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  #1  
Old 08-01-2015, 10:06 AM
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Default Lets discuss port area

Ive been building boxes for a while now and ive had pretty good success. I know that alot of people think its bull, but i normally use the 12-16 sq in of port per cu ft. I know its not the law, but more of just a general guideline.

After talking with another old school builder locally, we exchanged ideas as when he builds he does so normally using somewhere in the neighborhood 6 in per cu ft or even less. In his opinion, the smaller amount of port area causes the sub to move therefore keeping itself cool. Its also understood that with his port area being so small, that the tuning on his boxes is normally extremely low (26 hz and under). He also claims that a port this small will create the hair trick like "pressure" that we are all looking for, cause more vehicle flex, and do so all with a pretty flat response. Having been around long enough i know that making the port smaller will increase velocity possibly causing port noise and and likely have reduction in overall spl output.

So the question is, as loud and impressive as my builds have been, is there something that im missing by using too much port area? Open discussion here.... all ideas and experiences welcome.
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Old 08-01-2015, 11:28 AM
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Less the mach velocity. I'm tuned at 30hz with only 10inch port area. However cause I'm tuned so low no port noise cause 30 hertz sine wave moving slower then 40hz. Is my understanding
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  #3  
Old 08-01-2015, 11:44 AM
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Quote:
Originally Posted by djscrubb View Post
Less the mach velocity. I'm tuned at 30hz with only 10inch port area. However cause I'm tuned so low no port noise cause 30 hertz sine wave moving slower then 40hz. Is my understanding
Interesting thought. Ill have to look into that. What does your setup consist of?
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  #4  
Old 08-01-2015, 12:15 PM
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you'll run into problems with too little port area before you run into problems with too much port area. Within reason, you can't have too much port area. You won't give an 8" sub the amount of port area for an 18, but you'd be fine with giving a 12" sub like 18" of port

here's a copypasta

Quote:
Originally Posted by Dmac1991 View Post
quoted from over on SMD. Excellent read on port area. Port area. what does it affect? - Page 2 - SPL - SOUND PRESSURE LEVEL - SMD Forum


"Minimum port area can be found from a few variables of your system such as cone area in total, xmax and tuning frequency.

Using this:

Calculate the minimum port diameter from the following equations:

Sd = pi*(Dia/100)^2/4
Vd = Sd*Xmax/1000
Dmin = 100*(20.3*(Vd^2/Fb)^0.25)/Np^.5

where,

Dmin = minimum port diameter (cm)



For vented type speakers operated at full power that is, full excursion, it is possible to end up with audible noise from the air rushing in and out of the vent. For a given speaker system, the smaller the vent opening the greater the velocity of air in the vent and the greater the vent noise.

The main reason this ever became a topic is because Richard Small (the S in T/S Parameters) wanted to know the minimum velocity the air from the port could have without causing noise. This happens to be about 5% the speed of sound. That formula should get you close to it, but with modeling software you don't really need to use it, just make sure your port velocity is down.

There are other negatives from port compression such as added heat from inadequate air transaction and poor frequency responses due to it as well.

The Xmax is there to have it know your upper limit of excursion.

If you use less power you'll have less excursion to cause less air to move. So you could use a smaller port area, but when designing you design around your speaker not your amplifier, because what if you get a bigger amp.



If you could have too much port area then playing speakers at low volumes would sound absolutely horrible as the port area needed gets exponentially smaller as you use less power.

Unloading is based on port location not area.

Area can make positioning it hard to keep from unloading happening, but doesn't cause it.

No such thing as too much port area (at least within reason), there is such thing as too long of a port though, and since area and length are tied to tuning, you can definitely get yourself into a spot where your port is too big, because its too long.

Cone control problems will arise from not enough port area, not from too much.

Not having enough port will cause port compression. This makes your box start to act like a leaky sealed box. Output around tuining will drop, impedance will rise, output from your amp will decrease.

If your port is too long you run into trouble with pipe resonance.

Pipe resonances will produce extra harmonic waves and in extreme conditions it can adversely affect your overall output.


I believe I talked with Triticum about this at one point and I think I said that you don't want your vent length longer then 1/4th the length of the frequency you're tuning to.


And also impedance and tuning of your box changes when adding power."
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  #5  
Old 08-01-2015, 01:00 PM
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Originally Posted by FocusOn20s View Post
you'll run into problems with too little port area before you run into problems with too much port area. Within reason, you can't have too much port area. You won't give an 8" sub the amount of port area for an 18, but you'd be fine with giving a 12" sub like 18" of port

here's a copypasta
good read there.
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  #6  
Old 08-01-2015, 04:40 PM
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So do more guys use sd to determine how much port area they use? Got some insightful pm's. Thanks fellS
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Old 08-01-2015, 07:03 PM
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Higher frequencies require more port area to get proper efficiency. Too little port area causes a mirade of issues, most worse than the audible issues of port noise. Too much internal enclosure pressure is what causes surrounds to prematurely fail, there is less heat transfer as well. Think about it like this, a small port might move air at a higher speed but how much air is actually exchanging? A small diameter fan moves less cfm at a higher pressure than a larger fan that moves more cfm at a lower pressure so what fan would have better cooling properties? The answer is simple, heat dissipation needs more air to flow rather than higher pressure to work efficiently. A smaller port doesn't cool better than a larger one, it actually promotes heat build up rather than a cooling trend due to less exchange. Try this little experiment, scream with your mouth barely open, the air is moving at a higher speed exiting your mouth but is your mouth cooler and how loud is it when compared to screaming with your mouth all the way open at the same frequency? Small ports shrink bandwidth and have premature rolloff in higher frequencies. The key is to have big port area and low tuning. This allows for efficiency across a larger bandwidth ergo less rolloff throughout so a flatter response curve depending on the woofer and tuning frequency combination. Now, to further the idea of ports and cooling (doesn't really do a whole lot in reality) we will try and look at the way a port works. The port can only move as much air as the woofer itself can move. If the cone area and stroke of the woofer displaces less air than the volume of the port you won't transfer any air at all. However you will introduce cooler air into the front of the port that will allow rear port air to transfer some heat out but it is dependant on port area. More port area means more surface area and that allows more efficient heat transfer. So in conclusion of the whole heat transfer vs port area your friend is trying to talk to you about, he is 100% wrong on every aspect of his reasoning based on actual physics. You are doing what you should and that is why you've had such success in your builds. Honestly if you want the best possible cooling for your build go inverted... If you want the best bandwidth with a ported enclosure go big port area and ridiculously low tuning. The only time you have too much port area is if you are not generating any internal enclosure pressure at all and the woofer is basically reacting as it would in free air.
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The Following 5 Users Say Thank You to e46inperrycounty For This Useful Post:
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  #8  
Old 08-01-2015, 07:12 PM
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Quote:
Originally Posted by e46inperrycounty View Post
Higher frequencies require more port area to get proper efficiency. Too little port area causes a mirade of issues, most worse than the audible issues of port noise. Too much internal enclosure pressure is what causes surrounds to prematurely fail, there is less heat transfer as well. Think about it like this, a small port might move air at a higher speed but how much air is actually exchanging? A small diameter fan moves less cfm at a higher pressure than a larger fan that moves more cfm at a lower pressure so what fan would have better cooling properties? The answer is simple, heat dissipation needs more air to flow rather than higher pressure to work efficiently. A smaller port doesn't cool better than a larger one, it actually promotes heat build up rather than a cooling trend due to less exchange. Try this little experiment, scream with your mouth barely open, the air is moving at a higher speed exiting your mouth but is your mouth cooler and how loud is it when compared to screaming with your mouth all the way open at the same frequency? Small ports shrink bandwidth and have premature rolloff in higher frequencies. The key is to have big port area and low tuning. This allows for efficiency across a larger bandwidth ergo less rolloff throughout so a flatter response curve depending on the woofer and tuning frequency combination. Now, to further the idea of ports and cooling (doesn't really do a whole lot in reality) we will try and look at the way a port works. The port can only move as much air as the woofer itself can move. If the cone area and stroke of the woofer displaces less air than the volume of the port you won't transfer any air at all. However you will introduce cooler air into the front of the port that will allow rear port air to transfer some heat out but it is dependant on port area. More port area means more surface area and that allows more efficient heat transfer. So in conclusion of the whole heat transfer vs port area your friend is trying to talk to you about, he is 100% wrong on every aspect of his reasoning based on actual physics. You are doing what you should and that is why you've had such success in your builds. Honestly if you want the best possible cooling for your build go inverted... If you want the best bandwidth with a ported enclosure go big port area and ridiculously low tuning. The only time you have too much port area is if you are not generating any internal enclosure pressure at all and the woofer is basically reacting as it would in free air.
this is why I think I legit never felt my SA Ultra ever get warm. It was legit cold through 45 mins of heavy play.

The box had like 18.5" of port per cube. 2.5 cubes tuned to 30. It was made for a HDD 12 but HDD motors were out of stock so I ordered a Ultra to play with. 1200 rms after rise and that sucker banged.
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  #9  
Old 08-01-2015, 07:57 PM
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I prefer aero due to the inherient increased efficiency and less displacement so I can build smaller gross and still achieve both desired tune/port area while maintaining desired net volume as well as increased efficiency over slot/square. Rocking 14.6" port per cube aero currently at 33hz in 8 net for the HD 15's on the bc5500 at .5 and everything is happy. Next enclosure will be around 13.5" per cube at 26hz tuning in 8 cubes or 15.2" per cube at 26hz in 8 cubes... haven't decided how big I want to go gross yet but trying to keep the enclosure as small as I can.
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Old 08-01-2015, 08:08 PM
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Quote:
Originally Posted by djscrubb View Post
Less the mach velocity. I'm tuned at 30hz with only 10inch port area. However cause I'm tuned so low no port noise cause 30 hertz sine wave moving slower then 40hz. Is my understanding
I think this is where the time spent learning to use WinISD or BBP really pay off, nothing like seeing your port velocity.
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