Question - I'm working on summing the new measurements, and have a correction factor of -9.8db for the port. Since there are two 75mm ports, should I be compensating somehow, ie should I only be reducing by half that, 4.9db?
No, doubling the power (what effectively you do by adding second port) is +3 dB, so reduce correction factor by 3. Also, dBs are logarithmic unit, you cant divide them like regular numbers.
Also, correction factor is proportional to the square root of poort and surface ratio, so doubling the surface reduce it by 1.41, square root of two, if we drop the decibels and look at the "natural" ratios.
Also, correction factor is proportional to the square root of poort and surface ratio, so doubling the surface reduce it by 1.41, square root of two, if we drop the decibels and look at the "natural" ratios.
OK Vox, that makes sense, thank you. If I'm reading you correctly I need to reduce the correction factor by 3db, which would bring me from -9.8 to -6.8db. Will work on it tomorrow and do the summing and see what happens.
OK, finally sat down to do some computer work, and I want to show the results. The new configuration is a 42L box (earlier was 84L) with two 75mm ports, 36mm deep. 36mm is the baffle thickness, so I couldn't go shorter without arduous gymnastics but it seems alright. According to the above exchange, I compensated the port -6.8db, which is the original -9.8 reduced by 3db because there are two ports. No baffle step, but it's looking much better. The first photo is woofer/port/summed. The second shows sealed woofer vs ported. The sealed response is lovely and smooth but DOA in the bass department so I definitely think the ports are necessary. Third shot shows the earlier response in the 84L box, and you can see the 1-2db gain in the midbass ending with a plummeting response below 80hz, which it seems would eliminate the need for a HP filter on the woofer?
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Looking good! Maybe tune port a bit lower if the system is going to be used in medium-to-smmall rooms or near the wall.
I would always use some HP on bass reflex speaker in PA/high-SPL conditions to limit the woofer excursion. With flea power tube amp it's very optional, though.
One of (if not the) task(s) of the HP filter is to reduce cone excursion. I wrote that before. Whether you go closed or ported, it’s mandatory. You also will have to consider the acoustic summing of sub and mains, don’t forget about that. The phase swing on a ported system plus additional highpass that is not tuned on that acoustic system can become something of a rollercoaster.
For this and for a few other reasons I’d pick a closed box anyway. Better damping of the rear volume and no port resonances to name a few. Plus the cabinet size can be reduced further.
For this and for a few other reasons I’d pick a closed box anyway. Better damping of the rear volume and no port resonances to name a few. Plus the cabinet size can be reduced further.
I added two sims, one of your ported system, black line = ported, tuning optimised to 60Hz (the summing of two ports adds 6dB not 3) and red line = one closed box 30l with active HP Q=1 fc=95Hz. What you can observe is that max output above 100Hz of both systems aren't far apart, 3-4 dB at 100Hz in favour of the ported enclosure. But this is sub territory still, so of practically no concern.
Also interesting is that the group delay of the ported system becomes problematic at 60-70Hz (audibility threshold of 0,8/f). Which is not a secret with higher tuned ported systems by the way.
One last thing, cone movement reduction. At just below 50Hz the closed box is behaving better in any way (meaning: for a given SPL, cone movement is less with the closed system than with the ported system). The active HP takes care of any excess cone movement. The red curve shows a lot more distance from the respective grey curve than the black curve from the grey curve belonging to the ported enclosure. I didn't notice any attention for better midrange performance (copper rings or caps or low Le design) in this driver, so I'd do a lot to keep cone movement to a minimum. But these are just my 2ct.
Also interesting is that the group delay of the ported system becomes problematic at 60-70Hz (audibility threshold of 0,8/f). Which is not a secret with higher tuned ported systems by the way.
One last thing, cone movement reduction. At just below 50Hz the closed box is behaving better in any way (meaning: for a given SPL, cone movement is less with the closed system than with the ported system). The active HP takes care of any excess cone movement. The red curve shows a lot more distance from the respective grey curve than the black curve from the grey curve belonging to the ported enclosure. I didn't notice any attention for better midrange performance (copper rings or caps or low Le design) in this driver, so I'd do a lot to keep cone movement to a minimum. But these are just my 2ct.
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Is'nt the consideration of cone movement moot, if they are only intended for low power? For sure in most cases high pass is optimal, but kind of cool if it just works with the subwoofers crossover.
Cheers!
Cheers!
Thanks Mark, I'm taking all of this into account, I'll keep messing with the AR approach but I'm still leaning toward BR, though as I come to understand what you and others are saying about a closed box I may change my mind. Can you clarify for me: When I used the calculator to modify the port volume, it gave me -9.8db for one port, then I subtracted -3db for the second port as per Vox's point above, bringing it to -6.8 Are you saying that I should be using -3.8 as my compensation for two ports? That's going to change things for sure.
The simple approach to this is overlaying the port output on the woofer output output at very low frequencies <20Hz. They have to be the same, since the woofer is in fact shorted by the port at frequencies 1/2 of fp.
Your port curve however is (you guessed it) 3dB lower than the woofer curve. Remember that doubling a radiating area leads to doubling the SPL at some distance (about 5 times the size of the radiator).
Your port curve however is (you guessed it) 3dB lower than the woofer curve. Remember that doubling a radiating area leads to doubling the SPL at some distance (about 5 times the size of the radiator).
I've read guys saying to just drag the port curve down until it meets the woofer at 20 hz or so, but that was for a single port. I'm a little confused by what you wrote, excuse my ignorance. If the compensation calculator tells me to reduce the port by 9.8db, it's only thinking I have one port. I understand the power is doubled, I'm just not clear if I should be reducing now by 6.8 (adding the 3db for the second port) or 3.8 (adding 6db)? It seems like you're saying to go with the second option as I have done? I'll compare that to the manual version and see how they look.
Correction to the above ^ I should have said "go with the first option", ie use -6.8 as the correction factor.
True, my apologies for the confusion. How far apart are the ports? Are you sure you measured just one and the other one didn’t interfere? If they are close together, measure inbetween. Actually I wouldn’t know why the traditional comparing of areas wouldn’t work in your case, otherwise than the above issue.
You could also use the klippel-method to measure combined driver/ports response, positioning the mic between all drivers and ports looking for the deepest low frequency cancellation.
See:
https://www.klippel.de/fileadmin/klippel/Files/Know_How/Application_Notes/AN_38_Nearfield_Measurement_with_Multiple_Drivers_ and_Ports(2).pdf
It requires several measurements to find the correct spot, usually next to the driver surround. I was successful. Keep in mind it's a nearfield measurement that will only be valid for low frequency range (some 100 Hz probably) and that needs eventual baffle step correction.
See:
https://www.klippel.de/fileadmin/klippel/Files/Know_How/Application_Notes/AN_38_Nearfield_Measurement_with_Multiple_Drivers_ and_Ports(2).pdf
It requires several measurements to find the correct spot, usually next to the driver surround. I was successful. Keep in mind it's a nearfield measurement that will only be valid for low frequency range (some 100 Hz probably) and that needs eventual baffle step correction.
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the ports are only 7cm apart, but I measured with the mic dead flush to the baffle so I don't know how much the second port is in play, but the Umik is omnidirectional so it's probably getting a boost from port 2, I hadn't considered that. I'll measure between the ports maybe a few cm from the baffle, what do you think? Can you tell me what this means?:
"why the traditional comparing of areas wouldn’t work in your case"
Thank you for this stv, really interesting method and makes a lot of sense. I'll re-read it later or tomorrow and give it a shot. It's a great point in my learning to try different things and compare, I've begun to enjoy sitting in front of the much despised PC (cheap Lenovo, I'm a Mac user) playing with squiggly lines 🙄
REW runs perfectly on my Macbook. Wish VCAD would do too, but an old laptop or Parallels helps me out.
I think measuring with the mike close to the baffle and inbetween ports will do fine. My guess is that you can tune a little bit lower.
I think measuring with the mike close to the baffle and inbetween ports will do fine. My guess is that you can tune a little bit lower.
This (of course) was because you cannot trust your port measurements right now. Chances are you measured output of both ports already.
REW runs fine on my Macbook too, but it's easier to move files from there to Vituix or any other program within the same computer🙂
I understand now, thank you. I'm going to run a few sweeps tomorrow with the mic at different positions to see which gives me the highest port output, and I'll just go with that. I suspect that measuring between with the mic not too far off the baffle will bump the response a little, but we'll see.
I really appreciate all the input from you Mark, and everyone else who's chimed in on this thread.
I understand now, thank you. I'm going to run a few sweeps tomorrow with the mic at different positions to see which gives me the highest port output, and I'll just go with that. I suspect that measuring between with the mic not too far off the baffle will bump the response a little, but we'll see.
I really appreciate all the input from you Mark, and everyone else who's chimed in on this thread.
Alrightie, so I went and shot a whole bunch of sweeps just to compare what happens in various positions/combinations. In the first pic, from top to bottom, we have:
1. Single port, flush w/baffle
2. center between ports, 2.5cm from baffle
3. center between ports, 7.5cm from baffle
4. Port+woofer, 35cm
5. Port+woofer, 50cm
6. Port+woofer, 1 meter
What you can see is that the single port measurement is the loudest, and measuring between the ports has more of a cancelling effect than additive, which is what I would expect, so I'm discounting them.
Moving to the lower three (combined), What I see is general agreement with the summed curve from earlier,which is shown in the second pic, with a bit of a sag above the port resonance, but not too bad. I'm concluding that my earlier summation is probably pretty close. The 1m measurement is showing reflections after 8.5ms, but the gated response does me little good right now.
I guess the midbass sag is telling me I need to tune the ports a bit higher, maybe 80-90, but I want to see if I can make this work.
1. Single port, flush w/baffle
2. center between ports, 2.5cm from baffle
3. center between ports, 7.5cm from baffle
4. Port+woofer, 35cm
5. Port+woofer, 50cm
6. Port+woofer, 1 meter
What you can see is that the single port measurement is the loudest, and measuring between the ports has more of a cancelling effect than additive, which is what I would expect, so I'm discounting them.
Moving to the lower three (combined), What I see is general agreement with the summed curve from earlier,which is shown in the second pic, with a bit of a sag above the port resonance, but not too bad. I'm concluding that my earlier summation is probably pretty close. The 1m measurement is showing reflections after 8.5ms, but the gated response does me little good right now.
I guess the midbass sag is telling me I need to tune the ports a bit higher, maybe 80-90, but I want to see if I can make this work.