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    Length error and kerf application

    Hey guys, Mobile here.
    I wanted to do a write-up of kerf ports because I know some people have issues with knowing the length to make them for multiple panel usage for bass reflex style enclosures.
    So, what is a kerf and what is it's purpose?

    The kerf port is a bend within the dimensional length of a panel used on the enclosure to allow for increased compression and velocity with the ability to lower distortion and resonance build up within the enclosures port created from port noise. It also allows the ability to reduce resonance peaks within the compression chamber if the other side of the port consists of a compression volume, normally where the drivers are installed.

    So, the basic look of a kerf is shown here:



    This is utilizing a kerf on both sides in and out of the enclosure, much like a precision port does, but using a slot style layout. SO, in order to do this kerf on each side, you will see in the pic, that the front and the ports are actual one piece on each side. SO, each piece involves 2 kerfs on a single panel!

    Now, if you have a CADD program of the such, this will make this much easier, but for hand sketches, a compass will work just fine as well, with a transparent plastic ruler.
    We need to know the angle that these kerfs are to be formed into. These angles in the example are both 90 degrees, which is the most common kerf bend used.

    We know that (2)90degree bends are needed. Now, we need to know the radius of those bends. This can be sketched up using 12 sections of equal length to make up the 90 degree angle composing of 13 cuts. 12 sections usually give a good distance between each cut to allow for proper flexibility and stability of the wood. SO, using 12 sections, we have to seperate them by length. We will say that we need a 2" radius kerf on the bend.

    Now, there is a template I put together that accounts for this bend and the length at the same time. Here is the example I sketched of the template.



    It has a 1.4% error on average for each inch of the kerf's radius. So, by the time you do say a 12" radius kerf, you are still only looking at less than 17% error. That is a pretty big kerf. The error only exists due to the simplicity of the template for ease of use. This error is a fraction of a quarter of an inch in most cases.

    So, say in the example that the wood before the kerf begins is 6" and the wood after the bend is 4". We know that is 10" already, so we just have to account for the bend length. The reason for this is to know how long to make the panel so we account for the length in the bend by not cutting is too short or too long, though cutting it too long is a better deal, it is best to get it as accurate as possible.

    So, if you notice in that pic, the 1 2/16" (1 1/8) dimension. That is the dimension from that point to the center of the circle, and then continues the 2" radius of the circle to the point where the 6" length stops and the kerf begins. This was calculated by taking the inner circle to half of the radius of the outer circle, and then drawing a line from the end of the kerf (at the bottom part of the pic), to the circles edge and continuing until it touches the horizontal line. This is where the measurement of the kerf starts. So, this is saying to add 3 2/16 to the length of 10" to make it 13.125" long. This will have a 2.8% error from the actual bend, which is a fraction of a fraction.

    The lengths of the cuts are found by taking the length of the actual kerf, and dividing it by 12. In this case, the length is 3 2/16(3 1/8) or (3.125"). So, the cuts should be about 1/4" apart for this to bend correctly.
    Now, we do the second kerf bend, say if it is the same radius at the first, then we will have another 3.125" to add and end up being a total length of 16.25 inches long to account for both kerfs.

    At this point, you then have to measure the distance between the kerfs to begin the cuts. Say if the example pic has a distance of 7" between the bends, then that distance is accounted for between the beginning and end of the kerfs respectively.

    In the example pic above of the double kerf ports, the layout is as follows if you follow the template and the 12 cuts for each 90 degree bend:

    The front is 14 inches, the kerfs are 4 inch radius, and the space between is 7 inches and the part after the inner kerf is 2.25 inches (example). Then it would look like this:



    Hope that helps.
    Last edited by Moble Enclosurs; 05-16-2012 at 12:01 AM. Reason: Changed the 4" radius to 2" as per template. Typed it wrong. All set now :D





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    Re: Length error and kerf application

    And that error is not an overall length error, it is onyl the error of the length from the center of the circle to the end of the kerf. So, that 17% is 17% of a percentage of the full length. That is why it will be an acceptable error.





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    Re: Length error and kerf application

    Very descriptive, and helpful. 'Like'



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    Re: Length error and kerf application

    This is great. Thank you!

    Is it true that effective port length starts at 2/3 of the radius inward? Or should one completely disregard any flare or any portion of a kerf's flare as usable port length? Err, unless the radius happens to be much much shallower of an angle, rather than 90 degrees I suppose?




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    Re: Length error and kerf application

    I wonder if the amount of replies are reflecting the numbers of members that are out building and attempting their hands at single or double kerfed enclosures :p




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    Re: Length error and kerf application

    Quote Originally Posted by Ronny View Post
    This is great. Thank you!

    Is it true that effective port length starts at 2/3 of the radius inward? Or should one completely disregard any flare or any portion of a kerf's flare as usable port length? Err, unless the radius happens to be much much shallower of an angle, rather than 90 degrees I suppose?
    It does make a difference, but not so much with the response curve as much as the output it can create. So, yes, the length is included, but to a point dependent on the curve angle of the kerf, just as aeroports use on a smaller scale.

    SO, if the kerf is based on a 90 degree angle, the 2/3 rule is not exact, but very close to what can be usable. In fact, if the bend is smooth and the degree is constant, a 50% length of the kerfs effective overall length can be included in the port length, BUT you also have to consider the flare area and parabola effect that it has on the sound coming out of it. It is less directive than a standard port area of constant volume in that it disperses the sound much like that of a typical horn with the exception of the ends of the kerf having a higher velocity than the actual middle of the port, so there does exist some nulled areas in a kerf, but if you want to utilize them, do it in an SUV type vehicle for the best coupling .





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    Re: Length error and kerf application

    Quote Originally Posted by Moble Enclosurs View Post
    Hey guys, Mobile here.
    I wanted to do a write-up of kerf ports because I know some people have issues with knowing the length to make them for multiple panel usage for bass reflex style enclosures.
    So, what is a kerf and what is it's purpose?

    The kerf port is a bend within the dimensional length of a panel used on the enclosure to allow for increased compression and velocity with the ability to lower distortion and resonance build up within the enclosures port created from port noise. It also allows the ability to reduce resonance peaks within the compression chamber if the other side of the port consists of a compression volume, normally where the drivers are installed.

    So, the basic look of a kerf is shown here:


    This is utilizing a kerf on both sides in and out of the enclosure, much like a precision port does, but using a slot style layout. SO, in order to do this kerf on each side, you will see in the pic, that the front and the ports are actual one piece on each side. SO, each piece involves 2 kerfs on a single panel!

    Now, if you have a CADD program of the such, this will make this much easier, but for hand sketches, a compass will work just fine as well, with a transparent plastic ruler.
    We need to know the angle that these kerfs are to be formed into. These angles in the example are both 90 degrees, which is the most common kerf bend used.

    We know that (2)90degree bends are needed. Now, we need to know the radius of those bends. This can be sketched up using 12 sections of equal length to make up the 90 degree angle composing of 13 cuts. 12 sections usually give a good distance between each cut to allow for proper flexibility and stability of the wood. SO, using 12 sections, we have to seperate them by length. We will say that we need a 4" radius kerf on the bend.

    Now, there is a template I put together that accounts for this bend and the length at the same time. Here is the example I sketched of the template.


    It has a 1.4% error on average for each inch of the kerf's radius. So, by the time you do say a 12" radius kerf, you are still only looking at less than 17% error. That is a pretty big kerf. The error only exists due to the simplicity of the template for ease of use. This error is a fraction of a quarter of an inch in most cases.

    So, say in the example that the wood before the kerf begins is 6" and the wood after the bend is 4". We know that is 10" already, so we just have to account for the bend length. The reason for this is to know how long to make the panel so we account for the length in the bend by not cutting is too short or too long, though cutting it too long is a better deal, it is best to get it as accurate as possible.

    So, if you notice in that pic, the 1 2/16" (1 1/8) dimension. That is the dimension from that point to the center of the circle, and then continues the 2" radius of the circle to the point where the 6" length stops and the kerf begins. This was calculated by taking the inner circle to half of the radius of the outer circle, and then drawing a line from the end of the kerf (at the bottom part of the pic), to the circles edge and continuing until it touches the horizontal line. This is where the measurement of the kerf starts. So, this is saying to add 3 2/16 to the length of 10" to make it 13.125" long. This will have a 2.8% error from the actual bend, which is a fraction of a fraction.

    The lengths of the cuts are found by taking the length of the actual kerf, and dividing it by 12. In this case, the length is 3 2/16(3 1/8) or (3.125"). So, the cuts should be about 1/4" apart for this to bend correctly.
    Now, we do the second kerf bend, say if it is the same radius at the first, then we will have another 3.125" to add and end up being a total length of 16.25 inches long to account for both kerfs.

    At this point, you then have to measure the distance between the kerfs to begin the cuts. Say if the example pic has a distance of 7" between the bends, then that distance is accounted for between the beginning and end of the kerfs respectively.

    In the example pic above of the double kerf ports, the layout is as follows if you follow the template and the 12 cuts for each 90 degree bend:

    The front is 14 inches, the kerfs are 4 inch radius, and the space between is 7 inches and the part after the inner kerf is 2.25 inches (example). Then it would look like this:


    Hope that helps.


    Or you could find the length the easy way with no error.





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    Re: Length error and kerf application

    Yea, I don't like to google my findings though, I create my own. If something works better than what I come up with, by all means, use it. But I am a person of self-improvements and though I do appreciate your "finding", I am working to come up with something different for every aspect of my work. Be glad I mentioned I do have an error rather than saying, hey guys this is perfect and you should use it all the time, only to find people coming back to me and saying I was wrong.
    I like the way I do my own work, and though this concept you have is simple geometry and is well known to anyone who understands circumference (especially for the pi part of the formula used above), I wanted to try something of my own and found it exciting that I created something interesting to use that works. Just expanding the opportunities of what is already out there. That is what a good designer does. .





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    Re: Length error and kerf application

    Quote Originally Posted by Moble Enclosurs View Post
    Yea, I don't like to google my findings though, I create my own. If something works better than what I come up with, by all means, use it. But I am a person of self-improvements and though I do appreciate your "finding", I am working to come up with something different for every aspect of my work. Be glad I mentioned I do have an error rather than saying, hey guys this is perfect and you should use it all the time, only to find people coming back to me and saying I was wrong.
    I like the way I do my own work, and though this concept you have is simple geometry and is well known to anyone who understands circumference (especially for the pi part of the formula used above), I wanted to try something of my own and found it exciting that I created something interesting to use that works. Just expanding the opportunities of what is already out there. That is what a good designer does. .
    Sorry to offend but your method is overly complicated, and hard to follow, especially when you mix up radius and diameter in your write up (A 90 degree arc with a 4" radius would not be 3.125"). I admit that I grabbed the image with a Google search simply because I didn't feel like drawing the image myself or typing out a long winded explanation of the formula. Geometry works best, if your intention is to help people, why make things harder?

    Here is the even easier formula to find the length of a 90 degree arc (since that is the angle you will use most often).

    L = πR/2




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    Re: Length error and kerf application

    Quote Originally Posted by vaiboy View Post
    Sorry to offend but your method is overly complicated, and hard to follow, especially when you mix up radius and diameter in your write up (A 90 degree arc with a 4" radius would not be 3.125"). I admit that I grabbed the image with a Google search simply because I didn't feel like drawing the image myself or typing out a long winded explanation of the formula. Geometry works best, if your intention is to help people, why make things harder?

    Here is the even easier formula to find the length of a 90 degree arc (since that is the angle you will use most often).

    L = πR/2
    I didn't notice until now that I did say a 90 degree angle of a 4 inch radius is 3.125", lol. I meant to type as shown in the following pic after the type up, the 90 degree angle of a 2" radius is about 3.125. And by the formula (3.146), this is very close to what I came up with. The 4" mentioned was the portion after the bend, so it was a mistype on that part that has been corrected to match the picture of the template I was referring to (thanks for catching that ). And, as mentioned, I wanted to come up with something different, and came across this as a way to get very close to the geometry of the original concept and figured I would share it. I find it fun to experiment and share what I come up with. I am not offended, just need to make sure you understand why this was posted, and that it was mentioned the error was only due to the simplicity of the template. The purpose of this is not educational but informational. It is a post to what I made up that almost works perfectly to the formula, and I found it pretty interesting that my template is very accurate and just wanted to share it is all. No worries. Anyone can follow the standard length of the 1/4" of the circumference formula for 90 degree bends, and 1/2 the circumference of the 180 degree bends, etc and as an acoustical professional, I relate that to phase degree and directivity, so that geometry is very understood, But I just wanted to see if I could come up with my own way to figure it out and I came pretty darn close I might say! I like my template...... and plan to improve it if mathematically possible. These are the things that I do. I dont follow the ts parameter standards, nor any program for any of my other calculations, but the difference with those are, they have taken me over 4 years to complete and are more accurate than the general ways.
    In this case, I came up with this yesterday in about 10 minutes. It is much like the documentation you read about acoustics and enclosures from major engineers and hobbyists from online, where a lot of it consists of trial and error and testing results along with explainations. This is no different in that sense.

    Just like there are many ways to design, there can be other ways to do other things in enclosure design, and if I can get this template to show a parallel accuracy, I will be even more proud of it and share that as well as an update. I like to stay active in my field and not just conjoin to a standard and not understand the reasoning of why the things work the way they do.

    Ever take toys apart when you were a child and try to put them back together only to find that sometimes you have pieces left over? And sometimes you do actually figure it out in different ways and it makes you feel good? That is what I do with math, geometry, acoustics, etc and everything with my profession even today. This is only an example and I may admit, I posted it quickly, and should work on it to make sure it has no errors in the end result, but I was excited and like to share all of my flaws and accomplishments to everyone so they know I am real and I am the type of person that wants to improve or improvise.





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