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Air velocity and chuffing  

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DKLexx1980
(@dklexx1980)
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19/10/2019 6:31 pm  

Hey Guys...

I'm dreaming about designing my own 5.1 set but im a complete novice speaker builder. Im trying to do my home work before jumping in the the shark infested waters which is speaker design😀. I don't intent to start from scratch for my first build, rather im planning to redesign some builds Ive found here and there on the web. 

I have a question regarding chuffing. When I play around with WINisd, it can show me the rear air velocity for a given port size - but what is/are the velocity/velocities that are acceptable. 

In one of 123Toids videos he says its 17 m/s without chuffing in his design. I can't remember the size of the port though. On audiojudgement's page he has an article says 10 m/s for non-flared and 30m/s for flared ports. So the question becomes how fast can the air flow be without creating any chuffing? Do any of you guys know how to calculate this. Are there any "rules of thumb". 

Hope one of you can help or at least point me in the direction of an answer.

Thanks in advance.

DKLexx

Regards
DKLexx


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Kevin Kendrick
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19/10/2019 7:01 pm  

Good luck getting a hard and fast answer on that one. Some say 10% (35 meters per second)of the velocity of sound in air, some say 5% (17 meters per second). I've always aimed for 26 m/s or less which lands right in between the two. Have built numerous ported enclosures and never had a problem with that rate. Flared definitely helps. Good luck on your builds!


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DKLexx1980
(@dklexx1980)
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19/10/2019 10:24 pm  

@kevin-kendrick

Hey Kevin

Thanks for the reply even if it's not what I wanted to hear 🤔 😉 🤣 This is what i keep running into. DIY Speaker design is a lot of "some people say" 😊 and not so much 2+2=4.

So just to follow up - your build are different port sizes (diameters)? and speaker sizes? and 26 m/s works well for all of them? if yes - I guess we just invented the official "kevin-kendrick 26 m/s chuffing rule of thumb". 👍 

Let's see what/if other people have something to say on this matter. If not im gonna run with your suggested 26 m/s 😉 . 

Thx for the input

Regards
DKLexx


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Kevin Kendrick
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19/10/2019 10:53 pm  

@dklexx1980

Lol, you nailed it. Some people say this and some say that but seldom will you be able to lock it down.

To your follow up on your question, yes, lots of different sized cabinets with different sized ports, some round, some slots, and the 26 m/s worked without issues. Don't go giving me credit for the 26 m/s rule though, others have used it before.

Here's an example of a slotted port I built recently (these are test cabinets for some desktops I may start selling in the near future). This one was a bit of a challenge due to the length I needed for the desired tuning (sidewall removed to show it). I pushed the driver to it's mechanical limits and there was no audible chuffing from the vent.

20190714 204006

 


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Lindelium18
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20/10/2019 8:40 am  

If you would like to follow along in my post, I’m currently trying to deal with some chuffing issues on a build I’m working on. The post is called, “The New Build - The Alyens.”  Essentially, I have built the enclosure to a worst case scenario with a long, non-flared port tuned DEEP. I think it’s tuned to something like 28hz in a 2” x 8.5” port. At anything above 125 watts, chuffing is audible to me from about 35hz to about 22hz.  I think, off the top of my head, air velocity was audible at about 12 m/s.  However, here comes the complication; right now, the sub is the only speaker in the enclosure so, with other music, the chuffing may not be as audible. So, my next step is going to be a round over on all parts of the port where the air changes direction.  We will see if that does it. 


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123Toid
(@123toid)
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20/10/2019 12:34 pm  

@dklexx1980

There's a few reasons why you won't hear a hard and fast rule.  I'll give a few reasons why.

1.  If you look at the graph in WinISD you will notice that the speed of air is directly related to the frequency at which it is playing.  Keep this in mind.  If you plan to cross it over by 80hz, you can simulate that filter in WinISD, which will lower your air velocity considerably.

2. Since it is frequency dependent, some people are less concerned about lower frequency 30hz and below.  Some of this is dependent on the use of the speaker.  If you are designing it for use of Home Theater full range tower, you will be more concerned, as you will probably get a lot more power going through it in quick burst.  Where this may not be as big of any issue depending on your music preference.

I typically shoot for 17m/s, especially if doing a theater build.  One way to get around this is to have a huge roundover.  You are typically looking at something like a 1 1/2" roundover bit depending on the size of the port.  And this roundover would be on both sides of the port. 

Honestly, simulation can only get you so far.  You may need to create a test box to test different ports and or roundovers to see which you prefer with your specific setup.  Of course, if you have an exact model in mind or graphs for us to look at, we would definitely take a look at them. 


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Kevin Kendrick
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20/10/2019 3:20 pm  

@123toid

Thanks for adding examples Nick on why it wouldn't be hard and fast, should help others.


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DKLexx1980
(@dklexx1980)
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20/10/2019 4:52 pm  

@kevin-kendrick

Hey Kevin.

Thanks again. I will keep the 26m/s in the back of my mind while im designing the front and rear speakers. And if it doesn't work I will simply blame you 😝 😜 😆 - I guess in the end there is nothing for it than to try it out as see if it work or not.

Thanks again.  

Regards
DKLexx


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DKLexx1980
(@dklexx1980)
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20/10/2019 4:56 pm  

@lindelium18

Hey...

I saw you post before I wrote mine. I will follow along and see what you will come up with.

Thanks

Regards
DKLexx


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Kevin Kendrick
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25/10/2019 4:55 pm  

Here's a write up some may find interesting. If you're curious about the pro's and con's of different port geometries and/or how flares impact their performance, take the time to look this over. It was published by Scott Hinson on his Facebook DIYRM page. He's one of the more knowledgeable guys I know in the DIY community and what I like most about his work is his scientific approach to what he is discussing.

https://drive.google.com/file/d/1SU56ScKuPIr7SftWRHB7eeKdjmnnMh3b/view?fbclid=IwAR0KNTkw_cYFeV1E8fJ2bkuCzqOumgoE-x0zxRO7hG6rfGJlLkUh9adwqo4  


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