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DCR (dual chamber reflex) which port method is correct
DCR (dual chamber reflex) which port method is correct
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Old 23rd June 2018, 10:28 PM   #1
scott68 is offline scott68  United States
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Default DCR (dual chamber reflex) which port method is correct

I've build a DCR (dual chamber reflex) box with 40 cu in in one chamber and 84 cu in in the driver chamber. I understand it is a little better to have more in the speaker chamber. There seems to be two or more methods to figure the port size for my 3" speaker. Which is the correct method (or another?) plus I will make the third middle port the same but maybe test and adjust it to be shorter.
1. Simulate the 3" TS parameters in just the 84 cu in chamber with/without stuffing using 2 ports in the sim.
2. same but use the total volume of 124 cu in?


Would it be the same to simulate with one port, and use that figure for the other two?


The DCR box may or may not lower the f3 but I understand that it may have a little less distortion at high volume which is important for a small driver.
Thanks for any help?
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Old 24th June 2018, 12:21 AM   #2
xrk971 is offline xrk971  United States
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DCR (dual chamber reflex) which port method is correct
I don’t know how to simulate them one chamber at a time, but the rule of thumb is to have the larger driver chamber as V and each subsequent chamber reduced by 1/2 so V and 1/2V total. The vents are all the same in each chamber and the connector vent. Although I find the connector should be about 1/3 the main vents. I simulate it all in Akabak so no approximations needed.
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Old 24th June 2018, 01:00 AM   #3
Michael Chua is offline Michael Chua  United States
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Quote:
Originally Posted by scott68 View Post
1. Simulate the 3" TS parameters in just the 84 cu in chamber with/without stuffing using 2 ports in the sim.
That's the correct way.

Whether it's one port or two, just duplicate them for the lower chamber and the middle one.

Quote:
The DCR box may or may not lower the f3 but I understand that it may have a little less distortion at high volume which is important for a small driver.
The DCR does not lower the f3. You tune the DCR to whatever f3 you want, just like in a ported box.

The difference in a DCR is the bottom chamber acts like a bandpass sub. Since the bandwidth is higher than the f3 in the upper chamber, it results in an additional "punch" in the upper bass. That's where the DCR makes a difference to the quality of the bass.

DCR (dual chamber reflex) which port method is correct


The plots above are from my latest DCR. The Blue plot is from the upper (larger) chamber. The Green plot is from the lower (smaller) chamber. Both taken at the mouth of the ports.

More information is at: LARK DCR
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Old 24th June 2018, 02:00 AM   #4
scott68 is offline scott68  United States
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So are you saying that I use the total vol of the box (124 cu in) to figure the port then make 2 more ports the same size (maybe the connector port might be smaller?) Should I enter, in the simulator, one port or 2?


Thanks
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Old 24th June 2018, 02:28 AM   #5
freddi is offline freddi  United States
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DCR (dual chamber reflex) which port method is correct
the old way (first to my recollection by George L. Augspurger) was to have an alignment (or total volume) divided into two chambers, one , twice the volume of the other. The whole volume using two external vents determined tuning.

In the classic DCR, the inner chamber vent was the same size as the other two. At system fb, the partition essentially becomes invisible and the entire volume is radiated by those two ports. A slow sweep of the system I think will show the ports working separately then together in phase This gave no extension vs a single volume with same parameters and tuning. It was characterized by a small notch in response and in driver excursion about an octave up from system resonance.

When the DCR article was first published, it was felt it did something rather special. Later, with better modeling, it was better explained.

Michael, I'll have to check your DCR. Hornresp should do some of them. Which driver T-S might work best in those type? (I've only made one for a VIfa 8" about 20 some years ago) Besides some B&C 15pzb20, I've the current Eminence B102 which might make a nice double chamber reflex.

here's what I get with 48 liters for B102 - look ok? what would you do different? (I'all be adding a Morel CAT 378 tweeter) https://i.imgur.com/o1jjmPt.jpg

Last edited by freddi; 24th June 2018 at 02:45 AM.
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Old 24th June 2018, 04:53 AM   #6
Michael Chua is offline Michael Chua  United States
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Hi scott

I think there's some confusion. I'll take freddi's Eminence B102 as an example.

Ported Box Volume = 60 liters (see attachment)

Divide by 3 = 20 liters

Top chamber = 40 liters

Bottom Chamber = 20 liters

Use 40 liters to tune box. In the B102, it's 3" dia, length=3.56" for F3=60Hz.

Use this port dimensions for all three ports.

Quote:
Originally Posted by freddi View Post
here's what I get with 48 liters for B102 - look ok? what would you do different? (I'all be adding a Morel CAT 378 tweeter)
Hi fred

I'm afraid I'm not familiar with hornresp. I based my sim on the B102 spec sheet. Is your 48 liters the total volume?

BTW, I have the CAT 378 too. It's a wonderful tweeter.
Attached Images
File Type: gif EMINENCE B102 60L.gif (39.6 KB, 269 views)
File Type: gif EMINENCE B102 40L.gif (40.0 KB, 264 views)
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Old 24th June 2018, 05:15 AM   #7
freddi is offline freddi  United States
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DCR (dual chamber reflex) which port method is correct
yeah - 48 - if ever built then might go a bit larger and somewhat lower tuning - I think a K-tube for some instances would beat a 378 - the tube is a lot louder and subjective dispersion wider https://i.imgur.com/ZUWOcVl.jpg

Last edited by freddi; 24th June 2018 at 05:24 AM.
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Old 24th June 2018, 05:25 AM   #8
scott68 is offline scott68  United States
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Now I really confused. I appreciate all the comments and I have already built and separated the chambers in 1/3 and 2/3. Maybe I misinterpreting the comments since I think one comment was to use the total volume to figure the ports and another to use just the large chamber. Did I misunderstand that? Let me ask a different way:
1. Using a bass reflex simulator, do I enter the larger chamber (84) or do I enter the total (124)?
2. Then do I enter a "1" for 1 port or "2" for 2 ports. I know then to use the port's length and diameter for all three ports.


Thanks
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Old 24th June 2018, 07:25 AM   #9
freddi is offline freddi  United States
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DCR (dual chamber reflex) which port method is correct
I usually think of a DCR having approximately the same LF response, internal volume and tuning as a single chamber reflex with the same volume and tuning for a given driver. I think there are other approaches and tuning juggling but am not familiar.


in the case below, hornresp says my 48 liter DCR has about the same systerm tuning (around 48Hz) as a single chamber reflex when both have the same total vent (to the external world) area and vent length. It will be interesting to hear from Michael and others. You can see system tuning is about the same, but at octave above tuning, the DCR has a lot different behavior. (assuming the connecting vent is not heavily damped)

does this make sense?

Click the image to open in full size.
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Old 24th June 2018, 08:12 AM   #10
David McBean is offline David McBean  Australia
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Quote:
Originally Posted by scott68 View Post
Now I really confused. I appreciate all the comments and I have already built and separated the chambers in 1/3 and 2/3. Maybe I misinterpreting the comments since I think one comment was to use the total volume to figure the ports and another to use just the large chamber. Did I misunderstand that? Let me ask a different way:
1. Using a bass reflex simulator, do I enter the larger chamber (84) or do I enter the total (124)?
2. Then do I enter a "1" for 1 port or "2" for 2 ports. I know then to use the port's length and diameter for all three ports.


Thanks
Hornresp can show you in 'real time' how changes to the chamber and port tube sizes will affect the system response.
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