PVC pipe is pretty cheap and easily cut by even cheap hacksaws. PVC is basically just vinyl.
If 15 cm is what you need, start off by cutting 25 cm off in sections until you get a sound that you like.
Warning: The shorter you make the pipe, the more bass you will hear in the midbass-but your deep bass gets cut off. You have to find the middle, otherwise you will have a "boom box" type sound that will sound delightful for a short time, but rob you of the pleasure of deeper lows.
If 15 cm is what you need, start off by cutting 25 cm off in sections until you get a sound that you like.
Warning: The shorter you make the pipe, the more bass you will hear in the midbass-but your deep bass gets cut off. You have to find the middle, otherwise you will have a "boom box" type sound that will sound delightful for a short time, but rob you of the pleasure of deeper lows.
I think what kelticwizard means is that by starting with a longer pipe, it's easy to alter it by cutting a little off at a time. So go ahead and start at 25cm - you can easily shave a little off, but you can't easily lengthen it.
You will make 2 pipes. The first will be the test pipe that you continually shave down to find your ideal tuning point. You will reach a point where you've cut too much off because it doesn't sound as good anymore. So then you know what length pipe sounded best, and can cut the second pipe to that spec. voila.
So start with a pipe that will be long enough to cover any sane lowest tuning frequency (15.72cm = 30Hz, 25.21 = 25Hz, 42.69 = 20Hz). If you want an absolute opinion, then sure, go with 25cm. You will almost definitley be lower than your target point, and can work your way up. Cut 2cm off at a time which raises the tuning frequency by 1Hz approximately.
You will make 2 pipes. The first will be the test pipe that you continually shave down to find your ideal tuning point. You will reach a point where you've cut too much off because it doesn't sound as good anymore. So then you know what length pipe sounded best, and can cut the second pipe to that spec. voila.
So start with a pipe that will be long enough to cover any sane lowest tuning frequency (15.72cm = 30Hz, 25.21 = 25Hz, 42.69 = 20Hz). If you want an absolute opinion, then sure, go with 25cm. You will almost definitley be lower than your target point, and can work your way up. Cut 2cm off at a time which raises the tuning frequency by 1Hz approximately.
How will I go about this without having to keep unscrewing the driver each time? Shall I just make the hole a fairly tight fit so I can slide the tube out to cut it, but not so it's loose?
I can't see an easy way without removing the driver each time. You could keep the test port on the outside of the enclosure, but the tuning would be different, not because of where the port is, but because the enclosure volume will have changed. To keep it consistent, I would go through the hassle of removing the driver each time. Maybe keltic has an alternative?
Which reminds me, all equations thus far have not taken port dispacement into account (and the bracing and driver displacement was also just a guess). So when I say 25cm = 25Hz, the tuning frequency will actually be rasied when accounting for port displacement.
It's feasible that even a 25cm port may be above the tuning frequency you desire. If you don't care what the figure is (after all, it's merely academic in this situation), then go with the longest port possible to start with. 40cm. If you want to methodically reverse-engineer it to arrive at real numbers, then give us the dimensions of your bracing and we can take it from there...
Which reminds me, all equations thus far have not taken port dispacement into account (and the bracing and driver displacement was also just a guess). So when I say 25cm = 25Hz, the tuning frequency will actually be rasied when accounting for port displacement.
It's feasible that even a 25cm port may be above the tuning frequency you desire. If you don't care what the figure is (after all, it's merely academic in this situation), then go with the longest port possible to start with. 40cm. If you want to methodically reverse-engineer it to arrive at real numbers, then give us the dimensions of your bracing and we can take it from there...
I didn't add any bracing 
The thickness of the MDF and size of each panel means there isn't any unwanted resonance.
The thickness of the MDF and size of each panel means there isn't any unwanted resonance.
gross internal volume = 89.7l
average 12" displacement = 3.4l
8cm x 25cm port displacement = 1.2l
net volume = 85.1l
tuning frequency = 23.77Hz
In conlcusion, starting with 25cm will result in tuning to 23.77Hz. A rather low, but ok place to start.
average 12" displacement = 3.4l
8cm x 25cm port displacement = 1.2l
net volume = 85.1l
tuning frequency = 23.77Hz
In conlcusion, starting with 25cm will result in tuning to 23.77Hz. A rather low, but ok place to start.
The tuning frequency doesn't directly show how low a sub will go. port tuning decides the point where the port will contribute to the output and is usually closely fit with the enclosure and the driver.
This whole exercise is optimistic as best. But i'm curious to see the results. It may work well for you.
This whole exercise is optimistic as best. But i'm curious to see the results. It may work well for you.
Ok. Well I'll give it a try when I get some pipe and let you know how it went.
Thanks for your advice 🙂
Thanks for your advice 🙂
Another method is to extend the PVC pipe outside the box and cut just a little off at a time. When you hear what you like, just cut a second pipe about 2 cm longer than that and put it into the box. This will compensate for the slightly higher tuning that results when you put the port inside the box.
Just that touch of difference is not going to be able to be heard anyway, so just adding that 2 cm difference is essentially academic. Nobody's ear is that sensitive.
Tuning the port while it extends outside the enclosure is the normal way-I have heard engineers for companies tell me that they do it that way. This avoids having to remove the woofer all the time. 🙂
Just that touch of difference is not going to be able to be heard anyway, so just adding that 2 cm difference is essentially academic. Nobody's ear is that sensitive.
Tuning the port while it extends outside the enclosure is the normal way-I have heard engineers for companies tell me that they do it that way. This avoids having to remove the woofer all the time. 🙂
Please disregard above post. I checked WinISD and that changes things.
If you start out with an 8 cm pipe that is 25 cm long, your pipe volume will be about 1250 cm³, or 1.25 liters.
If you deduct that 1.25 liters from a 76 liters box, you find that it makes a difference of less than 1 Hz. In fact, it makes a difference of only 0.25 Hz! So if you end up tuning to 30 Hz when your pipe extends outside the box, when you move the pipe inside the box it will be tuned to between 30 and 30.3 Hz-a totally inaudible difference.
No need to compensate for outside/inside tunings. Just extend your port outside the box, cut it until it sounds right, then place it inside the box with no modifications. You will be all set.
If you start out with an 8 cm pipe that is 25 cm long, your pipe volume will be about 1250 cm³, or 1.25 liters.
If you deduct that 1.25 liters from a 76 liters box, you find that it makes a difference of less than 1 Hz. In fact, it makes a difference of only 0.25 Hz! So if you end up tuning to 30 Hz when your pipe extends outside the box, when you move the pipe inside the box it will be tuned to between 30 and 30.3 Hz-a totally inaudible difference.
No need to compensate for outside/inside tunings. Just extend your port outside the box, cut it until it sounds right, then place it inside the box with no modifications. You will be all set.
Right I'm going to tune the port to about 17 CM so that it's about 28 or so Hz. No point messing about, I've just gotta make a decision! 🙂
OK this is kinda urgent as I'm going to get the piping tomorrow. Standard drain pipe is 7CM here so what length should it be for a reasonably low tuning frequency of 30Hz as you suggest?
Superb. I'll get that done tomorrow then, along with some other minor tweaks I have planned 🙂
I have been told by numerous sources that car drivers work best in ported enclosures, so hopefully all should go reasonably well!
Just one more question regarding port placement - should it be on the front side panel facing into the room, rear side panel facing into the corner it's sitting in, or what? Thanks.
I have been told by numerous sources that car drivers work best in ported enclosures, so hopefully all should go reasonably well!
Just one more question regarding port placement - should it be on the front side panel facing into the room, rear side panel facing into the corner it's sitting in, or what? Thanks.
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