Ported my cab

[Phil Starr]

OK brief explanation of how the ports work, because if they stick out of the cab front and back they won't at all.

The ports hold a lump of air, at the tuned frequency the air in the port bounces backwards and forwards on top of the air in the cab, which kind of acts like a spring for the air in the port. This works in exactly the same way as when you blow across the neck of a bottle where the air in the mouth of the bottle bounces on the volume inside.

Now if you change the mass of the air in the port or the size of the air in the cab it will resonate at a different frequency. The trick is to tune the cab so it kicks in just as the speaker starts losing bass output so you get just a little more bass.

Now you can see why the ports need to be identical, if you have two masses they will each have different weights of air in and will resonate at different frequencies. Now it's not only the length of the port that matters (long ports=more air and lower tuning) but also the area of the port. By making the area smaller you end up with a smaller port. So a single shorter port is going to tune to the same frequency as a longer double port.

So if you make long ports right through the cab they won't be connected to the air in the cab and they won't resonate when the speaker hits the tuned frequency, you won't have a port you'll have a passageway through the cab. You can bend the ports inside the cab if you want, you could even have them sticking out of the front of the cab, but I think your best solution would be to have a single port.

Edited May 3, 2016 by Phil Starr

[luckydog]

+1 for PS's post above.

Choose the port resonance to suit the driver, and make sure it holds enough air in the port to cope with the normal flux of air, cos this limits the level at which the port will work - though at 4" diameter that should be fine.

Without going Einstein, it's best to calculate and design it, because it's unlikely to end up optimal by trial and error even though that is fun to do to hear the effects.!

LD

[Beer of the Bass]

Would two ports of different lengths really have two discrete tuning frequencies, or just a single tuning frequency?
I'm more or less thinking out loud here, but my cabs have 3 identical ports. With all three open they're at 50hz, and with one blocked the tuning drops closer to 40 Hz. That makes me think that the tuning frequency is a property of the whole cab/port system and not of the individual ports. If that's the case, wouldn't dissimilar ports still give you a single resonance but just make it harder to model?

[KingBollock]

[quote name='Maude' timestamp='1462257598' post='3041435']
I'm confused.
Are your ports now going straight through the cab now if they stick out the back? If so then you cab is now sealed again just with a smaller internal area surely, unless I've misunderstood you, which is quite likely .
[/quote]
Yeah, that's what I keep picturing. But I am assuming that he's actually cut the ports in the back of the cab.

[alexclaber]

The way to understand ports is that they're just like a mass bouncing on a spring. The bigger the port volume, the greater the mass. The bigger the cab volume, the softer the spring. The bigger the port area, the harder the spring. A heavier mass lowers the tuning frequency. A softer spring lowers the tuning frequency.

If your port is too small in area then the port develops turbulence and instead of having a neat and tidy lump of air vibrating back and forth (like having a subwoofer whose cone is made of air), you get air blowing/chuffing/whistling back and forth. If you increase the port area that stiffens the spring and raises the tuning frequency, so you then need to lengthen the port to increase the mass and bring the tuning frequency back down. Making the port volume bigger makes the cab volume smaller (assuming the port is within the cab) so that raises the tuning frequency again. And round and round you go until you get where you want to be!

A port should be tuned to help out where the speakers start rolling off. Below the port tuning frequency it starts to cancel out the output from the woofers, so you have to be careful not to tune too high or you lose more than you gain.

[Ghost_Bass]

If the ports are too big for the cab's depth you can use a 90º curve to bend the tube upwards inside the cab and get the desired lenght.

[Twincam]

Lol the ports are just in the back of the cab.
I actually can't believe some people would envision that they were right through the cab lol!

They are just to big for the 9inch depth.

[Twincam]

[quote name='alexclaber' timestamp='1462273762' post='3041580']
The way to understand ports is that they're just like a mass bouncing on a spring. The bigger the port volume, the greater the mass. The bigger the cab volume, the softer the spring. The bigger the port area, the harder the spring. A heavier mass lowers the tuning frequency. A softer spring lowers the tuning frequency.

If your port is too small in area then the port develops turbulence and instead of having a neat and tidy lump of air vibrating back and forth (like having a subwoofer whose cone is made of air), you get air blowing/chuffing/whistling back and forth. If you increase the port area that stiffens the spring and raises the tuning frequency, so you then need to lengthen the port to increase the mass and bring the tuning frequency back down. Making the port volume bigger makes the cab volume smaller (assuming the port is within the cab) so that raises the tuning frequency again. And round and round you go until you get where you want to be!

A port should be tuned to help out where the speakers start rolling off. Below the port tuning frequency it starts to cancel out the output from the woofers, so you have to be careful not to tune too high or you lose more than you gain.
[/quote]

Yes which is why they stick far out the back currently once again they are not through the cab lol just out the back. If I decided to place them internally at a bend I would recalculate the cab area.

[Twincam]

[quote name='CamdenRob' timestamp='1462260001' post='3041443']
Could you extend the ports, bend them over the top, then flare them out a bit so it looks like a giant double trumpeted gramophone?
[/quote]

I so wanted to do this but the maths did not add up. There was cool looking flared pieces in b&q I could of modified and painted. The other daft but fun idea was to have multiply smaller ports but coming through were the old Te combo head was.

Edited May 3, 2016 by Twincam

[Twincam]

Wow well that worked well just tested the cab although not at very high volume I did turn it up a bit.
The booming has totally gone. It's very clear while also seemingly going a lot deeper than it did originally and then with the mk1 porting lol.
Reacts well with eq changes etc.

However transporting and not getting laughed at might be an issue. (note a fellow Basschat members atk in bits on the floor)

[URL=http://s330.photobucket.com/user/Twincam2008/media/Mobile%20Uploads/WP_20160503_14_33_30_Pro_zpszqw2a2y1.jpg.html][IMG]http://i330.photobucket.com/albums/l415/Twincam2008/Mobile%20Uploads/WP_20160503_14_33_30_Pro_zpszqw2a2y1.jpg[/IMG][/URL]

[3below]

You will need a mesh cage over those exposed ports

Porting upwards into the space where the head was could be a 'cool' solution. Is there sufficient space and length?

Edited May 3, 2016 by 3below

[Ghost_Bass]

[quote name='3below' timestamp='1462297970' post='3041924']
You will need a mesh cage over those exposed ports

Porting upwards into the space where the head was could be a 'cool' solution. Is there sufficient space and length?
[/quote]

Now that i think of it the head space could have been used as a port bt itself. Simply cutting an acces to the inside of the cab near the back and plugging tha back of the head space would make a pretty good shelving port. Only calculation needed would be to know how much area to remove from the top of the box.

[Twincam]

[quote name='Ghost_Bass' timestamp='1462811907' post='3045876']


Now that i think of it the head space could have been used as a port bt itself. Simply cutting an acces to the inside of the cab near the back and plugging tha back of the head space would make a pretty good shelving port. Only calculation needed would be to know how much area to remove from the top of the box.
[/quote]

Indeed I did think of this. I'm new to calculating cab area, port length and other specs so I did it simply. It's only a cab used in the house anyhow. Works very well. Needs a bit of distance from the wall though.