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post #31 of 46 Old 08-14-2019, 12:15 PM
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Quote:
Originally Posted by iH8usrnames View Post
No. Cabinet size, port diameter and length are fairly straight forward.
I'm calling it a manifold but I suppose that would be incorrect as it is a single driver feeding it.


I made this CAD drawing to give you a sense of what I hope to accomplish.


That'll work fine. Don't skimp on manifold size. If you want some overkill, which based on your past projects you might, you can add a "C" shape to the manifold to help with output. It's a minor difference and I haven't tested it but a few of the guys who play with slot loading swear by it. Quick & dirty will work fine too.

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post #32 of 46 Old 08-14-2019, 12:45 PM
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Looks like you have some room to work under the addition, how about a horn sub.

Edit: You could even build a THT LP and just keep it outside. Use it as a table. Just have to put some sort of a cover over the mouth when not in use.

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post #33 of 46 Old 08-14-2019, 12:48 PM - Thread Starter
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Originally Posted by noah katz View Post
If everything is permanently mounted, why the reference to hauling out PA speakers?

Maybe there are some reasonably sized PA speakers with 10" or 8" woofers.
When you mentioned PA speakers in envisioned powered 12" drivers cabinets - much larger than I would ever want.



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Barring that, I think the concepts shown in both your manifold and ported sketches are feasible.

My guess is that you could go down in manifold cross sectional area to about half the cone area w/o issue.

Maybe that could be determined more precisely by modeling as a bandpass with a tiny front volume.

I don't think IB rattling the house would be an issue, especially if you HP filter it to your 30 - 40 Hz requirement.
I am about to illustrate some serious ignorance.
If I make sure the manifold mouth is equal to, or greater than, the cu^in displacement of the cone (Area of cone * xmax) then the effect of the manifold should be negligible.
Napkin math:
18" diameter = 254" sq
8mm xMax = .33" (if a speaker has an 8mm xmax I assume its 8mm each direction?)
254*.33=83.82 cu inches of displacement

The minimum required opening would be 4.75" x 18". I could hedge my bet and go 6"x18" and have 108" opening.

Am I completely wrong?
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post #34 of 46 Old 08-14-2019, 12:51 PM - Thread Starter
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Originally Posted by BllDo View Post
Looks like you have some room to work under the addition, how about a horn sub.
Correct me if I am wrong, a horn sub would require a very large opening through the crawlspace wall.
Via the drawing above I believe the could limit the penetrations to 8" diameter hole for port and a 6"x18" hold for the manifold mouth.
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post #35 of 46 Old 08-14-2019, 12:53 PM - Thread Starter
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Originally Posted by Chris Popovich View Post
That'll work fine. Don't skimp on manifold size. If you want some overkill, which based on your past projects you might, you can add a "C" shape to the manifold to help with output. It's a minor difference and I haven't tested it but a few of the guys who play with slot loading swear by it. Quick & dirty will work fine too.

Chris
I'm not sure I have ever had an idea that might work on this forum - usually I am woefully incorrect. YAY!

I am not sure what you mean by C shape but will do some forum searches to find out.

If this all works out, this will be the cheapest damned sub I could ever imagine building.
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post #36 of 46 Old 08-14-2019, 01:07 PM
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Quote:
Originally Posted by iH8usrnames View Post
Correct me if I am wrong, a horn sub would require a very large opening through the crawlspace wall.
Via the drawing above I believe the could limit the penetrations to 8" diameter hole for port and a 6"x18" hold for the manifold mouth.
The Tuba 18 (link) has a pretty small foot print and relatively small opening. It's not going to go super low, but a pair should play plenty loud for a patio theater.

Alternatively, you could build a horn and just leave it out doors. This is a THT LP sub, you could put it right up next to the house and use it as a table. Just need to cover the opening when not in use.


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post #37 of 46 Old 08-14-2019, 02:15 PM - Thread Starter
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Quote:
Originally Posted by BllDo View Post
Alternatively, you could build a horn and just leave it out doors. This is a THT LP sub, you could put it right up next to the house and use it as a table. Just need to cover the opening when not in use.
I specifically do not want to leave something on the patio nor am I looking for concert level volume. I appreciate the value of horns but not the right fit for this application.
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post #38 of 46 Old 08-14-2019, 02:19 PM
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Quote:
Originally Posted by iH8usrnames View Post
If I make sure the manifold mouth is equal to, or greater than, the cu^in displacement of the cone...

You can't make an area equal to a volume

The way I'd think about it is in terms of air velocity, and I wouldn't think it's an issue until you approach what would cause compression in a port.

Using this calculator http://www.modalshop.com/vibration-calculator?ID=1036 , I get 2 m/s for 8mm (.315 in.) at 80 Hz, which is the xtreme case (which I doubt you have the power for).

The active area for an 18" driver is about 190 sq in, so a 108 sq in opening would increase the velocity to 3.5 m/s, which I'd have zero concern about.

That's not the whole story, as port compression is related to air flow, whereas there's also the mechanical load on the driver caused by restricting the driver, but I expect that's negligible compared to the driver mass and spring stiffness of the driver suspension and box until you get to much smaller openings.
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post #39 of 46 Old 08-14-2019, 04:32 PM - Thread Starter
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Quote:
Originally Posted by noah katz View Post
You can't make an area equal to a volume

The way I'd think about it is in terms of air velocity, and I wouldn't think it's an issue until you approach what would cause compression in a port.
I was meant:
if the cone can displace 100 cubic inches of air peak to peak.
and the manifold opening is 100 sq inches or greater,
then I would imagine the effect of the manifold to be completely negligible.
If the manifold opening was smaller, that would be an issue.

Clearly, I am wrong.
I can totally accept being wrong, it means I am learning something.

Quote:
Using this calculator http://www.modalshop.com/vibration-calculator?ID=1036 , I get 2 m/s for 8mm (.315 in.) at 80 Hz, which is the xtreme case (which I doubt you have the power for).

The active area for an 18" driver is about 190 sq in, so a 108 sq in opening would increase the velocity to 3.5 m/s, which I'd have zero concern about.

That's not the whole story, as port compression is related to air flow, whereas there's also the mechanical load on the driver caused by restricting the driver, but I expect that's negligible compared to the driver mass and spring stiffness of the driver suspension and box until you get to much smaller openings.
Now we are talking fluid dynamics - wish I had paid attention in school.

The basic takeaway is, its a viable idea so long as the mouth of the manifold is notso small it creates significant resistance to the driver?
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post #40 of 46 Old 08-14-2019, 05:42 PM
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Quote:
Originally Posted by iH8usrnames View Post
if the cone can displace 100 cubic inches of air peak to peak.
and the manifold opening is 100 sq inches or greater,

I don't understand why you're comparing a volume to an area.



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Originally Posted by iH8usrnames View Post
The basic takeaway is, its a viable idea so long as the mouth of the manifold is notso small it creates significant resistance to the driver?

Right.

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post #41 of 46 Old 08-14-2019, 05:53 PM
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You're roughly correct, and for the purposes of what you're doing it falls into the negligible category. When in doubt make 'er a little big and you'll be fine. Have fun!

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post #42 of 46 Old 08-14-2019, 05:58 PM
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In order to have all of the sound flying out of 1 hole you'd have to make a 6th-order BP (or a horn.)
https://www.the12volt.com/caraudio/s...ofer-boxes.asp


Calculating air-velocity is a 5th dimensional problem set (with additional variables beyond that...)

It isn't just simply area or volume.
There is a time component and a Hz component (+ driver/box mechanics).

cone area x xmax x2 = displacement per full stroke.
That displacement x Hz = total-volume displaced per second.
If you divide that by the port area, that will give the air-velocity through that area normalized to per-second (with no consideration to the driver/box mechanics.)

Note: that would be for both directions (full stroke). Divide by 2 to calculate the velocity in a single direction (half stroke).
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post #43 of 46 Old 08-14-2019, 07:01 PM - Thread Starter
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Quote:
Originally Posted by noah katz View Post
I don't understand why you're comparing a volume to an area.
Here is the logic, as flawed as it may be.

If I have a solid form 12" on all sides and I need to fit it through an opening, the opening would have to be a minimum of 12" x 12".

If the cone would displace 100 cu in, then a 100 sq in opening would allow that volume to flow through the opening unimpeded.

I did not say it was correct, just my thought process.
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post #44 of 46 Old 08-14-2019, 08:25 PM - Thread Starter
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This is what I have come up with, however, I do believe there is something wrong with the WinISD file representing the PA460 driver.

Using the PA460-8 driver I created the following:
7.75cuft cabinet tunes to 34hz and high pass filter set to 25hz.

A 6th order bandpass cabinet:
Rear chamber is 7.75cuft and tuned to 34hz
Front chamber is 1.5cuft and tuned to 150hz and a port opening of 18"x6"this results in a port length of 0
High pass filter set to 25hz.

I figured a 6th order cabinet where the front chamber is the anticipated volume of the manifold, and the port size is equal to the anticipated opening of the manifold would be a useful simulation. <-- no doubt I am wrong.

If anybody would like to look at the WinISD files they can be downloaded here:
May have to right click and "save link as" to download as a file, otherwise just opens the text in a new page.
http://dale-murray.com/web/PA460-8_M...Simulation.wpr
http://dale-murray.com/web/PA460-8_Vented.wpr

I am trying to learn and appreciate all the feedback.
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post #45 of 46 Old 08-14-2019, 09:44 PM
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Quote:
Originally Posted by BassThatHz View Post
That displacement x Hz = total-volume displaced per second.If you divide that by the port area, that will give the air-velocity through that area normalized to per-second (with no consideration to the driver/box mechanics.)

It's the velocity for a cycle that matters, not the total volume per second.



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Originally Posted by BassThatHz View Post
Note: that would be for both directions (full stroke). Divide by 2 to calculate the velocity in a single direction (half stroke).

Same comment as above


Quote:
Originally Posted by iH8usrnames View Post
I figured a 6th order cabinet where the front chamber is the anticipated volume of the manifold, and the port size is equal to the anticipated opening of the manifold would be a useful simulation.


That sounds right to me, but there are others more qualified to comment than I.

Maybe do a search on bandpass and ping the experts from those threads.

Noah

Last edited by noah katz; 08-14-2019 at 09:50 PM.
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post #46 of 46 Old 08-14-2019, 09:48 PM
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Quote:
Originally Posted by iH8usrnames View Post
I figured a 6th order cabinet where the front chamber is the anticipated volume of the manifold, and the port size is equal to the anticipated opening of the manifold would be a useful simulation.
Correct (with a port length of zero or the of depth of the plywood.)

That's the only way to use both sides of the driver and put all the frequencies out of a single hole (without going horned.)

Quote:
Originally Posted by iH8usrnames View Post
Using the PA460-8 driver I created the following:
7.75cuft cabinet tunes to 34hz and high pass filter set to 25hz.

A 6th order bandpass cabinet:
Rear chamber is 7.75cuft and tuned to 34hz
Front chamber is 1.5cuft and tuned to 150hz and a port opening of 18"x6"this results in a port length of 0
High pass filter set to 25hz.
I haven't crunched the numbers, but if that is what it models then it should be correct.

Just be careful setting the HPF that far below tuning, but as long as it still has xmech clearance or not vastly exceeding xmax then it should be "ok".
A EQ cut can be applied if there is a narrow band where excursion gets out of hand if it is above the HPF, that way max-power can still be applied to the upper frequencies (mid-bass).

The only way to get louder bass without increasing the size of the holes is to increase the air velocity. This of course also requires using larger cones or more cones (or both), and either more power or more efficient drivers (or both).
There is no cheating the laws of physics. There are limits and prices to pay.
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