Just curious as to how much the volume of a box can be reduced over a ported arrangement apart from the port itself, if there is a rough percentage figure I'd be grateful to know.
Many thanks.
Many thanks.
I don't believe they reduce box volume. You get a smaller box purely because you don't need a port.
The downside is a steeper roll off below tuning and a slight loss in efficiency
The downside is a steeper roll off below tuning and a slight loss in efficiency
Yes I reckon so. One article is suggesting a further 10% reduction will achieve the same response but I'm not sure.. but yes, steeper roll off.
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A PR system does have a couple of advantages over vented systems, one of which I believe your question is hinting at. A PR system can be smaller in total volume for the same tuning and still "sound good". This is because, for a fixed tuning frequency, as you reduce the cabinet volume, the length of the port must increase (and quickly!). So, you can dial up a small vented subwoofer box in your favorite modeling program and set the tuning to 20Hz. Everything looks good until you notice that the port length is on the order of 30-40 inches or more! This is especially true if you use a large enough port to avoid port power compression (which happens when the port air speed exceeds about 25m/s). Apart from being difficult to physically fit into the box, such a long port will have a self-resonance frequency that might be as low as 120 Hz and you will definitely hear it. Many people use too small a port in their vented designs because of the length problem. There is a tradeoff between port length, air speed, and port diameter and some choices have to be made.
Now, getting back to your question about PRs - a PR does not have any of these problems. The PR functions the same no matter the tuning frequency (within the mass limits of the PR). There is no "self resonance" with a PR, so you can make the box smaller without this kind of penalty. As the box volume is reduced, the PR mass must increase and eventually you run into the limit of the PR's suspension to support the weight. As a result, in practice you can indeed create a low-tuned PR system that is smaller than the analogous vented box. I would guesstimate that it could be as much as 30% smaller, but this really depends on the driver parameters as well, since the smaller box will influence it's contribution to the response. However, as an example, Peerless makes some 12" XXL series drivers that have very low Q and which make excellent PR subs when used with Peerless PRs in about 1 cubic foot tuned to just over 20 Hz. Try that with a vented box!
A PR has one other small advantage compared to a vented box - it does not completely unload the woofer below tuning. This is because the total excursion of the PR is limited by the PR suspension. This affords the PR a small amount of "protection" against driver over excursion below tuning that is superior to a similar vented box. I wouldn't count this as a major advantage, but it's one just the same.
Hopefully this has provided some insight for you on PR systems. PRs have some drawbacks compared to vented boxes, like the additional cost of the PR (almost as much as another driver in many cases) worse transient response (not really an issue in a subwoofer application) and cabinet vibration when a single PR with high mass loading is used. If you can use a large box, go with a vented system. When small size is important, a PR is the way to go.
-Charlie
Now, getting back to your question about PRs - a PR does not have any of these problems. The PR functions the same no matter the tuning frequency (within the mass limits of the PR). There is no "self resonance" with a PR, so you can make the box smaller without this kind of penalty. As the box volume is reduced, the PR mass must increase and eventually you run into the limit of the PR's suspension to support the weight. As a result, in practice you can indeed create a low-tuned PR system that is smaller than the analogous vented box. I would guesstimate that it could be as much as 30% smaller, but this really depends on the driver parameters as well, since the smaller box will influence it's contribution to the response. However, as an example, Peerless makes some 12" XXL series drivers that have very low Q and which make excellent PR subs when used with Peerless PRs in about 1 cubic foot tuned to just over 20 Hz. Try that with a vented box!
A PR has one other small advantage compared to a vented box - it does not completely unload the woofer below tuning. This is because the total excursion of the PR is limited by the PR suspension. This affords the PR a small amount of "protection" against driver over excursion below tuning that is superior to a similar vented box. I wouldn't count this as a major advantage, but it's one just the same.
Hopefully this has provided some insight for you on PR systems. PRs have some drawbacks compared to vented boxes, like the additional cost of the PR (almost as much as another driver in many cases) worse transient response (not really an issue in a subwoofer application) and cabinet vibration when a single PR with high mass loading is used. If you can use a large box, go with a vented system. When small size is important, a PR is the way to go.
-Charlie
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Charlie, this is so much clearer than a dozen or so sites I looked at, and also for your knowledge and tips. Thanks for taking the time to explain.
Confession time, altho I posted this in the subwoofer section (because of the topic), the PR application I was thinking is for a 4" full range driver! http://www.ejjordan.co.uk/drivers/
The passive radiators wd be 2X peerless 3.5" in each box here Speaker Building Supplies from Madisound
The reason for thinking to go this route is the box volume (12 litres vented)plus port size. imo a quite large box for such a small driver. So I'm thinking 8 litres - I'd place the PR's on each sides of the cab. One issue is the PR's mass is fixed and might not even work with the driver, plus I'd have to work out what the ideal box volume is and hope its around what i want, because I can't tune the PR's. The other more serious issue is transient response below below tuning frequency as its much higher than normal.
Confession time, altho I posted this in the subwoofer section (because of the topic), the PR application I was thinking is for a 4" full range driver! http://www.ejjordan.co.uk/drivers/
The passive radiators wd be 2X peerless 3.5" in each box here Speaker Building Supplies from Madisound
The reason for thinking to go this route is the box volume (12 litres vented)plus port size. imo a quite large box for such a small driver. So I'm thinking 8 litres - I'd place the PR's on each sides of the cab. One issue is the PR's mass is fixed and might not even work with the driver, plus I'd have to work out what the ideal box volume is and hope its around what i want, because I can't tune the PR's. The other more serious issue is transient response below below tuning frequency as its much higher than normal.
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