You may be kerfing all wrong.

[gckless]

So, I'm going to copy and paste this from another forum, simply because T3mpest wrote this better than I ever could.

"Well I've been doing quite a bit of research on ports recently. I've run nothing but passive radiators in my own vehicles now for about the last 3 years and before that, did mostly sealed setups, a few ported, but not that many. Done some ported builds for friends, but without measuring their gain, it was mostly a 1 shot guess and frankly, as long as it was loud, they didn't care.. Builds I did that didnt' turn out as I hoped was still very impressive to them.. Anyways, in researching ports I ran across this little gem of a paper.

http://koti.kapsi.fi/jahonen/Audio/Papers/AES_PortPaper.pdf

For those with the time, I suggest reading it, even if you just do a few pages at a time, there is a LOT of good info in here.. I'll summarize some of the important stuff that I found interesting. I'm mostly using this post as a launching point for discussion.. I've noticed in the few years KERFING has become VERY popular. I remember back in 06 when I first got into audio, I don't remember anyone doing it, nowadays seems like it many box builders go to tricks on here.. Not speaking down on it or them, but the implications of some of this paper I found quite interesting.

One thing I know me and Murphy talked about via PM's one time, and it does seem to be true is that many people in the audio industry do things based off of incomplete knowledge. They either have no real world experience and use textbook knowledge as gospel or have no textbook knowledge and do things that they were always told were the "best way". As with most things in life, the best way is somewhere in between.

Early research into port speeds found that ports are compromised with airspeeds as low as 10m/s with anything beyond that being bad.. For those unfimilar with box modelling, most good mid level 1-2k powerhandling 15's in a 3.5 cube box tuned at 30hz with reach that airspeed with about 200 watts of power applied. So either the ports we use in car audio are woefully undersized, OR there is more to the story.. Again, as I said earlier the truth lies in between, BUT it leans a bit toward the latter conclusion in this case The issue with these studies as pointed out in the JBL paper is two fold. First, being that this was early research, they weren't using big speaker.. Consequently, they weren't using big ports. Small ports compress with lower airspeeds than larger ports do, so testing with a 6.5inch speaker with a 2inch aero port isn't going to give you the same real world results as a tantric hd on 3k..

How high can vent velocity go you ask? It seems like for larger ports, 8-10inch aeros and equivalent slot ports, (100+sq inches) 35m/s seems to be the cutoff before vents REALLY begin to hit a brick wall and completely resist getting louder. For that same 3x13.5inch vent that you see with most 15's, that's around 2500 watts of input power, so things are getting a bit closer to life using that approximation as anyone using that kind of power would likely go with an SPL vent for a 15, which tends to be a 4-6inch vent, which wouldn't EVER hit that brick wall point..

Anyways so basic main takeaway here is that bigger ports=better.. I think we all knew that though lol, don't worry things get more interesting, promise.

After research was done on port size and they found small ports had major airspeed issues at relatively low SPL and airspeeds, the next step was to try using roundovers and flares.. Research on flares showed pretty big differences, with flared ports having much better performance than non flared ports, although at some point, the center of the pipe still becomes the weak link.. The JBL paper deals with how larger ports work and how SPL is effected, their results are rather interesting.

I guess to understand port compression the first thing you need to know is why a port compresses.. Essentially, air flowing in a port flows slowest at the edges and fastest in the center. That's why when people do hairtricks, they use the center of the window, airspeed is the highest in the center of the window. the edges are slower because the edges of the enclosure cause friction slowing it down. As the air gets compressed it can eventually slow down enough that it actually begins to get caught up inside the port and reverse direction.. The eddies end up working like a traffic jam, stopping your port from producing output.. That's why areo ports are more effecient, a circle has the highest amount of radiating area in comparison to how much surface area the air has to drag across. Things are most turbulent at the ends of the port, so a flare gives you more area for the air to expand where you need it most and slows the airspeed at the beginning and ends..

Anyway here are some neat things in the paper that box modelling programs WON'T show you or tell you.

1.Higher tuned ports perform better at high airspeeds than low tuned ports.

This is because as I said earlier, when eddies begin to form the air gets turbulent and goes the wrong direction.. If the air in the port is constantly changing direction, the eddies essentially have no time to form as when the begin to, the port changes direction and they are ripped apart. Many people tune low because it "lowers airspeed", which it does.. The issue the box programs won't show you is the lower tuned port NEEDS to be a lower airspeed to perform as well. This has been verified in other places as well. Here is a graph a HT guy came up with years ago, he did ALOT of good work on this stuff for an amateur. He eventually built a program called "flare it" based on his real world data As you can see, as port speed goes down, it gets easier and easier to run into chuffing and compression.

2.Turns are a bitch. As I mentioned earlier, ends of the pipes are where ports have the most problems, but anytime you ask air to change direction, whether it's due to lack of boundary conditions at the end of a port, or a turn in a box.. Your in for a bad time. Your creating another weak spot in the port for things to get turbulent.. I see a lot of fancy looking boxes on here tuned at 28hz for the "lowz" with all kinds of crazy looking bends, 45's, etc, etc.. Anything they can do to make the port long enough and makes things as smooth as possible. However, I'm starting to wonder if many of these boxes wouldnt perform better by shortening the port just a few inches.. Sure it's not as fancy looking, but your not trying to bandaid a bad port design either.

3. At high SPL's big exit flares are actually a disadvantage! One of the most enlightening things I learned from this paper was that. At high SPL's you NOT going to have a port big enough to completely stop all eddies.. They begin forming on small ports as low as 10m/s, even on a big port by 20m/s, your going to have some issues.. The problem with flares is two fold. First, the first eddies are still going to form at the ends of the port, in this case they will be in the flare, probably near the middle, usually we use 1/2 the flare length as port length, so that's near the "end" of the port. With a big flare, you now have a eddy forming in what is still the "inside" of your port. At least without a flare, it's forming outside the enclosure. Conversely, on the inside of a box a big flare is a good thing, it keeps the eddy outside your port and inside your box.

4. I know that last point was a bit crazy sounding, but I'll go ahead and post the overall results of how to "best" use flares, and their effects.. Remember what I said about incomplete knowledge, it applies here. Many guys use big kerfed ports because that's what most big supersubs use, because many of them are based off of home theatre designs with 90mm of xmax to hit 20hz.. Useage for music is a bit different, and or ports should be too.

Big flared ports tended to work best at low output levels. The ports have the higher Q, so they ring the loudest when played at lower volume. If your trying to get the response you see in your box program at low power levels (think pure SQ apps) a big flare is the way to go.

At moderate output levels moderate flare rates work the best. Think flares that expand to around 1.5x the center of the port size. The JBL paper gives a nice formula for this, but I simplified it in a way that makes a bit more useable, and works out similarly in most cases. Anyways, moderate flares perform decent at low volume, and decent at high volume, they were the best overall compromise..

Last no flare tended to perform the best at high output when an adequate port size was used.. While it compresses the most at low volume levels, it compressed the least from that point as you approached high output levels. I think for daily streetbeating, this may be the a good option, if you can fit a big straight unflared port that is lol. If you build a box that has a peaky lowend according to a box program, it will work as follows.. At low volumes, the peak will be reduced due to compression at high volumes, the peak will get more pronounced as opposed to a box with a large flare. More SQ oriented at low SPL's and more peaky when things get roudy.. Only issue is fitting a big enough port in the first place with no flare.

Last interesting thing I learned about ports involves port symmetry. JBL found ports work best when they are symmetrical. This includes the baffle.. For any of you aero guys, try placing a baffle around the end of the port that is inside the box, assuming you don't have an external areo. Doesnt' seem like it needs to be a big one.

Anyway hopefully this paper helps some people, it helped me in planning my next box. My next ported box is going to be a completely straight port, as I alluded in my last paragraph. By the time the box was big enough to get a decent tune without bending my port, it was a peaky port design. Knowing that the peak will be reduced in magnitude at low volume levels helped me out a lot. At high volume who cares, no "SQ" at mid 140's anyway, other things to worry about at that point.

So for everyone else, what are your thoughts and opinions? Any SPL guys currently using a big flare willing to up their port size another inch or two and try burping without a falre? Perhaps use a big flare only on the inside of the box, unflared on the outside, seems to be a bit of conflicting info in the paper based on port symmetry vs varying flare needs depending on where the port is located. They also found that with flared ports at high levels the port not only compressed, but the tuning changed, because the eddies occurring within the larger flare length shorten the port by quite a bit, as opposed to no flare which causes the eddies just outside the port, hence no tuning change."

Full thread: http://mobileaudioforum.com/forum/showthread.php?6626-Port-compression-and-sizing&highlight=kerf

The paper is worth a read.

Reason I brought it over here is because there might be a few people here that are actually willing to test some of these findings. Anyone want to try a big kerf on the inside and not the outside? Maybe increase port size and run no flare? Opinions, thoughts, blatant harrassment?

[Triticum Agricolam]

I came across that paper a while ago, but I haven't found the time to read it just yet. I really need to.

One quick comment on what you posted, there is no doubt that large, straight ports definitely offer the most ideal solution. The problem is the combination of large ports, straight ports, and box dimensions that will fit inside a vehicle quickly become mutually exclusive goals. So now you have the play the game of "where do I compromise so this will actually work". Personally I'm of the belief that more port area fixes a lot of other port design sins. Enough area will get your port velocities down to where a lot of the problems you described either go away or are reduced. Its my opinion that a lot of folks could benefit from more port area even if they aren't getting any noticeable port noise.

Of course the flip side to more port area is it ends up taking up more, in some cases a lot more, of precious box volume and as your port gets larger it has to get longer and you can start running into pipe resonance issues which you really do not want. As with most things in life, everything is a trade off.

[gckless]

I came across that paper a while ago, but I haven't found the time to read it just yet. I really need to.

One quick comment on what you posted, there is no doubt that large, straight ports definitely offer the most ideal solution. The problem is the combination of large ports, straight ports, and box dimensions that will fit inside a vehicle quickly become mutually exclusive goals. So now you have the play the game of "where do I compromise so this will actually work". Personally I'm of the belief that more port area fixes a lot of other port design sins. Enough area will get your port velocities down to where a lot of the problems you described either go away or are reduced. Its my opinion that a lot of folks could benefit from more port area even if they aren't getting any noticeable port noise.

Of course the flip side to more port area is it ends up taking up more, in some cases a lot more, of precious box volume and as your port gets larger it has to get longer and you can start running into pipe resonance issues which you really do not want. As with most things in life, everything is a trade off.

Totally agree, no arguments there, there is a delicate balance. Though I will say that most people have the "cone area is king" mentality ingrained in their heads. While it is true some, and probably most, of the time, there are definitely people that would benefit from maybe downsizing slightly and perfecting their enclosures.

But, not arguing, just trying to spread some info

[slowfkncar]

First thought that came to my head when i read thos was jacob (sundownz) jeep with the single 12" kerfed to hell box pulling well over 150db lol. Interesting read actually!

[Phaeton]

whoa. im going to need to try this now. thanks for sharing man! intense stuff.

[Krakin]

Commenting to read through later, seems very interesting. Too bad we tend to work in a car audio environment where it is next to impossible to achieve what is on paper and then you realize it's audio and 100% correlation does not exist....

[bkolfo4]

^^^that is correct.

We do things in car audio because you can get away with it and many times the "correct" design is just not practical.

[_paralyzed_]

V-tec ports yo. We need diaphragms that open and close based on output. And while we're at it nuclear powered amps...

[Ahmed Johnson]

Commenting to read through later, seems very interesting. Too bad we tend to work in a car audio environment where it is next to impossible to achieve what is on paper and then you realize it's audio and 100% correlation does not exist....

you one smart young'n

[Krakin]

Commenting to read through later, seems very interesting. Too bad we tend to work in a car audio environment where it is next to impossible to achieve what is on paper and then you realize it's audio and 100% correlation does not exist....

you one smart young'n

Without knowledge there can't be progress. Also a lot of it I just repeat from books I've read on the subject.