speaker cabinets

[Matt]

in our drama studio, we have some biggish sort of speakers that we want to raise up off the floor away from the kids, but not wanting to use stands or drill into the walls, so I was wondering if anyone could give me advice on how to modify the cabinet (quite strong in itself) to be hung from some drift wires on our RSJs.

 

would be v.grateful for any advice

[Pete McCrea]

Not meaning to cause any problems, but are the RSJ's load rated? They probably have the load capacity if they are part of the structure, but you need someone suitably qualified (Civil/Structural Engineer) to say that they can take the load you might want to put on them, and not be taken past design capacity. And when you start to consider the forces that wind and snow loading could put on them, it might not be enough to allow you to fly the cabs off them. Or they might be designed to take compression only, and not bending moments, so again lobbing the cabs off them could over stress them.

[Joshwah]

As said above you first need to check if the RSJ can take the load and you need a qualified person to do this.

If it is possible to then depending on what speakers you have, then there may be various flying options such as frames or fly wires. What speakers have you got??

[paulears]

unless the cabinets are designed with hanging points fixing is a problem. a frame, probably box section type could be made up, but load ratings would be interesting to calculate. it's the old risk assessment again. Many years ago I lifted a cabinet by flying it from plasterboard fixings through the top panel. When I raised them one speaker promptly fell - the entire top panel was chipboard, stapled to the upright sides! and the other one half fell when one of the cheapo fixings snapped. if you look inside most cabs with proper hanging points, you see an internal stress spreading arrangement. The handles are not even safe hanging points either. An over engineered frame seems the only safe way to fly cabinets - most companies who have coded welders can certify them with a weight limit. in this area they are used to working for the offshore gas industry and are happy with this type of thing.

 

last bit - even if you have hanging points, make sure you use rated hardware. diy shop shackles and turnbuckles rarely have ratings. eyes are a particular problem - eyes should be closed and welded - cheap ones are just bent closed, and open up when loaded. Rawlbolts are often properly coded - especially the ones designed for vertical loading.

[Jivemaster]

Unless the speakers are designed to be flown, they will likely fall apart if flown. They could poss be put in a flying cage or cradle but you're very much on your own for risk assessments structural calculations etc.

 

remember the acoustics will vary with location also

[Matt]

well we have just had the rsjs looked at because we have had new bars installed, and they said we can hang around 4Tons on them before worrying, and I doubt 20 par cans and 2 mac 500's get that far :blink:. Regarding the problem about the cabinets, the front does come off allowing for changing of drive units etc... so I am surei could build a internal cage strong enough to sturdy the whole lot up, and we arent bothered about the acoustics of it all, we just wanna get them off the floor and away.

 

cheers anyway, I'll keep these points in mind.

[Ike]

well we have just had the rsjs looked at because we have had new bars installed, and they said we can hang around 4Tons on them before worrying

Any structural engineer worth his salt will give you an exact SWL for them which will be a tiny factor of the breaking strain. Without a SWL and certificate you really shouldn't even hang your coat off them.

 

As for making a flying frame up yourself I wouldn't suggest doing so, it will take a fair bit of maths to be able to calculate the strength required in the various components and anyone who has to ask the question originally posed does not, IMHO, have the skills to be able to do this.

 

I mean no offense in what I say and have in the past flown things from unrated points in 'home made' frames however you have to bear in mind that you will be going against the Lifting Operations and Lifting Equipment Regulations and risk (quite literally) pulling the building down around your ears.

[Pete McCrea]

Was it a qualified Engineer who looked at them? I'm a nats wisker from finishing a civil engineering degree, and now understand better what goes on in a design. Did the person take into account wind loading+snow loading+ dead loading on the RSJ before applying your proposed loads, or did they pull the figure out of a catalogue and say 'oh that 185x185x20UC will take 4tons at 5 metres'. Fair enough if a suitably qualified person has done the calcs and signed it off, otherwise be aware.

[Matt]

I wasnt there at the actual time when they did it, I was somewhere else, cant quite remewmber where...

 

but the inspection was provided by the education authority on behalf of the council so I guess it must have been a properly qualified bloke who did it, as far as I know, all I was told from the report that as long as we dont try to hang anything more than 4tons on any one rsj at any one time, or try to hang 4tons worth of a whole rig over the lot of it, we would be fine...

 

I'm just going by that, so its not really my neck if I was given the wrong info (I hope you'd agree on me being mis-informed if anything does happen). I am sure the report is in the filing cabinet at work somewhere, I'll try to dig it out.

 

but thanks for the advice on the cabinet stuff... thought I'd better bounce my idea around a bit before putting into practice, you reckon if I got a local welding company (personal friends of mine), to make me up a cradle like structure, that would be ok, the speakers only weigh like 20kg anyway, abut from about 4m that would be quite a headache.

 

Matt.

[Jivemaster]

A flying cage is an external basket of steel to support the cab and restrain any part should it become loose.

[Matt]

so a cradle big enough to get the cab in, one way, and then secure it somehow to the frame, and then I'd just use drift wires (has loads of swl500kg) to attach to rsjs right?

[Martin]

so a cradle big enough to get the cab in, one way, and then secure it somehow to the frame, and then I'd just use drift wires (has loads of swl500kg) to attach to rsjs right?

Yes that's just about it. Some speakers attach to their frames via the bolt holes on the bottom that are designed for attaching a stand adaptor. This frame is then usually mounted in a yoke and suspended via a hook clamp just like a lantern. This enables you to easily 'focus' the speaker. It's worth checking if the speaker manufacturer make one of these flying frames for your speakers.

 

Paulears wrote:

The handles are not even safe hanging points either.

 

I have had this argument with someone who wanted to fly some rather heavy active Meyers from their handles. IIRC these were plastic handles too. His argument was that if the handles were strong enough to pick them up with your hands then it must be okay to fly them from their handles. :blink: If this wasn't enough, he wanted to fly them from the dress circle handrail :( In the end I think they ended up stand mounted.

[Ellis]

You mentioned that the front panel comes off - you will need secondary suspension for anything that may become detached in use.