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Author Topic: Rigging Question re: isolation dampers  (Read 4990 times)

Scott Carneval

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Rigging Question re: isolation dampers
« on: February 01, 2012, 11:32:36 am »

Recently put in a system at a nightclub/lounge in the first floor of a mixed use building.  Club on first floor, leased offices on second floor, and condos on 3rd-6th floor.  The neighbors on the 5th floor complained about the noise.  You could hear actual harmonics, not just bass, due to the close proximity of the stairwell.  We brought in an acoustical consultant to discuss options, and he recommended vibration dampers on the chains hanging the speakers.  The problem is they're only rated at 50lbs and the speaker weighs 84lbs.  I've always used a safety factor of 5-7 when specing rigging gear, which would mean that I would need a load rating of 420-588lbs.  Is it possible that the manufacturer calculates a safety factor into the rating of their products?

Here's a quote from the acoustic consultant:

For isolating your 84 pound hanging speakers I recommend two Mason WHD rubber hangers in size A-Green, rated for up to 50 lbs each.  See this link.
 
http://www.mason-industries.com/masonind/_doc/pdf/WHD.pdf?CFID=34714436&CFTOKEN=29561702
 
Insert one of these in each hanging chain.  Do not be tempted to use one with a higher capacity.  These work best when the load on them is close to their maximum capacity.  While the speaker near the DJ booth is probably most important, I would go ahead and isolate at least any other of the JBL speakers within 20 feet of the stairway wall.


He's a well respected consultant and he's been in the industry a long while, so I'm not sure what gives here.
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Tom Young

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Re: Rigging Question re: isolation dampers
« Reply #1 on: February 01, 2012, 01:11:40 pm »

Recently put in a system at a nightclub/lounge in the first floor of a mixed use building.  Club on first floor, leased offices on second floor, and condos on 3rd-6th floor.  The neighbors on the 5th floor complained about the noise.  You could hear actual harmonics, not just bass, due to the close proximity of the stairwell.  We brought in an acoustical consultant to discuss options, and he recommended vibration dampers on the chains hanging the speakers.  The problem is they're only rated at 50lbs and the speaker weighs 84lbs.  I've always used a safety factor of 5-7 when specing rigging gear, which would mean that I would need a load rating of 420-588lbs.  Is it possible that the manufacturer calculates a safety factor into the rating of their products?

Here's a quote from the acoustic consultant:

For isolating your 84 pound hanging speakers I recommend two Mason WHD rubber hangers in size A-Green, rated for up to 50 lbs each.  See this link.
 
http://www.mason-industries.com/masonind/_doc/pdf/WHD.pdf?CFID=34714436&CFTOKEN=29561702
 
Insert one of these in each hanging chain.  Do not be tempted to use one with a higher capacity.  These work best when the load on them is close to their maximum capacity.  While the speaker near the DJ booth is probably most important, I would go ahead and isolate at least any other of the JBL speakers within 20 feet of the stairway wall.


He's a well respected consultant and he's been in the industry a long while, so I'm not sure what gives here.

I applaud your awareness and concern here.

I would have to crack the books and read up on how Mason hangers are specified to see whether the specified rating is related to mechanical isolation or for suspension weight.

I suggest you discuss this with the consultant. It could be a 5-15 minute discussion and they should be willing to do this.

If you do end up having to use isolators that do not provide as much suspensions-safety wiggle-room (safety factor) as you would like, you can add a safety (rated wire rope or chain) to each suspended ldspkr (or cluster) which does provide the appropriate rating. As long as the safeties do not "short circuit" the mechanical isolation provided by the hangers, they should be fine from both your and the consultant's points of view.

HTH-
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Tom Young
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Scott Carneval

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Re: Rigging Question re: isolation dampers
« Reply #2 on: February 01, 2012, 01:37:17 pm »

I applaud your awareness and concern here.

I would have to crack the books and read up on how Mason hangers are specified to see whether the specified rating is related to mechanical isolation or for suspension weight.

I suggest you discuss this with the consultant. It could be a 5-15 minute discussion and they should be willing to do this.

If you do end up having to use isolators that do not provide as much suspensions-safety wiggle-room (safety factor) as you would like, you can add a safety (rated wire rope or chain) to each suspended ldspkr (or cluster) which does provide the appropriate rating. As long as the safeties do not "short circuit" the mechanical isolation provided by the hangers, they should be fine from both your and the consultant's points of view.

HTH-

Thanks for the input.  I emailed him my concerns about the same time I posted this, and here is his reply:

I know what you mean about the safety factor and I am sure there is such in the design of these things.   The rated capacity range they state is purely from the standpoint of the required load to get the compression in the rubber in a range where it is providing isolation, with loading to the maximum providing the best isolation.  It is not directly related to structural strength which is much greater.   For instance when someone says that a one-inch static deflection spring has a capacity of 200 lbs, that means that 200 lbs is the load necessary to achieve the one-inch deflection required for the isolation.  I will see if I can get a statement from Mason regarding structural safety factor. 

I'll wait for confirmation from Mason, but it sounds like these should work just fine.  Another red flag that went up is that he recommended two of these, but my speaker is hung with three chains...
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Brad Weber

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Re: Rigging Question re: isolation dampers
« Reply #3 on: February 01, 2012, 01:44:02 pm »

They're not addressing the failure load of the hanger, the hanger itself can support a much greater load without a complete failure as there is a steel washer that is larger than the opening on top of the neoprene element that would provide a much greater failure capacity should the neoprene fail.  The capacity they are addressing is for the isolation element and its related capacity and deflection.  The greater the spring constant (the stiffer the spring or in this case, neoprene) of the isolation element, the less deflection obtained for a given load.  In this case, for the recommended hanger apparently a 50lbs. load corresponds to a 0.35" deflection and a lighter load would result in correspondingly less deflection.  The "Red" version of the same hanger uses a stiffer neoprene and requires a 90lbs. load to get the same 0.35" deflection of the neoprene and would theoretically have a 0.19" deflection with a 50lbs. load.  The "White" version an even stiffer neoprene that requires a 180lbs. load for a 0.35" deflection and theoretically just a 0.097" deflection for a 50lbs. load.

Since it is the deflection that relates to the isolation provided, they may be looking for the full deflection possible with that isolator, thus wanting it operating near its maximum load rating.  If you applied a 5:1 to 7:1 factor to the load then the isolation element would be so stiff that you would get virtually no deflection and thus virtually no isolation.

If you wanted the isolation element to have a cpacity for the full failure load but also wanted the same isolation then you might go to a very high deflection spring or combination hanger that is severely underloaded.  Say you wanted the same 0.35" deflection with a 50lbs. load but wanted a unit rated for 500lbs., then you might look at a hanger rated for a 500lbs. capacity and 3.5" deflection.

It also sounds like your 84lbs. speaker load is also spread across multiple hang points, so the load per point is probably under the 50lbs. capacity of the recommended product.
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Tom Young

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Re: Rigging Question re: isolation dampers
« Reply #4 on: February 01, 2012, 03:07:46 pm »

I'll wait for confirmation from Mason, but it sounds like these should work just fine

I forgot to mention that Mason's field offices have very good folks on staff who can also answer such questions.

So their response will be very interesting. Can you let us know what they say ?

[/quote] Another red flag that went up is that he recommended two of these, but my speaker is hung with three chains...[/quote]

Guess he missed that. Good thing you had your eyes open.
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Tom Young
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Ivan Beaver

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Re: Rigging Question re: isolation dampers
« Reply #5 on: February 01, 2012, 05:13:21 pm »

I forgot to mention that Mason's field offices have very good folks on staff who can also answer such questions.

So their response will be very interesting. Can you let us know what they say ?

 Another red flag that went up is that he recommended two of these, but my speaker is hung with three chains...

Guess he missed that. Good thing you had your eyes open.
It could be that the 3rd chain is the pull back and depending on how it is angled and where the fly points are-there may not be much weight on that 3rd point.

And if there is not much weight, then there would not be enough weight to even compress the isolators.

Totally agreed that the isolators need to be compressed in order to work.
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Scott Carneval

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Re: Rigging Question re: isolation dampers
« Reply #6 on: February 02, 2012, 07:05:12 pm »

We got a response back from the distributor:

A perfectly valid question. I attached a sample test from Mason depicting the WHD hanger under much more of a load than the 5 to 7 times of structural capacity that the engineer is concerned with.
Mason tested this product up 825 lbs, right before a failure, over 16 times your supported weight.

The first picture is no load, the second is with 600 lbs and the third is with 825 lbs. Hope this helps a graph is included.



If you look at the graph, the test time is today, at 2:58pm, suggesting that they performed this test specifically to answer my inquiry.  That struck me as a little odd. 

Also, there is an anomoly in the graph at about 700lbs, it looks like it started to give way at this point. 

Due to size limitations I've attached the graph and the first picture to this post, the other two pictures will follow.
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Scott Carneval

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Re: Rigging Question re: isolation dampers
« Reply #7 on: February 02, 2012, 07:08:13 pm »

Here are the pictures at 600lb load and 825lb load.  Notice the severe angle of the wire rope, with no bushing or guide in place.  I don't think that is the proper way to do it, but who am I to question.
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Tom Young

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Re: Rigging Question re: isolation dampers
« Reply #8 on: February 03, 2012, 07:26:02 am »

Here are the pictures at 600lb load and 825lb load.  Notice the severe angle of the wire rope, with no bushing or guide in place.  I don't think that is the proper way to do it, but who am I to question.

You are correct: this is not the right way to do this for a permanent install.

You need a properly rated and sized thimble to protect the wire rope strands from becoming kinked, then knicked and (over time) failing.

Although Mason isn't a rigging company, they really should be expected to know this. We do, after all.

« Last Edit: February 03, 2012, 07:33:55 am by Tom Young »
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Tom Young
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Charlie Zureki

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Re: Rigging Question re: isolation dampers
« Reply #9 on: February 03, 2012, 08:28:56 am »

Hello Scott,

   Yes, as Mr. Young posted, they should have set a good example of rigging, even though, they are not a rigging company per se...

   I wouldn't be too concerned with the way they rigged their lab experiment. Keeping in mind that it was only a temporary rig.... with an extremely small amount of vertical displacement on the wire rope and other components..

  Their main concern was in testing to affirm that their bracket/isolation bushing could handle the mass you've indicated without failure. As you've noted, there was a hefty margin for safety before any indication of failure was to occur.

   I wouldn't use wire rope through the bracket's hang/mounting hole...I'd use the largest shackle's pin that would fit through the hole without rheeming the hole...."bell" up, and "moused" off with pin wire. 

   At the 700lb measurement....it was probably a deformation in the bushing housing/bracket. Again, it is well within the 6 to 1 parameters that you were looking for.

   Always... get, and keep the manufacturer's specs on any materials that are used in rigging any job.  If there are no spec sheets available...pass...and purchase materials with proper spec sheets.

   Good Luck,
   Hammer

   

   

   
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Re: Rigging Question re: isolation dampers
« Reply #9 on: February 03, 2012, 08:28:56 am »


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