If I were you, I'd make the cabinet a little deeper for the internal volume to be 16-18 liters. By using a port tube of the same length, you can obtain at least 5 Hz deeper bass extension.
As Zaph noted in the design page, his port tuning was limited by the cabinet depth and volume because he chose to use a premade cabinet. Since you build it yourself, you can use an ideal box volume for the woofer.
As Zaph noted in the design page, his port tuning was limited by the cabinet depth and volume because he chose to use a premade cabinet. Since you build it yourself, you can use an ideal box volume for the woofer.
Jay_WJ said:
You can accomplish the same thing by decreasing the diameter of the port opening, which will automatically allow you to shorten the length of the pipe for this downsizing. What is wrong with that?
John L said:
No, you can't obtain the same effect simply by reducing the port diameter. If you doubt, try Unibox or winISD sims. Reducing the port dia and lowering the tuning freq as a result without changing the box volume will produce even higher F3 in this case for the ER18 driver.
If tweaking the port dia would work like that, then people would have no need to use a large box. They would only need to decrease the port dia to get deeper bass. Let alone the increased port air speed issue...
I hadn't realized the cabinet was undersized. Can I get the same effect going for a taller rather than deeper cabinet?
A little taller baffle should be okay. If you want to go this route, extend the bottom edge instead of the top edge. The tweeter's diffraction pattern will be more affected by a change of top edge.
Jay_WJ said:
You left out an important part of my post. Changing the diameter of the port is only one part of the equation. The length must also be changed when you alter the diameter. That is why you can use your modeling software to alter the diameter, and the length of the tube will automatically change to accomodate it.
Look, I'm not an expert on this, but I have yet to see anything that states that the diameter and length of the vent, working in concert with each other cannot be altered to acheive the same goal. I thought it was all about air flow here, and the ability of the cabinet to pass the signal through that opening. The length of the tube is just as important as the opening.
Please point me to the literature which states that altering the two dimensions won't acheive the same goal. If wrong I will gladly alter my thinking, ok?
John,
You also missed a point in my post. I didn't necessarily mean that the length of the port should stay the same. What I meant is that you can't have the same response by simply changing the port size (both diameter and length) without changing the box volume.
BR response depends on not only the port size but also the cabinet volume. You can't have the same effect by changing the port size alone. That's what I meant.
You also missed a point in my post. I didn't necessarily mean that the length of the port should stay the same. What I meant is that you can't have the same response by simply changing the port size (both diameter and length) without changing the box volume.
BR response depends on not only the port size but also the cabinet volume. You can't have the same effect by changing the port size alone. That's what I meant.
Hey John L and Jay WJ
Changing the port diameter and maintaining the same tuning frequency necessitates changing the port length. This is either increased length for a larger diameter port or a decreased length for smaller diameters.
On top of this you have to consider that with decreased port diameter the air velocity within it also increases and could be audible at some listening levels. Conversely larger diameters mean lower air velocity around the port tuning and less or no noise but the length can quickly becomes unmanageable. The trick is finding a balance that doesn't result in a huge long port nor one that's creating audible port noise.
Adjusting the box volume changes to resonance of the air volume within it. Larger volumes lower the box resonance. The box resonance cannot be altered by the port resonance, however the port resonance frequency derives itself from the box resonance frequency so box volume influences port tuning.
Changing the port diameter and maintaining the same tuning frequency necessitates changing the port length. This is either increased length for a larger diameter port or a decreased length for smaller diameters.
On top of this you have to consider that with decreased port diameter the air velocity within it also increases and could be audible at some listening levels. Conversely larger diameters mean lower air velocity around the port tuning and less or no noise but the length can quickly becomes unmanageable. The trick is finding a balance that doesn't result in a huge long port nor one that's creating audible port noise.
Adjusting the box volume changes to resonance of the air volume within it. Larger volumes lower the box resonance. The box resonance cannot be altered by the port resonance, however the port resonance frequency derives itself from the box resonance frequency so box volume influences port tuning.
Thanks for the additional info, ShinObiwan. We're talking about knowledge that is already very well established.
To avoid confusion, what I meant above by "the same response" or "the same effect" is the response obtained by changing the box volume, which can't be replicated by changing only the port size, let alone the air velocity issue.
To avoid confusion, what I meant above by "the same response" or "the same effect" is the response obtained by changing the box volume, which can't be replicated by changing only the port size, let alone the air velocity issue.
I think we are waltzing around the same basic answer, but looking at it differently. For example thinking that the box volume must be changed to accomodate the change in diameter and length is one way. I look at it in a completely different context. In other words I look at a change in port parameters being the end result. Pick the size of the box you need first and tailor the other part to match it.
Now, Shin is correct in that we should use reason, and flexibility, on this. Naturally I would try to shoot for the initial parameters that the program recommended. But if the recommended tube length is too long, clearly it must be shortened, or bent. And bending is out of the question IMO.
So it is imperative that the length be shortened so the air can easily be moved through the tube, as before. But to accomplish this the diameter of the tube MUST be decreased to allow the same air to move back and forth at the same amount.
If you enter a change in your program, and keep the tuned frequency the same, the program will automatically make the necessary adjustments for you.. somehow I suspect most people believe that is it imprtant to have the port diameter as large as possible, and this is a fallacy. It's not necessary.
That is all I am stating here. The parameters are adjustable, and must be tailered for the size of the box.
Now, Shin is correct in that we should use reason, and flexibility, on this. Naturally I would try to shoot for the initial parameters that the program recommended. But if the recommended tube length is too long, clearly it must be shortened, or bent. And bending is out of the question IMO.
So it is imperative that the length be shortened so the air can easily be moved through the tube, as before. But to accomplish this the diameter of the tube MUST be decreased to allow the same air to move back and forth at the same amount.
If you enter a change in your program, and keep the tuned frequency the same, the program will automatically make the necessary adjustments for you.. somehow I suspect most people believe that is it imprtant to have the port diameter as large as possible, and this is a fallacy. It's not necessary.
That is all I am stating here. The parameters are adjustable, and must be tailered for the size of the box.
To be more informative, especially for drproton, below are simulated bass responses that can be obtained by changing the cabinet volume or the port size.
First, below is the response of the default 44 Hz tuning in a 14 liter cabinet:
Now below is the response obtainable by increasing the box volume to 17 liters with the port size unchanged, which I suggested:
As you can see, the port tuning frequency changes from 44 Hz to 40 Hz, and -3 dB point from 48 Hz to 44 Hz. And the -6 dB point, which should be in-room usable bass, changes from 42 Hz to 37 Hz.
Next is the response obtained by changing the port size (both dia and length), but using the default 14 liter cabinet, to achieve the same, low 40 Hz tuning as above:
Now the -3 dB point increases even higher, from 48 Hz to 52 Hz. But the -6 dB point remains about the same (about 41 Hz). Notice the difference between this 14 liter and the 17 liter versions. Although the tuning frequency stays the same, the amplitude of port contribution is bigger in the 17 liter box than in the 14 liter box.
Now I've given enough info to prevent any confusion.
First, below is the response of the default 44 Hz tuning in a 14 liter cabinet:
An externally hosted image should be here but it was not working when we last tested it.
Now below is the response obtainable by increasing the box volume to 17 liters with the port size unchanged, which I suggested:
An externally hosted image should be here but it was not working when we last tested it.
As you can see, the port tuning frequency changes from 44 Hz to 40 Hz, and -3 dB point from 48 Hz to 44 Hz. And the -6 dB point, which should be in-room usable bass, changes from 42 Hz to 37 Hz.
Next is the response obtained by changing the port size (both dia and length), but using the default 14 liter cabinet, to achieve the same, low 40 Hz tuning as above:
An externally hosted image should be here but it was not working when we last tested it.
Now the -3 dB point increases even higher, from 48 Hz to 52 Hz. But the -6 dB point remains about the same (about 41 Hz). Notice the difference between this 14 liter and the 17 liter versions. Although the tuning frequency stays the same, the amplitude of port contribution is bigger in the 17 liter box than in the 14 liter box.
Now I've given enough info to prevent any confusion.
John L said:
No, that's exactly what I'm concerned about. You cannot "pick the size of the box you need and tailor the other part to match" the response that would be obtained in a different sized box, as I showed above.
Jay_WJ said:
Sigh,..........You can if you are a rugged individualist, and I am. I determine what I want, not some other entity. That entity can advise me, but I make the command decisions, not the program.
While you are technically correct here, it is the utopian/Perfect result. But in "real life" there are compromises and decisions that differ.
I guess you and I are looking at how the universe works from a different perspective. As a former tank company commander I gladly accepted advice from my platoon leaders and senior NCOs, but the responsibility was mine, and I made the decisions.
That is my point here, and I am not trying to be a contrarian, but we will just have to agree on the minute details, but not on the overview. Even in the world of DIY we are still faced with decisions that tend to take us away from the ideal results.
That is all I am getting at. Anyway, I will bow to your superior judgement and agree that you are correct. 🙂
John L said:
No, I'm not talking about an utopian/Perfect result. Using an ideal volume for some drivers are nearly impossible because the suggested volume is unreasonably large. But in the case of the ER18, it's not. 17 to 18 liters is a reasonable size for a standmounting speaker.
are we there yet?
Looks like the difference here is building a speaker following known theory and guidelines, vs. doing whatever you feel like doing while still using some sort of preformatted structure for some guidelines.
apples and oranges
another John L.
Jay_WJ said:
Looks like the difference here is building a speaker following known theory and guidelines, vs. doing whatever you feel like doing while still using some sort of preformatted structure for some guidelines.
apples and oranges
another John L.
Alright, I need to change what I said above a bit. In fact, as we know, there is no single ideal box volume for a driver. But there is a range of parameters that produces reasonably good results. And within this range, there can be a better choice over another depending on the situation.
What I meant in my posts is simply that a bit lower tuning in a little larger but still reasonably sized box provides a better result than tweaking the port size in the default box, since the OP has flexibility in building his own cabinet.
That's it. Actually there's nothing significant we want to argue about.
What I meant in my posts is simply that a bit lower tuning in a little larger but still reasonably sized box provides a better result than tweaking the port size in the default box, since the OP has flexibility in building his own cabinet.
That's it. Actually there's nothing significant we want to argue about.
I got the SR71 kit today. Looks great. I think I'll go for the 17L cabinet as suggested. I figure I can added solid volume to the cabinet if I want to try the 14L volume as well. To get the extra volume I increased the height by 2". There is now space on the back panel to mount the crossover between the port and the terminal cup, where it is easier to work at. Hopefully I can get the boxes built this weekend and post some pics. Before I start any building I gotta rig up a better dust collection system for the router. That MDF dust is nasty. Its so light it ends up all over the basement.
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