Too Much port Area, how do I know?

rufus.K

rufus.K

Oct 18, 2015
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#1
So Ive got this speaker cabinet. Its made by a well known brand, which I wont mention cos I dont want to hear why I shouldnt do this based on the brand... But it was/is a combo, one of their popular models with a 2ru airhead and one of their amps on top. Ive removed the head, and want to cut the air head off of it., make a standalone cabinet. No big deal there except hiding the surgery. While Im at it, Im thinking of adding another port/tube to it, to match the port area of another of their models with almost identical box volume. So how do I know, by listening to it, if Ive got tooo much free area in port volume. What are the basic characteristics of too much porting in reference to the sound I hear? I can and am willing to re-fill the hole if its just terrible in the end, as Im cutting the airhead off anyway. I like the cabinet well enough, but I really want to to do this for science's sake, and would like a little idea of what affect it will have, in general terms.
I realize I could model the cab with software, or be actually scientific about this, but then we wouldn't have the opportunity to have this little chat, would we...
 
#2
There's actually a lot of support for some WAC designs around here.

Many of the real successful ones just turn out to match the science in the end.

Some give a big hump in the middle bass which fool a lot of people into thinking it has real bass.
It's absolutely true that you can just ad hoc drill holes, and ad ports and you may find a sound you like better than stock. In your case since you can return to stock, I say give it a try.
Follow some science guidelines like don't make the port to small that it chuffs. If you burn out a speaker or two by underloading it, well then you can buy a new and possibly better driver. Worst you'd be is out some bucks, and you'll have a learning experience. In the end you can return to stop design.
 
#3
Does WAC stand for Without A Care?
Its is ported to begin with, just not as large as another later model, so hopefully it wont chuff. So more port area will raise the tuning from lower to mid bass, if I understand correctly? Im hoping for more articulation, rather than depth, when Im done, and its paired with another box Ive got.
 
#4
Well... porting a cab is supposed to enhance bass response. Changing the porting changes the bass response. At some set of frequencies, the port resonance enhances the base response. Change the porting and you change the set of frequencies that get enhanced. You should be able to hear the enhancement change. Before, it was somewhat louder in one frequency range. After, it is somewhat louder in a different frequency range. Like seamonkey put it, there's a hump in the sound somewhere. Change the porting, and you change where the hump is. Adjust to taste.
 
#5
Now with port shelves, I understand that that the airflow over the back of the shelf (like blowing accross a bottle) is where the interaction starts... Is that so with port tubes?
if I go creating a multiple of tubes, each with their own "mouth", this cant be better than if I were to enlarge one of the existing ones, yes?
Harder to return it to stock then, though.
 
#6
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rufus.K said:
I really want to to do this for science's sake, and would like a little idea of what affect it will have, in general terms.
I realize I could model the cab with software, or be actually scientific about this
Click to expand...

Can't have it both ways...
Or_wink.gif


Port area trades against port depth if you want to maintain the same box tuning. If you aren't exactly duplicating the cab volume of your target cab and using an identical driver, you're essentially just throwing darts without doing modeling and/or actually measuring.

rufus.K said:
Now with port shelves, I understand that that the airflow over the back of the shelf (like blowing accross a bottle) is where the interaction starts... Is that so with port tubes?
if I go creating a multiple of tubes, each with their own "mouth", this cant be better than if I were to enlarge one of the existing ones, yes?
Click to expand...

That depends on what tradeoff set you are going for. There's never a free lunch, ever.

If you would tell us what the cab in question is someone might do your homework for you BTW.
 
#8
rufus.K said:
Now with port shelves, I understand that that the airflow over the back of the shelf (like blowing accross a bottle) is where the interaction starts... Is that so with port tubes?
if I go creating a multiple of tubes, each with their own "mouth", this cant be better than if I were to enlarge one of the existing ones, yes?
Harder to return it to stock then, though.
Click to expand...
image.jpeg

Self noise from any kind of ports is a very serious problem in recording studio monitors, my son Henry goes that extra mile in his range of custom sub bass designs to avoid edges that can induce chuff and hooting.:-)
You will need to model the enclosure with software to see how port changes will change the frequency response, assuming you have the TS parameters for the driver.
 
#9
You also need to TEST the design to be sure it performs as you think the software predicts. I can't tell you the number of mistakes I have seen that get caught in the testing process.
 
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#10
Yeah, guess I cant have it both ways... was trying to fish out some rules of thumb. Youre right about off of this. So to divulge, it'd be butchering an Aguilar GS112, and getting closer to the diameter of an Aguilar SL112 port wise. I didnt know port area trades against port depth.
 
#11
Bassmec said:
View attachment 713871
Self noise from any kind of ports is a very serious problem in recording studio monitors, my son Henry goes that extra mile in his range of custom sub bass designs to avoid edges that can induce chuff and hooting.:)
You will need to model the enclosure with software to see how port changes will change the frequency response, assuming you have the TS parameters for the driver.
Click to expand...
Damn... great looking tube indeed
 
#13
agedhorse said:
You also need to TEST the design to be sure it performs as you think the software predicts. I can't tell you the number of mistakes I have seen that get caught in the testing process.
Click to expand...

A lot of the times the problem lies in the port length that has been predicted by the software. I use two software tools, one (AJhorn) to calculate the exact tuning frequency and Boxsim to calculate the length. Boxsim is one of the only tools that takes the position of the port (distance port to bottom, top, left, right and back side) into account which has a major influence on the port lenght. This is where it goes wrong most of the time.
 
#14
rufus.K said:
So how do I know, by listening to it, if Ive got tooo much free area in port volume. What are the basic characteristics of too much porting in reference to the sound I hear?
Click to expand...

It depends.
Really, if the ratio of port area, length and enclosure volume reach a certain point then the whole system will morph into a transmissionline system. You can already see this happening with designs that have a rather long shelf port, a standing wave will occur inside the port. If this standing wave is situated somewhere between 150-300hz then it's rather tough to get rid of it with lining/damping the enclosure. This standing wave will color the sound, making it "hollow" sounding in some case.
 
#16
okcrum said:
It is often done purposely, even with cylindrical ports. Compare all the similar sized 2x10s, and you'll see all kinds of tuned-in lower mid bumps. Warwick's, Avatar's, G-K, GB, etc.
Click to expand...

That lower mid bump isn't caused by a standing wave inside the port but due to the fact that they've used drivers with relative high Qts(Qes) and a to small cabinet for that driver where they didn't lower the tuning of the port to get a flatter response.
 
#19
Arjank said:
That lower mid bump isn't caused by a standing wave inside the port but due to the fact that they've used drivers with relative high Qts(Qes) and a to small cabinet for that driver where they didn't lower the tuning of the port to get a flatter response.
Click to expand...

Sorry, more to your point, the bump can be flattened out with cabinet tuning to some extent. Moving the cab tuning frequency changes both the amplitude & frequency of the center of the bump.
 
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