# DIY port noise question



## Moose

Yo guys, new to the forum and first time building a sub for HT. 

Here's the situation. Had an old Polk audio 10" sub (psw202) with a blown amp. So I bought a bash300 amp and a CSS SDX-10" woofer to retrofit in the box. Box dimensions are 14h x 13.5w x 12.5d. Rear ported with a 2" flared port. I had to cut the port down to fit the woofer in (it is quite deep). Port is now roughly 3" long.. Yikes I know too short.

I have port noise (chuffing) and need a solution before building a new box. I like how small this box is and it looks good. Should I put in a bigger port like a 3"? Should I extend the original port using bends?

Woofer calls for a 17" long port 3" in diameter. I obviously can't do this. So what can I do here?


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## Moonfly

First of, welcome to HTS :T

Everything about a cabinet is designed around the driver. If you change the driver you need to change the cabinet. If the port is chuffing its not because of it length, its because its too small. 

Scrap the cab and start a new.


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## Moose

Moonfly said:


> First of, welcome to HTS :T
> 
> Everything about a cabinet is designed around the driver. If you change the driver you need to change the cabinet. If the port is chuffing its not because of it length, its because its too small.
> 
> Scrap the cab and start a new.


What if I went to a 3" port? Or slotted port?


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## Moonfly

You need to know if the cabinet is large enough first anyway, but if you increase the port, you can reduce or even prevent the cuffing. Ideally you will want to flair the end of the port, this also helps prevent chuffing.

What is the size of the cabinet you have now?

Also, you will likely need a HPF to prevent the sub from bottoming out, what does that amp have for that purpose?

The SDX10 really needs a minimum of 50 litres and a 25hz tune, but going bigger will yield more performance.


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## Moose

Check original post for cab size. It would be flared on both ends if changed to 3" port.


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## Moose

Not sure on hpf for bash300. I think it only has a lpf.


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## Mike P.

> Box dimensions are 14h x 13.5w x 12.5d.


Is the box built from 3/4" material?

http://www.hometheatershack.com/for...55-diy-port-noise-question.html#ixzz1AHcEdibQ ​


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## Moonfly

Moose said:


> Check original post for cab size. It would be flared on both ends if changed to 3" port.


Sorry, I should have been clearer. This is what I was after:



Mike P. said:


> Is the box built from 3/4" material?
> 
> http://www.hometheatershack.com/for...55-diy-port-noise-question.html#ixzz1AHcEdibQ ​


The size we need to know is the cabinets internal volume, not just its physical dimensions.


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## Moose

.95 cubic feet of internal volume on box.

Box made with 3/4 MDF.


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## Moonfly

IMO, you should build a new box. Are you set on using this box, if so we can look at a port that will work, well not chuff anyway. Problem is, as you make the port larger to have to get it longer to tune lower, otherwise your going to end up with it tuned to high and it'll be pointless as a sub anyway. Dont forget, the larger the port, the more it reduces the working internal volume of the cabinet.

You really should build another box.


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## Mike P.

The net volume is about 23 liters, your tuning frequency is 48 hz with your current port.

Green is what you currently have.

Purple is your current cabinet tuned to 24 hz with a pair of CSS 10" passive radiators. To get the same response with a port it would need to be a 3" flared port 36" long. Pretty hard to shoehorn that in.

Yellow would be a new cabinet with a net volume of 50 liters tuned to 23 hz with a 4" flared port 33" long.

As Moonfly pointed out you really should consider a new cabinet for this sub and amp combination.


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## Moose

Maybe I'll build another box then. Thought I could use this one and it would sound good. It does sound decent just has port noise.

Maybe I'll build one out of oak or something that way I will still have the wood grain look. 

I've got winISD so I'll use what they gave me as a design. 

Do u really need a 36" port? I kinda wanna keep this sub a bit low profile, clean looking relatively small size not overly huge. Would you do 2 3" ports on the front or the back?


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## Mike P.

Yes, it would need a 3" diameter flared port 36" long to get decent low end extension without any port noise. You could always have most of the port sticking out the back of the cabinet if it would be hidden by your specific room placement.


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## Moose

Mike P. said:


> Yes, it would need a 3" diameter flared port 36" long to get decent low end extension without any port noise. You could always have most of the port sticking out the back of the cabinet if it would be hidden by your specific room placement.


Mike,

The CSS sdx-10 calls for a 3" port 17" long on the spec sheet. If its 36" long, that's gonna have to be a massive cabinet to get that port in there right? Or can we bend it? If I did passive radiators on the sides of the box, would this sub hit hard and low? And would I be looking at a shorter port overall? It would have more output wouldn't it with the passive radiators?


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## Mike P.

> The CSS sdx-10 calls for a 3" port 17" long on the spec sheet


Do you have a link to that?


http://www.hometheatershack.com/for...55-diy-port-noise-question.html#ixzz1AI8R7fz5 ​


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## Moose

Mike P. said:


> Do you have a link to that?
> 
> http://www.hometheatershack.com/for...55-diy-port-noise-question.html#ixzz1AI8R7fz5


http://www.creativesound.ca/pdf/CSS-SDX10-data-140408.pdf


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## Mike P.

With the Bash 300 amp the port would have an airspeed of over 42 m/s, that's too high for even a 4" diameter port. You would have to change the subsonic filter in the Bash amp to get the air speed down.


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## Moose

Mike P. said:


> With the Bash 300 amp the port would have an airspeed of over 42 m/s, that's too high for even a 4" diameter port. You would have to change the subsonic filter in the Bash amp to get the air speed down.


Alright meaning what? How does one change the subsonic filter? :s


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## Mike P.

You solder in two different resistors as shown in the documentation of the amp. For a 3" port the subsonic filter would need to be at 28.9 hz from the 1 db boost table to have an acceptable port air speed.















http://www.parts-express.com/pdf/300-750.pdf

As you can see in the graph this would limit the low end output compared to a 4" port with the stock subsonic filter that the amp has. (Yellow)

Yellow is 50 liters tuned to 23 hz with a 4" port with the stock subsonic filter.
Green is 43 liters tuned to 25 hz with a 3" port with the modified subsonic filter.

A 4" port allows you to get the most performance form the sub and amp combination. Using 4" PVC elbows to turn the port allowing it to fit is a simple process.


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## lsiberian

Is sealing the port an option? I've done it before and think it might work for this project. I need the T/S parameters though to see if it's an option.


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## Moose

lsiberian said:


> Is sealing the port an option? I've done it before and think it might work for this project. I need the T/S parameters though to see if it's an option.


Yo the t/s parameters are in that link I gave. I'm thinking about doing this too. But only if it works.


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## Mike P.

Here is your current cabinet sealed compared to 50 liters tuned to 23 hz.


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## Moose

Alright first off thanks mike and moonfly for your help with this port problem. I will be building a box using WinISD's suggestion, ported obviously. But should I go with a 4" port 12" long or go with a 3" and make it longer? I understand that when you make it longer it takes up box volume?

Also can you suggest somewhere to buy a port?

Thanks!


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## Moonfly

There are 2 main things to consider with a port. To keep air velocity down, you need to increase its size, so make it large enough so as to get an acceptable velocity. Now, as you increase the port size, the first port resonance will drop. You need to ensure this stays out of the operating zone of the subwoofer (1-120hz), ideally plenty clear, say no lower than 150hz, but 180 or above is better.

As you increase a ports size, you will need to increase its length to maintain a low tune, so this has implications for fitting into a cab. To make things easier, and given your building your own box, I would make a slot port, that way you have no issues trying to buy port. Of course, if you simply prefer round ports then go for it.

I would do your model in WinISD and first model with a round port. Note the cross sectional area of the port once you have a port design your happy with. If you then click the circle it will change to square, which represents a slot port. If your happy with the dims it gives then your good to go, if not, you can play with the dimension, just make a not of the cross sectional area, and ensure it stays close to what it was in your round port simulation.

If you have trouble, we will run the model for you and provide the dimensions you need :T Dont forget, the port isnt part of the internal volume of the cabinet, so when you work out its size, it will need to increase to account for room needed to house the port.


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## Moose

Moonfly said:


> There are 2 main things to consider with a port. To keep air velocity down, you need to increase its size, so make it large enough so as to get an acceptable velocity. Now, as you increase the port size, the first port resonance will drop. You need to ensure this stays out of the operating zone of the subwoofer (1-120hz), ideally plenty clear, say no lower than 150hz, but 180 or above is better.
> 
> As you increase a ports size, you will need to increase its length to maintain a low tune, so this has implications for fitting into a cab. To make things easier, and given your building your own box, I would make a slot port, that way you have no issues trying to buy port. Of course, if you simply prefer round ports then go for it.
> 
> I would do your model in WinISD and first model with a round port. Note the cross sectional area of the port once you have a port design your happy with. If you then click the circle it will change to square, which represents a slot port. If your happy with the dims it gives then your good to go, if not, you can play with the dimension, just make a not of the cross sectional area, and ensure it stays close to what it was in your round port simulation.
> 
> If you have trouble, we will run the model for you and provide the dimensions you need :T Dont forget, the port isnt part of the internal volume of the cabinet, so when you work out its size, it will need to increase to account for room needed to house the port.


My velodyne dps-12's are slot ported but the slot is about 10" wide. Can I buy a finished slotted port end made out of plastic so it looks nice? Obviously I'll have to do a rectangle port tube at least 12" long correct? I'll whip out that WinISD later on this weekend, but I am just stuck on whether I should do round or square. Obviously slotted is dbl the work but if it reduces chuffing then ill be happy.

If i did an 18" cube with twin 3" ports 20" or so long using 45's instead of 90's for better flow, would that make my hair stand on end?

I'm still learning this stuff so if I'm way out in left field, my apologies. I would most likely use 4 45's per port to give it a large half circle inside the box. Right or wrong here?


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## Moonfly

The shape of the port does not determine if it will chuff or not, only its size will. A slot port gives more flexibility on its dimensions as you can simply make them to your own chosen size. 90 degree bends are fine with low enough velocities but 45s would be better, just a little more work to produce with a slot port. To flair the ends of the slot port, you simply need a router and and round over bit, and this will flare your port ends no problem.


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## Moose

Moonfly said:


> The shape of the port does not determine if it will chuff or not, only its size will. A slot port gives more flexibility on its dimensions as you can simply make them to your own chosen size. 90 degree bends are fine with low enough velocities but 45s would be better, just a little more work to produce with a slot port. To flair the ends of the slot port, you simply need a router and and round over bit, and this will flare your port ends no problem.


I'm a finish carpenter so this is not new to me, I just am swamped at work so I was hoping to not have to build. Question, does slotted port length matter then? 18" cube could get away with 16L x 8W x 2H?


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## Moonfly

In Mikes model, a single 4" diameter round port 33" long is sufficient to ensure no chuffing will occur. There are several permutations available for a slot port, but one 13" x 1" would be roughly equivalent and would need to be 34 inches long.

With all ports, the length is tied into the ports size (cross sectional area) and the cabinet volume in order to attain a specific tune. As you increase the ports size, you need to keep its length increasing as well, because you need its length to match its cross sectional area in relative terms.


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## Moose

Moonfly said:


> In Mikes model, a single 4" diameter round port 33" long is sufficient to ensure no chuffing will occur. There are several permutations available for a slot port, but one 13" x 1" would be roughly equivalent and would need to be 34 inches long.
> 
> With all ports, the length is tied into the ports size (cross sectional area) and the cabinet volume in order to attain a specific tune. As you increase the ports size, you need to keep its length increasing as well, because you need its length to match its cross sectional area in relative terms.


Okay I get it. In the real world having a 32" long square port isn't realistic. Your box would need to really deep. I'm still not sure which one I'll choose yet.


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## Moonfly

You can bend a slot port just as you can with a round one. The port could be shortened slightly and still be ok, especially once the HPF is correctly set. Below are a couple examples of how you can intgrate a slot port should you want to, examples are also available in the member build database.


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## Moose

I've been working with winISD and its telling me I need about a 60L box. Can I modify the box dimensions over what the optimum ones are? I mean will it make that much of a difference? I'm looking for maybe an 18" or 16" cube or even a few inches longer rather than higher.


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## Mike P.

For what driver?


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## Moose

sorry should have specified, the CSS sdx10.


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## Mike P.

Post 19 shows the optimum net volumes depending on the air speed and subsonic filter setting. One is 50 liters and the other is 43 liters.


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## Moose

I read that, twice. In fact I read the entire thread again. I'm asking does box shape matter or can I design the box to be longer rather than higher?


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## Mike P.

OK, I get it. When you mentioned 60 liters I thought you meant net volume. You obviously mean total volume. Yes, you can the box to be longer rather than higher. Just make sure the port isn't to close to a wall.


​


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## Moose

Okay so as long as it gets within the 60L total box with about a 24" 4" port I'll be okay? So keep the port off the wall? I really dont want to modify the amp rather just use it as is.

How much do I need to account for the 4" port? winISD won't allow me to set custom box dimensions..


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## Mike P.

For a net box volume of 50 liters with the stock filters on the amp you need a 4" round flared port 33" long for a 23 hz tuning frequency. To the net volume you add the displacement of the driver (about 3 liters) and the displacement of the port and any bracing. I assumed you did that and ended up with a total of 60 liters. 

Not sure how you got a port length of 24". There is a box volume calculator and also a tube calculator ( use that for the port) at this link:

http://www.hometheatershack.com/forums/enclosure-volume-calculator/

Use it to figure out how much bracing volume and port volume you need to add to the 50 liter net volume to find out the total internal volume you need.

Here is the project file for the SDX10. Save it to the "Projects" folder in WinISD.

View attachment Moose SDX10.wpr


If you need any help just ask. :T


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