too as hope I can have a go at more projects in the future. 🙏
Let us know how it goes. Meanwhile, here are some more thoughts from my permanently addled brain! 
You could always cut a selection of lengths from a 2" plastic pipe and slide them, in turn, into a snug locating hole to determine by experiment which length audibly delivers the best bass response.
And we should consider that this 6" woofer may actually sound better in a sealed box!
Let's face it, given this T/S parameter unknown woofer, experimentation is the order of the day!
You could always cut a selection of lengths from a 2" plastic pipe and slide them, in turn, into a snug locating hole to determine by experiment which length audibly delivers the best bass response.
And we should consider that this 6" woofer may actually sound better in a sealed box!
Let's face it, given this T/S parameter unknown woofer, experimentation is the order of the day!
