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post #121 of 234 Old 08-02-2014, 05:32 PM
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Originally Posted by Mfusick View Post
I might have missed this, but did you ever have a thread with riser experiments and results?
I took readings for the riser but didn't see a huge difference, but I had the subs in the 1/4 in and up spots in the room. The response was fairly flat anyway. I will be getting carpet soon and all of the extra crap out of the room I'll take readings again and publish in my build thread. I'll do various ones with the risers vents in various states.
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post #122 of 234 Old 10-15-2014, 08:33 AM
 
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Quote:
Originally Posted by Mfusick View Post
Quote:Originally Posted by mtbdudex 

In a PM I asked Stew if he still had Home Theater Builder magazine July 2003 and September 2003 article's, unfortunately no.
https://www.avsforum.com/t/453984/hom...s#post_4452058










For some reason some of these pictures disappeared. I have no idea why.

I will repost.
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post #123 of 234 Old 10-15-2014, 08:35 AM
 
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post #124 of 234 Old 10-15-2014, 09:10 AM
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Just a another data point. Dennis has changed his approach to Risers as bass trap since 2003. He now designs the area as one large connected air volume with insulation. He uses fewer but much larger vents.
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post #125 of 234 Old 10-15-2014, 09:22 AM
 
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Originally Posted by BIGmouthinDC View Post
Just a another data point. Dennis has changed his approach to Risers as bass trap since 2003. He now designs the area as one large connected air volume with insulation. He uses fewer but much larger vents.
Thank's for the wisdom. Makes sense. By connected volume you mean the joists are not on the floor right? So it's basically one big volume. Or you mean you drill holes in the joists or something?
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post #126 of 234 Old 10-15-2014, 05:56 PM
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There is also this thread, which is a more readable transcription of the article, plus some collected addendums of changes since 2003: https://www.avsforum.com/forum/19-ded...ne-2003-a.html
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post #127 of 234 Old 10-15-2014, 06:12 PM
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Quote:
Originally Posted by Jeff in Canada View Post
I took readings for the riser but didn't see a huge difference, but I had the subs in the 1/4 in and up spots in the room. The response was fairly flat anyway. I will be getting carpet soon and all of the extra crap out of the room I'll take readings again and publish in my build thread. I'll do various ones with the risers vents in various states.
How about now?

I'm very curious about this, since most of the conventional wisdom of using vents is that it will make things notably better or, if done wrong, will make things notably worse. Getting a result that shows no real difference is not at all what I was expecting!
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post #128 of 234 Old 10-15-2014, 10:01 PM
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Quote:
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I can't verify it with math, but to me that much sand is insufficient. I'd skip it.
Okay, I'm very curious about the math as it pertains to the sub isolation. IF we assume that the box containing the sand is completely isolated from the riser and IF we further assume that the subwoofer is sitting only on the isolated sand square, then what would be the minimum amount of sand necessary to make a meaningful impact?
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post #129 of 234 Old 10-16-2014, 05:52 AM
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Originally Posted by Mfusick View Post
Thank's for the wisdom. Makes sense. By connected volume you mean the joists are not on the floor right? So it's basically one big volume. Or you mean you drill holes in the joists or something?
An abundance of Holes would work but it is easier to just use smaller joists.
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post #130 of 234 Old 10-16-2014, 05:57 AM
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Originally Posted by granroth View Post
Okay, I'm very curious about the math as it pertains to the sub isolation. ... what would be the minimum amount of sand necessary to make a meaningful impact?
I'm just paraphrasing and making assumptions from what I've gotten from Dennis Erskine's posts (so there's plenty of room for error). My feeling is that the sand should be at least an order of magnitude more massive than the sub. If that's true, a 50-pound sub needs at least 500 pounds (five cubic feet) of sand.
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post #131 of 234 Old 10-16-2014, 06:12 AM
 
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Originally Posted by BIGmouthinDC View Post
An abundance of Holes would work but it is easier to just use smaller joists.
I understand thanks.
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post #132 of 234 Old 10-16-2014, 07:46 PM
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Quote:
Originally Posted by HopefulFred View Post
I'm just paraphrasing and making assumptions from what I've gotten from Dennis Erskine's posts (so there's plenty of room for error). My feeling is that the sand should be at least an order of magnitude more massive than the sub. If that's true, a 50-pound sub needs at least 500 pounds (five cubic feet) of sand.
So if the two subs I'm planning on building weigh ~150lbs each (including the cab and driver)... I need a total of 3,000 lbs of sand in my stage?!? Ouch... I'm about 1,500lbs shy then. ...and I'm not hauling more sand. You can't make me!
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post #133 of 234 Old 10-16-2014, 10:49 PM
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Originally Posted by HopefulFred View Post
I'm just paraphrasing and making assumptions from what I've gotten from Dennis Erskine's posts (so there's plenty of room for error). My feeling is that the sand should be at least an order of magnitude more massive than the sub. If that's true, a 50-pound sub needs at least 500 pounds (five cubic feet) of sand.
Okay, I can see that. I do wonder if that's referring to using the sand as 'mass' in a different context, though.

Here's my (often flawed) thought process...

Subs are very commonly put on stages, which are nearly always stuffed entirely with sand. The thinking there is that if the stage was not filled with sand, that the vibrations from the sub would vibrate the stage like a massive drum.

Subs don't need to be on stages, though, and often are not. In those cases, I have not seen a recommendation to build a massive platform for the sub with multiple cubic feet of sand in the platform. Instead, the sub is either directly placed on the floor; or is directly placed on a floating floor; or is sometimes placed on a vibration reducing pad (like a Subdude). That implies that the use of sand as vibration-reducing mass isn't a direct requirement of using a sub, but is instead a specific recommendation for the use on a stage.

Now we get to risers. We don't want to fill a riser entirely with sand since a) it would be very unfun and b) it would disallow us from using the riser as a pressure absorber ("bass trap"). So risers are filled with fiberglass instead, to make sure the empty cavity doesn't resonate anyway.

But we want to put a sub or subs on the riser... and here's where it starts to get dicey. Is there any measured proof that just placing a sub on a riser (with no sand at all) would result in the vibrations resonating through the riser? I'm not saying that there's isn't -- I'd just like to see it if there is. Let's assume for now that there would be a negative result from doing that and so we want to isolate the sub from the rest of the riser.

We build a box that is slightly bigger than the sub and embed it in the riser with a gap of 3/8" or 1/2" all the way around (likely loosely filled with fiberglass). As such, this box does not touch the riser in any way. We then fill that box with sand. It would look something like this:



The layers of OSB on top would likewise be isolated. That is, there would be a gap all the way around for the OSB on top of the box, that would be filled with backer rod and caulk.

It seems to me that since this doesn't touch the riser, that the vibrations from the riser wouldn't be an issue regardless of how much sand is in the supporting structure. That sand would just be there to prevent the supporting box from resonating itself.

Am I missing something (again, as I often am)? If so, what?
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post #134 of 234 Old 10-17-2014, 05:28 AM
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Hmmmm... this is interesting & concerning... I am about to start my riser, and a tentative plan if needed, was to add a couple of subs later on, which would be located on the riser. But given the 'if needed later'... I wasn't planning on doing anything with Sand or Riser design up front.

One 'design' I am/was considering was to build 'corner' subs (i.e., DIY triangular boxes) based on the space available in my room. I wonder if I actually made the 'riser' with clipped corners/ a cut-out where the sub would go... and then the corner sub rested on the floor, decoupled from the riser... if that would work? I guess 'short term', if I didn't want a sub right now, I could construct essentially a removable riser piece the size of where the sub would go...

This would be a bit like the above diagram from Granroth, but instead of the decoupled box being filled with sand, having the decoupled box be the Sub-woofer itself?
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post #135 of 234 Old 10-17-2014, 07:10 AM
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Originally Posted by granroth View Post
Okay, I can see that. I do wonder if that's referring to using the sand as 'mass' in a different context, though.
This is probably true. My own justification for all the sand I used was not just for acoustics, but for isolation. I have filled my stage with sand - nearly two tons of it if I remember correctly. I have also filled significant portions of my riser with sand. All in I think I have something like five tons of sand in my basement - overkill? Probably.

I've arranged for subs in all four corners. The center portion of my riser is open (insulation, no sand) but I do not expect to try to make it into a bass trap - I don't think I could make it work predictably, so I will exhaust all other options before I try it.

The main reason I have for all the sand was keeping the low frequency rumble out if the foundation of my house. Concrete foundations conduct vibrational energy with very little loss, and a stick frame structure will exhibit resonances all over the place when driven from the foundation like that, so in the interest of keeping the rest of the house quiet, I'm trying to keep the foundation unperturbed.

I believe that the cavity resonances would color the sound measurably, if not audibly, and the benefit there is good, but not my pimary motivation.

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post #136 of 234 Old 10-17-2014, 11:02 AM
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Originally Posted by granroth View Post
Okay, I'm very curious about the math as it pertains to the sub isolation. IF we assume that the box containing the sand is completely isolated from the riser and IF we further assume that the subwoofer is sitting only on the isolated sand square, then what would be the minimum amount of sand necessary to make a meaningful impact?

I was going to put a sub on my riser and do something like this. Good thing I have some time to see where this goes
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post #137 of 234 Old 10-17-2014, 11:04 AM
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post #138 of 234 Old 10-17-2014, 05:49 PM
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The next level question related to having a subwoofer on a riser is this: is the vibration from the subwoofer cabinet the only real movement we need to decouple or dampen OR do we need to also be concerned about the sound coming from the subwoofer itself?

That is, say we do establish that putting a subwoofer on a logically isolated platform "in" a riser (sort of) would prevent the vibrations from the cabinet getting to the riser and thus not causing any resonation. But what about the fact that the sub is pumping out quite a bit of energy in the form of low frequency sound waves? Is that going to be just as bad as the direct vibrations since the sub is literally less than an inch from the riser?

The reason this question is important to me is that IF it's true that the vibrations from the sound waves alone is enough to cause problems with placing the riser on the sub, then going the extra distance to try to isolate it seems somewhat pointless.
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post #139 of 234 Old 10-17-2014, 08:08 PM
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I thought we might come to this question. It's a fair one, and I don't have a good answer for it. Over the last couple years I've noticed a fair number of threads or conversations pop up relating to dual-opposed subwoofer cabinets, which ostensibly exist to make the cabinet itself more inert or still. There are of course lots of other conversations about subdudes and similar, as you mentioned. There are those who believe that this is important, there are those who think it's irrelevant.

I suppose I've decided that it's important and that the cabinet vibrations are a problem - but if you asked me that academically, I'd have said no. http://billfitzmaurice.info/forum/vi...p?f=18&t=16945
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post #140 of 234 Old 10-17-2014, 09:32 PM
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I must agree with Bill on this one. The resonant frequencies of any well designed sub enclosure's panels are going to be far higher than the frequency played by the subwoofer, thus not responding resonantly. Now if the sub is jumping around on the floor, that is another thing. Just add some mass to the sub itself and don't worry about the whole sandbox thing.

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post #141 of 234 Old 10-17-2014, 10:50 PM
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Okay, that settles it for me, then. I removed my sandboxes from my riser design. Worst case, if I am detecting vibrations from my (eventually DIY) sub cabinets then I'll remake them... or at least create my own isolation pads of concrete or sand or something on U-boats. Hopefully it wouldn't get to that.

But let's keep on this track for a tiny bit longer. The Harman white paper on sub placement strongly suggests putting at least one subwoofer on the back wall. That research didn't assume you'd have the sub on a riser, though. Indeed, as I do searches for sub on riser strategies, I find myself coming up oddly short. That makes me wonder if it's somewhat rare to actually put subs on the back wall on a riser.

So is it rare? Do any of y'all have subs on "typical" (non sand-filled) risers and, if so, did you do anything special to make it work?
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post #142 of 234 Old 10-18-2014, 10:10 AM
 
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It's not rare. Lots of people put subs on their risers. Elevating a sub actually can help with smoother response sometimes.

Ideally you'd want two on the back wall, one elevated. But that is splitting hairs.
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post #143 of 234 Old 10-18-2014, 12:40 PM
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Okay, so if it is typical to put subs on risers, then the second part of the question becomes important: is anything typically done differently to prepare either the subs or the risers in order to do this?
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Make the riser strong (you should do this anyways) and stuff it with something (at least pink fluffy) so it doesn't resonate like a drum.

Two or three layers of OSB or ply would be a good choice for the top, do the joists every 12". That should be pretty solid and no problems.
Then just make sure you build super duper monster subs so gravity keeps em in place
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post #145 of 234 Old 10-18-2014, 07:11 PM
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Okay, to summarize:

1. Subwoofers may be placed on top of any well constructed and solidly built riser
2. There is no need to isolate the subwoofers from the riser, either via a separate internal riser or a product like a Subdude

If any disagree with that, then please speak now or forever hold your peace!
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post #146 of 234 Old 10-19-2014, 08:38 AM
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That is your summary. YMMV. Why waste time, money and a ton of effort () to isolate the stage subs with sand, if you don't do the same for the riser subs?

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post #147 of 234 Old 10-19-2014, 11:12 AM
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That is your summary. YMMV. Why waste time, money and a ton of effort () to isolate the stage subs with sand, if you don't do the same for the riser subs?
Well, that's a good point. Some more thinking out loud (all of which could be wrong)...

Looking at it from the perspective of "why not fill the riser with sand if we're going to fill the stage with sand?", I would argue that the riser stuffed with insulation is serving a purpose that a stage cannot. That is, it is a broadband absorber (pressure absorber for modal frequencies, with the vents) and filling it with sand would negate that benefit. A stage would be ineffective as an absorber and so it's filled with sand to essentially "remove it" from the room, from an acoustics point of view.

But now let's approach it from the perspective that we are saying that a riser doesn't need to be specially treated with subs, since a normal insulation filled riser would be fine. If that's true, then wouldn't it follow that a stage could also be filled with insulation and have the same result?

That suggests to me that filling a riser with insulation is NOT as effective at handling the subwoofers as filling it with sand would be. That further suggests that it's likely a matter of degrees.

So if we say that filling the riser with sand would be the ideal way of negating the energy from the subwoofers, may we also say that just filling it with insulation would be "good enough" in nearly all cases... and then filling a stage with insulation instead of sand would also likely be "okay" in most cases?
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post #148 of 234 Old 10-19-2014, 01:15 PM
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It's definitely a matter of degrees, on all counts if you ask me.

The only way I would disagree with what you posted here is to say, "What makes you think a stage is poorly suited to being a tuned absorber?"

The existing patterns of usage for stages and risers is a matter of tradition and myth. If subs are used on both stage and riser, there shouldn't be a difference in their construction. Here's how I understand the history and tradition, as it is, for these things - I think this will explain what you see in survey of existing home theaters. Stages were probably recommended as sand-filled long before anyone tried to use a riser as a bass trap. This has become a matter of pride and initiation to some degree, but still makes sense. Even in single row theaters, stages are useful. Second rows and the need for a riser are later developments, at least as far as I can tell and in terms of broad usage. Further, the use of multiple subs is a fairly recent trend. And the use of risers as bass traps has only been generally known (though mysterious) for about 10 years. So there's a great deal of inertia in this folk-culture of home theater design. And I say folk-culture since the hows and whys are often not understood, but a great deal of accepted best-practices are passed from one generation of builders to the next, with very few of us being actually capable of the engineering work that would be required to competently do the work from scratch.
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post #149 of 234 Old 10-19-2014, 01:15 PM
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Summary of other Notable Riser Threads

AVS makes it easy to create new threads and, as a result, we often have relevant information about specific topics spread over many threads over years. I'm going to try and aggregate a few earlier riser threads here.

The Riser Article

A lot of riser info comes from the seminal article in Home Theater Builder magazine in 2003 written by Dennis Erskine. That magazine is out of print and the article very difficult to find, so it's transcribed in the initial post there: https://www.avsforum.com/forum/19-ded...ne-2003-a.html

The thread continues with a few changes to the design since 2003, along with links to riser builds that followed Erskine's designs over the years. BIGmouthinDC likely has the most experience building these risers other than the Erskine Group itself and he contributes some practical wisdom in the thread, including why you might want to use a rubber pad (like Serenity Mat) instead of felt. SierraMikeBravo, from the Erskine Group, contributes

Erskine did an interview on Home Theater Geeks that touched on a number of topics, including using a riser as a pressure absorber. That portion of the audio interview is transcribed here

Broadband Absorber vs Helmholz Resonator

Back in 2011, user Felgar asked a question about designing a riser as a broadband absorber when the riser is not touching any walls, in the middle of the room: https://www.avsforum.com/forum/19-ded...nst-walls.html

The thread quickly took off and the next 71 posts contribute quite a bit to defining how the risers work, with interesting info from (at least) Felgar, Bigus, and Erskine. The question is raised if the Erskine design is a broadband absorber or a Helmholz Resonator. Felgar points out the Helmholz design by BasementBob is notably more complex. Bigus opines that if the Erskine riser is a broadband absorber, then the vents may be in an inefficient location.

Erskine steps in and clarifies that his design is not a Helmholz resonator but is rather a broadband pressure absorber that targets modal frequencies. He mentions that the riser must be against the back wall and at least one side wall. In his offered example, there are two vents in the back and one on each side. The vents are adjustable to allow for some level of tuning.

Erskine also provides a link to where he gets his linear bar grilles.

Bigus (and others) describe ways to create a more generic broadband absorber while Erskine goes into more details on how his design is specifically targetting modal frequencies.

More on Broadband for Targeted

Last year, user fulminis asked for a review of his rather uniquely designed riser: https://www.avsforum.com/forum/19-ded...ap-review.html

A vigorous discussion followed with input by such people as Erskine, SierraMikeBravo, Bigus, HopefulFred, TMcG, BIGmouthinDC, Mfusick, and others.

TMcG points to a calculator that would allow you to design a riser to capture a specific frequency. On clarification that a broadband absorber is desired, Bigus describes what makes a good broadband absorber.

BIGmouthinDC gave the many reasons why he likes putting internal supports in risers as well as what makes a quiet riser.

fulminis described a design that might be a hybrid between a targetted absorber and a broadband one. TMcG is dubious that it's possible to have both in one device, which Erskine confirms. Even as just a broadband absorber, SierraMikeBravo cautions that it might not as effective as hoped and explains why.

The question comes up on how to build a tall riser and Erskine steps in with plans on doing that.

Converting a Riser into a Broadband Absorber

In 2009, user mtbdudex decided to convert his riser into a bigger variant and to make it be a broadband absorber at the same time: https://www.avsforum.com/forum/19-ded...7-ft-deep.html

This thread is somewhat unique in that there are a number of "before and after" tests that show dramatic differences between an open vent and closed vent situation.

mtbdudex starts by noting that he will create multiple zones (three of them) in his riser rather than have one large cavity. The idea is to capture different peaks.

On advice from Ethan Winer, a total of fourteen 4'x14"vents were installed.

Multiple tests were done using REW both with the vents close and with them open, to see what kind of impact the riser was having as a broadband absorber and graphs of each were posted.

As an aside, the question of using standard fiberglass insulation vs Roxul Safe n Sound came up, and a GIK Acoustics rep answered that it wouldn't make much of a difference in this case, so use whatever is on hand.

Last edited by granroth; 10-19-2014 at 06:57 PM.
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post #150 of 234 Old 10-19-2014, 04:51 PM
 
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Strong post ^
I hit the like button. You've done your homework.
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