Hello,
I have bought a used pair of the McCauley 6174 quite some time ago, and did my best to use them in an open baffle configuration (with limited success...).
But now it's time to finally put them where they belong, into a vented box. I use Unibox by Kristian Ougaard for the calculations, the ideal size would be a bit over 500 Liters... but that's a bit more than I can fit! I played around a bit with the values and ended up with a 300 Liter box with 261 Liter internal volume (deducting driver and bracing), tuned to 20 Hz, F3 should be at roughly 30 Hz (if I did this all correctly):
And here is a rendering of what it should end up looking like:
Here the baffle and bracing only:
And I hear some readers screaming "what, OSB?".
Well, to be honest, I have my doubts about that idea now. I just happen to have a bunch of it left-over at the moment. The baffle is cut from an old but nice and strong oak-veneered 3/4" plywood, it's already being glued up to the bracing:
No OSB has been used yet, so still time to re-think. When I read about gluing OSB, I found a few people saying it doesn't glue well with wood glue. One point against using OSB. Further I just think that going through all this effort building the speaker (two, actually), why not spend another 150 Euros and use some Plywood for the rest instead?
I'd love to hear your thoughts on that, and any other thoughts of course as well!
P.S.: The vented port is still missing in the drawings.
I have bought a used pair of the McCauley 6174 quite some time ago, and did my best to use them in an open baffle configuration (with limited success...).
But now it's time to finally put them where they belong, into a vented box. I use Unibox by Kristian Ougaard for the calculations, the ideal size would be a bit over 500 Liters... but that's a bit more than I can fit! I played around a bit with the values and ended up with a 300 Liter box with 261 Liter internal volume (deducting driver and bracing), tuned to 20 Hz, F3 should be at roughly 30 Hz (if I did this all correctly):
And here is a rendering of what it should end up looking like:
Here the baffle and bracing only:
And I hear some readers screaming "what, OSB?".
Well, to be honest, I have my doubts about that idea now. I just happen to have a bunch of it left-over at the moment. The baffle is cut from an old but nice and strong oak-veneered 3/4" plywood, it's already being glued up to the bracing:
No OSB has been used yet, so still time to re-think. When I read about gluing OSB, I found a few people saying it doesn't glue well with wood glue. One point against using OSB. Further I just think that going through all this effort building the speaker (two, actually), why not spend another 150 Euros and use some Plywood for the rest instead?
I'd love to hear your thoughts on that, and any other thoughts of course as well!
P.S.: The vented port is still missing in the drawings.
For validation purposes, I've modelled the above driver/enclosure combination using VituixCAD. The following results were obtained, and the frequency response agrees quite well. The low-frequency cut-off point pedicted by VituixCAD is a little more conservative, being 31.7Hz versus 29.9Hz predicted by Unibox. It's interesting to see that the vent output is not contibuting that greatly to the total output.
If a reasonably powerful plate amplifier is available, and it has the ability to apply a 2nd-order peaking high-pass filter, then the following extension to the bass response could relatively easily be obtained. Here is an example of the response that can be expected if a 2nd-order high-pass peaking filter with Q=2.0 set to 20Hz is used with the vented enclosure. The low-frequency cut-off has been lowered to 21.8Hz, which is about 10Hz lower than before. The driver is still working well within its linear operating range while producing a peak SPL of around 113dB in its passband.
After adding a 4th-order 80Hz Linkwitz–Riley low-pass filter, the response looks like the following. Interestingly, notice how adding the low-pass filter has slightly lowered the low-frequency cut-off point, effectively increasing the low-frequency extension by almost 1Hz.
If the listener has an aversion to port chuffing, then a sealed enclosure of the same size can produce the following response. In this design, Xmax is reached at around 21Hz, for a nominal input power of 80W re 8 ohms, while the peak power required is around 300W at 21Hz.
If a more traditional 2nd-order low-frequency roll-off is desired, we can just apply some simple parametric EQ at 23Hz, consisting of +6dB of boost with Q=2.0. The resulting simulated response is shown below. In this design, we get a little bit more output between 22Hz and 32Hz.
After adding a 4th-order 80Hz Linkwitz–Riley low-pass filter, the response looks like the following. Interestingly, notice how adding the low-pass filter has slightly lowered the low-frequency cut-off point, effectively increasing the low-frequency extension by almost 1Hz.
If the listener has an aversion to port chuffing, then a sealed enclosure of the same size can produce the following response. In this design, Xmax is reached at around 21Hz, for a nominal input power of 80W re 8 ohms, while the peak power required is around 300W at 21Hz.
If a more traditional 2nd-order low-frequency roll-off is desired, we can just apply some simple parametric EQ at 23Hz, consisting of +6dB of boost with Q=2.0. The resulting simulated response is shown below. In this design, we get a little bit more output between 22Hz and 32Hz.
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Fantastic responses!
First a few things that may help to understand my initial plan:
At least that's the plan. I could use digital EQ on the Raspberry (through LSP Plugins) to boost the low end I suppose?
The idea to make it a closed box is interesting. From what I've heard, it should result in a bit more controlled response, if I'm not mistaken.
Since I haven't cut the port hole yet, I wonder if I should try a closed box? I think I tried modeling the response in Unibox, and it didn't extend very low at all. I suppose in your suggestion for the closed box you assume that the lowest octave is EQ'd for the closed box design?
I wonder if I could build an active analog EQ circuit from scratch? Might be a bit above my expertise, but I am not afraid to experiment.
Thanks for your help 🙂
First a few things that may help to understand my initial plan:
- The music source shall be a Raspberry Pi with the AroioDAC (already built).
- The main amp is a Musical Fidelity X-150, feeding two "satellite" speakers.
- It has a pre-amp out, which shall feed an active low-pass filter I already built. It's a forth-order with a cut-off frequency of 80 Hz.
- This low-pass filter then feeds a Musical Fidelity X-P200 power amp
- That power amp then feeds the two subs.
At least that's the plan. I could use digital EQ on the Raspberry (through LSP Plugins) to boost the low end I suppose?
The idea to make it a closed box is interesting. From what I've heard, it should result in a bit more controlled response, if I'm not mistaken.
Since I haven't cut the port hole yet, I wonder if I should try a closed box? I think I tried modeling the response in Unibox, and it didn't extend very low at all. I suppose in your suggestion for the closed box you assume that the lowest octave is EQ'd for the closed box design?
I wonder if I could build an active analog EQ circuit from scratch? Might be a bit above my expertise, but I am not afraid to experiment.
Thanks for your help 🙂
P.S.: The low pass filter I have built is using this module:
https://www.ebay.de/itm/176153661566?var=475601902563
https://www.ebay.de/itm/176153661566?var=475601902563
What type of 4th-order low-pass filter is it? Butterworth or Linkwitz–Riley?
If you have access to a variety of digital EQ plugins, then they could well be up to that task.
The closed-box enclosure having a more controlled response results from it having a 2nd-order low-frequency roll-off. In comparison, a vented-box enclosure has a 4th-order low-frequency response, which means that its transient response will take longer to settle.
That's what I described. Without the presence of significant EQ, the closed-box enclosure has a higher low-frequency cut-off point than the vented box of the same volume.
If you have already built an active low-pass filter, then an active 2nd-order high-pass peaking filter may not be that much more complex. I expect that most of the added complexity stems from enabling the variation in 1) the peaking frequency, 2) the amount of boost, and 3) the Q of the peak. That's when having good digital EQ/filtering options available can be a bit simpler in some respects.
Not necessarily. It could also be sign of a low Qts driver (which I think it is here).
I think it will indeed be much easier with digital EQ, at least for testing purposes alone. I will go that route.
Regarding your calculations, I tried to replicate these with my VituixCAD installation, but didn't get quite the same results. Upon comparing the driver data, I noticed that you used an RMS of 32.56. Mine was set to 8.646, a number I found here. I admit I don't understand every bit of Thiele/Small parameters, however, when changing this value the Excursion and Impedence graphs look significantly different for the closed box. Should this be a concern? I don't want to kill the driver... I already tortured them a lot when EQ'ing in the open baffle configuration...
The Rms value that I used is relatively large, corresponding to Qms=2.9, and leads to a lower value of Qts. So the amount of damping at resonance will be much greater than in your simulation, as shown by your driver impedance and excursion peaks being considerably higher.
The McCauley 6174 18-inch driver seems to have a variety of Thiele-Small parameters associated with it. Hence, ideally you would at try and measure your own driver if you want the most accurate simulations when adding filtering and EQ.
From what I can see in your curves, the SPL frequency response is quite similar. So even with such a large variation in the Qms (Rms) value, the simulation should still be relatively accurate in broad terms, if not in all the fine details. Keep in mind that, with a closed-box enclosure, one would generally add lots of fibrefill material. This will also have the effect of increasing the effective enclosure volume by 10–15%, which will also change the response by lowering the closed-box resonance frequency. All of these changes are a few Hz here and there, and maybe a dB or two near the cut-off frequency, which can of course usually be equalised away if desired, as the driver can handle quite a bit of power. Of course, similar things will happen to a vented-box enclosure if we don't have entirely accurate Thiele–Small parameters.
It would be handy to be able to perform some near-field measurements of the closed-box enclosure (e.g., using a low-cost microphone). This would help to ensure that the closed-box system with EQ is performing to your satisfaction. If you already have a microphone handy, then it's worth trying to carry out a set of such measurements. A measured impedance curve would also be highly informative to see.
What worries me about the simulation with my RMS value is, that the Xmax is reached much earlier than in yours. However, I like the idea of a 2nd-order low-frequency roll-off.
Anyway, I am thinking about building it as a closed box first, test it, apply some EQ, do some measurements, and then decide whether to leave it a closed box or install a port. It would be a bit messier to install a port after the box is built, but certainly not impossible.
I have RoomEQ Wizard and a measurement mic (the MiniDSP USB). I have done measurements before, so doing it is not the issue, doing it right and understanding it is another thing 🙂
Anyway, nobody commented on the OSB vs Plywood issue. I have decided to use Plywood, which I will get on Tuesday. So it will be a few more days before the box is done.
Anyway, I am thinking about building it as a closed box first, test it, apply some EQ, do some measurements, and then decide whether to leave it a closed box or install a port. It would be a bit messier to install a port after the box is built, but certainly not impossible.
I have RoomEQ Wizard and a measurement mic (the MiniDSP USB). I have done measurements before, so doing it is not the issue, doing it right and understanding it is another thing 🙂
Anyway, nobody commented on the OSB vs Plywood issue. I have decided to use Plywood, which I will get on Tuesday. So it will be a few more days before the box is done.
Still working on the boxes.
Meanwhile I have a question... is it a terrible idea to put the port on the short side of the box? I realized today that I cannot route the port hole later with the way I am building the boxes. The sides are proud of the baffle and back... to protect the speaker and also the speaker connector at the back. I'd like to keep it that way.... also gives me the option to put some type of "grill" or so in the front if I decide to do so.
Hopefully I will be satisfied with the closed box frequency response, and I won't have to worry about it. But if I do decide to add a port, it would have to be on the side...
Meanwhile I have a question... is it a terrible idea to put the port on the short side of the box? I realized today that I cannot route the port hole later with the way I am building the boxes. The sides are proud of the baffle and back... to protect the speaker and also the speaker connector at the back. I'd like to keep it that way.... also gives me the option to put some type of "grill" or so in the front if I decide to do so.
Hopefully I will be satisfied with the closed box frequency response, and I won't have to worry about it. But if I do decide to add a port, it would have to be on the side...
Doesn't matter what side of the box the port is on if it is not obstructed.
It is far easier to cover a port rather than add it after the build is finished.
I have several cabinets that alternate between sealed or ported depending on use.
I could do that too, as I have not yet glued on the sides.
Can you tell me how you cover your ports? Shouldn't it be a solid piece that's covering the hole, and airtight?