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When porting a cab, do you...?
- Thread starter m.oreilly
- Start date
yeah, calcing it out of the total volume. at least there will be less of a chance of 'chuffing'. this remake cab is for quieter situations/less back ache, or as an add on cab for the berg, though i doubt i'd really need it. if it doesn't pan out, a couple of bill's jack cabs would be neat. atm just trying to do a cheap two ten that will pass muster in low volume settings.
If it's "only" for lowish volume settings you maybe could get away with a bit smaller port area as well as a bit higher tuning to make it "sound" sort of loud/full/deep/whatever. Just know what you're going to be using it for/with and watch what happens to port air velocity as well as excursion below the tuning frequency when you put power to it.
billfitzmaurice
Commercial User
- Sep 15, 2004
- 16,860
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- Owner, Bill Fitzmaurice Loudspeaker Design
You can build a cab any size you want. The smaller it is the less low frequency extension you'll get. This box size is OK if you can live with a 58Hz F3. IMO doing so is a waste of the driver's capabilities.
I’ve been watching this thread as I’ve been working on plans for a build as well, using a Deltalite II 2512 and the “Larger 1x12 Bass Cabinet” suggestion from Eminence. They suggest a cabinet volume of 2.25ft3 with (2) 3” round ports that are 6.34” long, making a tuning frequency of 51.13Hz.
If I increase the cabinet volume to get more of the low frequency extention that Bill mentioned, do I need to increase the port tube length as well? I was under the impression that ports are tuned to a certain frequency based on driver specs and not cabinet size?
Thanks in advance to all for your patience and info! This is my first amateur hobby build and I’m obviously still in research and design mode. My main goals are 1) to keep myself occupied for a while and 2) to see what happens, good or... not so good.
5sg.
If I increase the cabinet volume to get more of the low frequency extention that Bill mentioned, do I need to increase the port tube length as well? I was under the impression that ports are tuned to a certain frequency based on driver specs and not cabinet size?
Thanks in advance to all for your patience and info! This is my first amateur hobby build and I’m obviously still in research and design mode. My main goals are 1) to keep myself occupied for a while and 2) to see what happens, good or... not so good.
5sg.
Thanks for the tip. I *do* want a lower tuning, so I'll leave the port length alone and work out measurements for adding more cab volume.
5sg.
A lower tuning does not translate into a bigger bottom.
A bigger box (to a point) is what does this.
You can see this by calculating two different alignments for the same driver. For the Deltalite 2510-II, these are the BB4 and Flat alignments.
BB4: tuned at Fs = 53 Hz, Vob=1.48 cuft, F3 point is 56 Hz.
Flat: tuned at 51 Hz, Vob=2.12 cuft, F3 point is 50 Hz.
By definition, the BB4 or SBB4 is tuned at the driver free air resonance (Fs). The box volume is determined by the remaining T/S parameters. I use a spread sheet, not WinISD to find these.
Using the numbers above with WinISD, overlay two graphs where you can see the difference.
2510-II is not a particularly good example to see this, as both are very close.
The vent area controls the tendency to chuff, but also controls the amount of noise made by the vent. More vent area makes more noise than a smaller area tuned the same. You can calculate the dB difference from D'Appolito's work, or simply take a nearfield RTA measurement with the mic in the vent.
Link Removed
I have just changed subnets, so DNS propagation may be 24 hours behind for the IP change.
The above RTA shows the response of a slot port vent using a swept sine wave. Note the asymmetric taper as the frequency goes up. This helps reinforce the drooping driver response, which is the whole point of vented boxes. Note the big gain from the vent when sitting on the ground compared to raised 31".
The trick to a vented box is getting the right vent/box combination for a given driver to maximize what you desire. Some desire low frequency extension (bigger box), others want a stronger response higher up, with a steeper cutoff (bi-amped top box).
WinISD will get you a starting point, but if you're dead serious about an optimal box, test gear and knowledge are required.
A bigger box (to a point) is what does this.
You can see this by calculating two different alignments for the same driver. For the Deltalite 2510-II, these are the BB4 and Flat alignments.
BB4: tuned at Fs = 53 Hz, Vob=1.48 cuft, F3 point is 56 Hz.
Flat: tuned at 51 Hz, Vob=2.12 cuft, F3 point is 50 Hz.
By definition, the BB4 or SBB4 is tuned at the driver free air resonance (Fs). The box volume is determined by the remaining T/S parameters. I use a spread sheet, not WinISD to find these.
Using the numbers above with WinISD, overlay two graphs where you can see the difference.
2510-II is not a particularly good example to see this, as both are very close.
The vent area controls the tendency to chuff, but also controls the amount of noise made by the vent. More vent area makes more noise than a smaller area tuned the same. You can calculate the dB difference from D'Appolito's work, or simply take a nearfield RTA measurement with the mic in the vent.
Link Removed
I have just changed subnets, so DNS propagation may be 24 hours behind for the IP change.
The above RTA shows the response of a slot port vent using a swept sine wave. Note the asymmetric taper as the frequency goes up. This helps reinforce the drooping driver response, which is the whole point of vented boxes. Note the big gain from the vent when sitting on the ground compared to raised 31".
The trick to a vented box is getting the right vent/box combination for a given driver to maximize what you desire. Some desire low frequency extension (bigger box), others want a stronger response higher up, with a steeper cutoff (bi-amped top box).
WinISD will get you a starting point, but if you're dead serious about an optimal box, test gear and knowledge are required.
well, it's for an electric bass guitar...a 4 string e a d g. i'll go with the eminence numbers (58hz) to start. less db drop in the range that counts, and i'll engage my qsc's filter to check unlikely destructive low end excursions when cab is tuned to this spec.
My suggestion is using a round port you can easily trim to length. This is a whole lot less grief than trying to size a slot port.
For a single port, you require 3.45" ID minimum to avoid chuffing.
Since you are placing the tuning frequency in the operating range of the cab, chuffing is a primary consideration.
A pair of 3" schedule 40 PVC is a lot easier to work, and purchase locally. Schedule 40 white is 3.042" ID and 7.268 square inches. A pair is larger area than a single 3.45" vent, so chuffing is a non-problem.
Without an RTA, you can lay the cab on its back and drop a few rice kernels onto the upfacing cone. The tuning frequency is where the rice jumps the least. You will hear the drone from the ports. There are a number of freeware signal generators you can get to make noise. The TrueRTA (www.trueaudio.com) Level 1 is freeware, and I think it might generate noise. I own the Level 4 version.
An incorrectly tuned port will severely cripple your design.
For a single port, you require 3.45" ID minimum to avoid chuffing.
Since you are placing the tuning frequency in the operating range of the cab, chuffing is a primary consideration.
A pair of 3" schedule 40 PVC is a lot easier to work, and purchase locally. Schedule 40 white is 3.042" ID and 7.268 square inches. A pair is larger area than a single 3.45" vent, so chuffing is a non-problem.
Without an RTA, you can lay the cab on its back and drop a few rice kernels onto the upfacing cone. The tuning frequency is where the rice jumps the least. You will hear the drone from the ports. There are a number of freeware signal generators you can get to make noise. The TrueRTA (www.trueaudio.com) Level 1 is freeware, and I think it might generate noise. I own the Level 4 version.
An incorrectly tuned port will severely cripple your design.
Have you taken the time to calculate the length?
A 6" ID is going to be a REALLY LONG duct.
This will subtract substantially from your internal volume.
You will have to recalculate the length again for the smaller volume, which changes your volume again. It is an iterative process.
The displacement of the duct assembly must be subtracted from the net internal cabinet volume. The displacement changes as you change the length.
Don't forget your duct correction factor for a round duct.
It varies, depending on where the duct is located on the baffle, in relation to a boundary wall.
If you keep the duct > 1 diameter from any boundary, you can use the mid-baffle correction factor.
A 6" ID is going to be a REALLY LONG duct.
This will subtract substantially from your internal volume.
You will have to recalculate the length again for the smaller volume, which changes your volume again. It is an iterative process.
The displacement of the duct assembly must be subtracted from the net internal cabinet volume. The displacement changes as you change the length.
Don't forget your duct correction factor for a round duct.
It varies, depending on where the duct is located on the baffle, in relation to a boundary wall.
If you keep the duct > 1 diameter from any boundary, you can use the mid-baffle correction factor.
sorry Bgavin, but i thought the displacement(airmass) of the duct should be add to the net internal cab volume, or am i wrong.
Subtracted, so the net volume is less when the duct is in place.
Adding the displacement value makes the internal net volume substantially larger than it actually is.
You can see this by running a continuous pink trace while pushing a long round duct into the cabinet. The deeper it goes in, the more the response changes.
For a duct a few inches long, this is mostly inconsequential, unless using a tiny box. The example I used above is 0.60 cubic feet with a substantial slot port. Changing the slot length changes the internal volume some 15~20%.
[ edit ]
I see my ~!@#$%^!!! DNS settings are still screwed, as the link to my website are down.
More bug shooting is on order when I get home tonight. The ISP says I'm having no end of line errors. Gotta love the phone company...
Adding the displacement value makes the internal net volume substantially larger than it actually is.
You can see this by running a continuous pink trace while pushing a long round duct into the cabinet. The deeper it goes in, the more the response changes.
For a duct a few inches long, this is mostly inconsequential, unless using a tiny box. The example I used above is 0.60 cubic feet with a substantial slot port. Changing the slot length changes the internal volume some 15~20%.
[ edit ]
I see my ~!@#$%^!!! DNS settings are still screwed, as the link to my website are down.
More bug shooting is on order when I get home tonight. The ISP says I'm having no end of line errors. Gotta love the phone company...
O.k, I thought to see this part of the duct as a massive, but it's not as you mentioned, while the driver and eventualy bracing need to be add to the netto volume.
Am I right?
Am I right?
Everything inside the box reduces the net internal volume.
Driver, ducts, bracing.
I did some RTA testing on a small box (0.6 cubic feet) to see if mattress topper did or did not affect the tuning.
It does not.
The mattress topper is pretty thick and dense.
There was some thought about it reducing the net volume, but the RTA said it does not. It would have been significant, considering the very small box volume.
Driver, ducts, bracing.
I did some RTA testing on a small box (0.6 cubic feet) to see if mattress topper did or did not affect the tuning.
It does not.
The mattress topper is pretty thick and dense.
There was some thought about it reducing the net volume, but the RTA said it does not. It would have been significant, considering the very small box volume.
to the OP, purely FYI
I put a deltalight 2510 in a 1.75 ft3 box tuned between 50 and 55hz. This had a nice compromise between size/low end extension/max spl due to excursion limits of the driver. Added two piezo's. Still one of the best 1x10's I have heard, though the mids required some boosting once my ear got used to mid-forward cabs.
I kind of wish I still had it now...
I put a deltalight 2510 in a 1.75 ft3 box tuned between 50 and 55hz. This had a nice compromise between size/low end extension/max spl due to excursion limits of the driver. Added two piezo's. Still one of the best 1x10's I have heard, though the mids required some boosting once my ear got used to mid-forward cabs.
I kind of wish I still had it now...
Everything in the box, no matter what it is (except for the foam lining) takes away from the space the driver has to work with, that's why I say using a big pipe in a small box like that can hit a point of no return. Whatever length you add takes away from internal volume....you can't make the pipe long enough without taking away too much space. The pair of 3 inchers mentioned should work fine and offer any tuning that would be needed.
Edit:
If they still chuff, add a 3rd one or go to 4" pipe and re-calculate the lengths needed . PVC or drainpipe is pretty cheap and easy to work with.
thank you for that
i've got it at 50hz (according to the winisd app). the two are in what is now a 3.0' (after driver and duct deduction). still have to finish up the baffle/paint it before i can test it.
