Paul, I can see the slight contradiction
but you said it best
Both noise free ports, and a smaller box are goals here
John, The group delay will be the same for the PR as a ported box would be. The peak is 46ms @16hz which quickly drops below 10ms at 17hz and doesnt do anything but get lower as frequency rises
but you said it best
Both noise free ports, and a smaller box are goals here
John, The group delay will be the same for the PR as a ported box would be. The peak is 46ms @16hz which quickly drops below 10ms at 17hz and doesnt do anything but get lower as frequency rises
I did some quick sims ...
Sealed 60L box with Linkwitz transform and 350w >>> 108db @ 20 Hz and with a rumble filter, xmax will never ever be exceeded
250L PR box with 2 x 18" PRs tuned very and 400w >>> 107db @ 20 Hz max SPL where <10 Hz will exceed xmax if you don't have a rumble filter
or 112 db @ 20 Hz if there is no signal below 11 Hz
if it were me, I'd be doing something more like this:
250L with 2 x 150mm ports with large flares tuned to 15 Hz >>> 115 db @ 15 Hz with a rumble filter where excursion below tuning will never exceed that above (fc @ 13 Hz, 6th order)
My main point is this: for movies, you only exceed the output of a sealed box by risking overexcursion, hence without a rumble filter you can't safely beat a sealed box. Why not go with a bullet proof design?
Hmmmmmm, I think you have a case of the "I just want to try it" factor with these PRs, am I right? Going with that theme, what would look really good is a reflective laminate on the PR. I've seen one which had a black high gloss veneer on a PR, it looked very good.
Sealed 60L box with Linkwitz transform and 350w >>> 108db @ 20 Hz and with a rumble filter, xmax will never ever be exceeded
250L PR box with 2 x 18" PRs tuned very and 400w >>> 107db @ 20 Hz max SPL where <10 Hz will exceed xmax if you don't have a rumble filter
or 112 db @ 20 Hz if there is no signal below 11 Hz
if it were me, I'd be doing something more like this:
250L with 2 x 150mm ports with large flares tuned to 15 Hz >>> 115 db @ 15 Hz with a rumble filter where excursion below tuning will never exceed that above (fc @ 13 Hz, 6th order)
My main point is this: for movies, you only exceed the output of a sealed box by risking overexcursion, hence without a rumble filter you can't safely beat a sealed box. Why not go with a bullet proof design?
Hmmmmmm, I think you have a case of the "I just want to try it" factor with these PRs, am I right? Going with that theme, what would look really good is a reflective laminate on the PR. I've seen one which had a black high gloss veneer on a PR, it looked very good.
Since it's not up and running yet, I think you need very many air holes in your bracing. For someone worried about port noise, you sure didn't concern yourself with air moving freely in your cab. At least you have the big openings for the PR's giving you enough space to get in there and do something about it, but getting all the shavings and dust out is going to be a pain.
I think a change in signature is in order.
I think a change in signature is in order.
I tend to agree with John, that the holes could be improved. To me they look too small. By forcing sound waves to move through such small holes, you may in fact alter the effective volume, or get some other unwanted effect. What you are trying to achieve is a compromise between structural rigidity of the bracing, and minimising the impact of the bracing, so that the braces occupy minimal volume, and the driver sees the entire volume as a single air volume. Add too much resistance through holes that are too small, and your box might start to behave like a smaller box. At which point this becomes a problem, I don't know.
I agree completely that those holes are small.... BUT my 3" hole cutter is more like a hole burner so I decided to just use the 1.25" bit. I'm fairly certian that there is a good deal of room in the box for air to move freely. If you look at the first and third pic I posted you can see that the middle section is very open and really needs no holes for air flow. Where the bracing is cut out for the driver there is also about 5"x 11" of open space on each side on the bracing panels you cannot see from the pics. Plus about 30 holes which should give a free space cross section of around 100in^2
It should be adequate.
The signiture stays.... It is indeed the golden rule(I didnt say I always obey it).
But, if you witnessed this enclosure in person, pushed it around a little bit, knocked on it, stuck your head inside and hummed a bit... I think you'd be satisfied... 1.5" thick MDF and all that bracing makes for one hell of a stiff, and heavy, box.
The amp is working. I feel like a moron really because the problem was a blown fuse. Who sends out brand new amps with blown fuses though anyway? Adire apparenly.
It should be adequate.
The signiture stays.... It is indeed the golden rule(I didnt say I always obey it).
But, if you witnessed this enclosure in person, pushed it around a little bit, knocked on it, stuck your head inside and hummed a bit... I think you'd be satisfied... 1.5" thick MDF and all that bracing makes for one hell of a stiff, and heavy, box.
The amp is working. I feel like a moron really because the problem was a blown fuse. Who sends out brand new amps with blown fuses though anyway? Adire apparenly.
Adequate on the sides, yes. From either side to the center, no. In fact your opening right at the driver has a greater crossectional area than the other hole in those 2 large braces combined. With that driver you're going to be moving alot of air, as will your PR's. I really do think you'll have enough resistance through the holes to cause the majority of the air to flow up through the opening at the driver, so we're talking about air flow from separate chambers, not acting as a single volume of air pressurizing and depressurizing.
Whether or not it's enough to make an audible difference is debatable, however, it's probably enough to affect your PR tuning. Who knows, maybe it will be beneficial and you invented a new kind of cab.
I understand the hole cutter was a "no way Jose". Much bigger and relatively few holes, freehand cut with a router or jigsaw is the way to go. Pretty internal braces isn't needed, a sledghammer won't break that cab apart. 1.5" mdf.....I hope you aren't planning a move any time soon and that it's supported by a concrete floor, the brace alone looks heavy.
Whether or not it's enough to make an audible difference is debatable, however, it's probably enough to affect your PR tuning. Who knows, maybe it will be beneficial and you invented a new kind of cab.
I understand the hole cutter was a "no way Jose". Much bigger and relatively few holes, freehand cut with a router or jigsaw is the way to go. Pretty internal braces isn't needed, a sledghammer won't break that cab apart. 1.5" mdf.....I hope you aren't planning a move any time soon and that it's supported by a concrete floor, the brace alone looks heavy.
My initial impression is to agree with John on this one, however, looking closer I can see there appears to be a clear air path to all air volumes without having to go only through those holes. If I've missed something, and the only path to a pocket of air is through those holes, then you definately need to get the jigsaw and make them bigger.
If you want louder, then perhaps you could consider something like what I'm planning - vented subs for LFE with midbass horns down to 40 Hz or so. This gives you more grunt for music, although to get down low you will of course still be displacement limited.
I still wonder if it's really worth tuning so low for your sub - you could get more output if you set a lower limit of 15 - 20 Hz
I still wonder if it's really worth tuning so low for your sub - you could get more output if you set a lower limit of 15 - 20 Hz
Bass,
I'll give it one more shot for you. Your 2 biggest braces would create 3 lengthwise chambers in your cab if they had no holes and the 2 smaller side chambers will contain your PR's. You have a driver cutout and the holes which are the only way for air to pass between these chambers.
The big brace on the left has only what appear to be 1" holes, about 30-35 of them. Let's say it's 40, which is about 30sqin of area. Do you think that just three 4" dia holes would be sufficient? No way, yet three 4" holes is about 20% more area, not to mention that more holes equalling the same area have significantly more resistance than one larger hole. Since it's more like low 30s # of holes, you've effectively got TWO 4" holes, plus the space near the driver AND THAT SIMPLY IS NOT ENOUGH, PERIOD. You are forcing much more air to flow up around the driver basket.
I'm not being critical, just trying to help. Get the jigsaw out because I don't think you want to drill a couple of hundred more of the small holes.
I'll give it one more shot for you. Your 2 biggest braces would create 3 lengthwise chambers in your cab if they had no holes and the 2 smaller side chambers will contain your PR's. You have a driver cutout and the holes which are the only way for air to pass between these chambers.
The big brace on the left has only what appear to be 1" holes, about 30-35 of them. Let's say it's 40, which is about 30sqin of area. Do you think that just three 4" dia holes would be sufficient? No way, yet three 4" holes is about 20% more area, not to mention that more holes equalling the same area have significantly more resistance than one larger hole. Since it's more like low 30s # of holes, you've effectively got TWO 4" holes, plus the space near the driver AND THAT SIMPLY IS NOT ENOUGH, PERIOD. You are forcing much more air to flow up around the driver basket.
I'm not being critical, just trying to help. Get the jigsaw out because I don't think you want to drill a couple of hundred more of the small holes.
John,
I appreciate your suggestion.
When I get the PRs I'm going to take the whole box apart to apply liquid nails and glue the 2nd wall of MDF to the first. I'll do something about those holes then.
My jigsaw is broken right now... which is why I didnt use it in the first place. I have two, but the one that works someone else has right now... I'll have to get it back soon.
Paul,
The "midbass" horns are a good idea. Its kinda PA style if you do it that way, but having a real sub below the horns makes it better.
I appreciate your suggestion.
When I get the PRs I'm going to take the whole box apart to apply liquid nails and glue the 2nd wall of MDF to the first. I'll do something about those holes then.
My jigsaw is broken right now... which is why I didnt use it in the first place. I have two, but the one that works someone else has right now... I'll have to get it back soon.
Paul,
The "midbass" horns are a good idea. Its kinda PA style if you do it that way, but having a real sub below the horns makes it better.
Don't take the box apart. Just 2 large rectangular holes in the center section of those 2 big braces is all you need. You've got the PR holes to work through, so don't let all that good work go to waste. Then it's a 20 minute adjustment plus some time to clean out the box instead of a rebuild.
I myself would measure results as-is, then cut the bigger holes. The effect may be able to be put to good use and learn something new, a box with aperiodic tuning of PRs.
I myself would measure results as-is, then cut the bigger holes. The effect may be able to be put to good use and learn something new, a box with aperiodic tuning of PRs.
It does sound like a usefull experiment to measure it both ways... Time to put that ECM to some use. I have done some recordings with it... and it is very dry
I need to take the box apart anyway to add liquid nails(to seal the cracks, although I havent heard a single airleak) and to also apply glue between the two outside panels. I'm doing double 3/4" mdf to make 1.5" walls and I only have screws holding the two boards together as of now, albiet very many screws. However I believe glueing the panels together will result in even more stiffness
I need to take the box apart anyway to add liquid nails(to seal the cracks, although I havent heard a single airleak) and to also apply glue between the two outside panels. I'm doing double 3/4" mdf to make 1.5" walls and I only have screws holding the two boards together as of now, albiet very many screws. However I believe glueing the panels together will result in even more stiffness
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