BillFitzmaurice.info

Flying speaker cabs – an APPROACH to safety and liability mitigation

Establishing an appropriate chain of professional considerations with respect to flown/suspended speaker systems, as a principle, is defined here-in as an APPROACH. The APPROACH involves analysing the mechanical stress/integrity issues for safety, but also involves very much more. Let’s start with insurance.

…generally speaking, and written from a mechanical bias…

Product liability insurance: A designer (also the manufacturer) may design a product/goods, which they intend to sell to customers. The product or goods have to be safe for the foreseeable uses they may be put to. The designer/manufacturer should consider risk assessment in the design and product implementation, taking into account how the product may be used, and in their product literature should specifically statehow theproduct is to be used, and any ancillary products they can provide to facilitate it’s safe usage or implementation. If there are any residual risks, including such as death if improperly used, then that warning should be attached to the product, or the literature associated with the product. This would specifically apply to the aspects of mechanical integrity of flown/suspended speaker cabinets. The mechanical integrity of the speaker cabinet and-or assembly of cabinets has to be established on its/their own account, independently of any other supplementary products/services the designer/manufacturer may also supply. A single speaker cabinet therefore has to have the mechanical integrity for any loads that may be applied upon it derived from or of its own mass in a dynamic system / environment, i.e. seismic structural vibration or drop/shock loads. An assembly of speaker cabinets and the structure that supports them also has to have the mechanical integrityforany loadsthat may be applied upon them derived from or of the combined mass in a dynamic system / environment, i.e. seismic structural vibration or drop/shock loads. Various mechanical testing procedures will form part of the substantiation of the mechanical integrity of the individual speaker cabinet, and then in a separate consideration, the system of cabinets and support structure. These integrity appraisal methods / reports / tests will be examined should there be a claim against the designer manufacturer in respect of their product. If the product is found to have the satisfactory integrity, the manufacturers product liability insurance will pay out. If it doesn’t have satisfactory integrity, then the insurance won't pay out. Health and Safety Executive in the Uk will bring criminal prosecution for gross negligence.

Determining acceptable mechanical integrity performance of the product is the domain of Suitably Qualified and Experienced (SQEP) engineers, and the expensive part is the testing of representative product, by certified testing houses. There may or may not be a recognised design standard for integrity of flown speaker mounting/anchor points, i.e. European, American or Internationally recognised standard, however, a strict literature search in combination with talking to vibration / force certified testing houses will begin to establish the standards / methods/ approaches that would be robust enough to substantiate and validate integrity. There will be knowledge about this in the current/existing speaker supply industry, and even from the professional or personal knowledge and experiences of a wider community in general, hence why publishing an APPROACH on the forum is useful.

That’s the mechanical integrity / product supply liability part done (see CAVEAT).

After that there may also be duties with regard to Structural engineers, and Suitably Qualified and Experienced (SQEP) Riggers for flown speaker installations. Each will have their insurances specificto what they do,which will be a combination of professional indemnity and/or public liability. Incidentally, professional indemnity insurance can mitigate the liability of the designer/manufacturer (i.e the original equipment manufacturer (OEM) product Vendor) but they would have to employ at least one professional in an appropriate discipline by definition to obtain it. Related to that concept, in a basic rigging safety lecture I found online they had the following definition of a qualified person:

qualified person: a person who, by possession of a recognized degree in an applicable field, or certificate of professional standing, or who, by extensive knowledge, training, and experience, has successfully demonstrated the ability to solve or resolve problems relating to the subject matter and work.

An APPROACH therefore should define ‘who’ needs to do ‘what’ (*) for everything to be in place such that should anything happen, it is the insurance companies and the legal process with whom the book stops, and not with any individual in jail or with a life on their hands because they did something wrong or were negligent. If the APPROACH is followed, the chances of anything going wrong will be greatly reduced.

(*) The ‘why’ and ‘how’ anyone involved in the application of the APPROACH does what they do may be part of their SQEP status which they or a similarly SQEP person only can do.

The application of the APPROACH is particularly important for the BFM community, because of their fanatical appreciation of easy to make, high performance, low cost PA equipment, that is only mechanically substantiated so far by common practice and practical judgement as to be reasonably safe to be ground / stand supported, and even then any risks associated with such usage are borne by each individual manufacturer / user, at each and any point of usage.

The manufacturer / producer / installer needs to satisfy themselves that they have met their duties. Suffice to say, all professional disciplines exercised within an APPROACH to flying loudspeakers, including specific installations, should be subject to checking by suitably qualified and experienced (SQEP) persons, where appropriate; eg. This does apply to mechanical stress calculations in a formal / professional working environment. This checking should extend to the APPROACH itself, as defined and developed (and published here), which should also be subjectto development, and re-definition, such that anyone who wishes to go through the process of making and selling flying cabs or offering flying installations may understand what they are getting into from the best information available

The CAVEAT is that I am not SQEP enough to specify an APPROACH that someone might realistically apply. The actual applied approach is in the end owned by the producer/installer who receivesfinancialpaymentforgoods/services sold. An online community of nearly 5,000 people ought to be able to co-develop an approach that should be successful.

I always assumed that Bill did not publish rigging specs or plans to avoid potential liability, or b/c he simply did not desire to sell such designs. Rigging often involves the venue and it's structural designs (if you hang slings from the rafters) or ground standing trusses which depend on the floor or ground conditions. There are a lot of variables which go into a safe implementation. I believe there are ANSI standards for suspending speakers, published along with other stage rigging standards, also OSHA regulations in the USA, but I do not have reference links.

Charles,
Well written

Let's look at approach then.
Many people will know that quite a number of commercial cabs have "points" for attachment of hardware in their cabs, intended for the purpose of "flying" the cabs.
This suggests that these cabs are (at least in part) designed to be flown, ie their structural integrity is such that using the points will suspend the cab satisfactorily in the flying system.
As far as I am aware though, the points in these cabs are not certified for rigging by the manufacturer, and should not be flown until certified.
The existence of points does not automatically mean that the cabs can be flown.

Then we have Bill's designs.
Which, obviously, have no integrated "points" for flying.
And, the design of the cab has no integral consideration for flying. The design of the cab is purely based on acoustic considerations.

In this situation, any flying system will basically have to wholly support the structure of each cab, rather than relying on the integrity of the cab.
At least to me, that means that each panel (top,bottom, sides, rear) would be required to be somehow constrained by the suspension system (because technically the box could fall apart).

So, the engineer designs a system to constrain each cab, and signs off on that system once manufactured.
The engineer remains responsible for the engineering of the flying system until the system is destroyed (because it is no longer required). * See below

The (suitably qualified) rigger flys the system, with consideration to load points as decided upon by a structural engineer in a given venue.

In the event of an incident (ie part or all of the cabs fall down);
The rigger is held responsible if:
The system was not installed as designed, or not rigged to load points as calculated by the structural engineer.
The designer (*) of the rigging system is held responsible if the flying system fails, eg the system was found to be of inadequate strength, or (eg) missed in inadequate weld in the system when signing off.

The owner of the cabs and flying system cannot claim no fault. They are always responsible too.

The architect and structural engineer may be responsible if the building itself does not actually meet the load points as specified (and, this case say, the building collapses).

Of course, on a country to country basis, applicable laws also vary, so any flying system has to satisfy all aspects of said laws in the country in which it is used.

Lastly, I would like to say that any attender of an event, such as a paying customer, in this day and age, has an expectation that a flown system will not fall, and kill or injure them...

dswpro wrote:I always assumed that Bill did not publish rigging specs or plans to avoid potential liability, or b/c he simply did not desire to sell such designs.

I'm an acoustical engineer, not a structural engineer, so I'm not qualified to design or recommend rigs for flying.

JBL has a good paper on some engineering aspects of flying speakers. It is in PDF format and can be found here. Of coarse for informational purposes only.

Grant,

Thanks for the compliment. I’ve been working up to this for a while, feeling a bit of a duty or inner compulsion I suppose. I’ve got a few more thoughts, but I don’t like quoting people on a line by line basis, so I’ll just lump it all together here following, if that’s ok.

On your point about wholly supporting the cab by other means – a suspension system –I take your point - I saw it in one of your responses in CAVE’s build thread. I like the idea, but I just don’t get the feeling that commercial speaker manufacturers have a metal frame inside secured to every individual panel – it is probably possible to do on most commercial designs (rectangular boxes), but it doesn’t make commercial sense and isn’t necessarily mechanically required, if the cab assembly process is repeatable and controllable. The cab is normally the setter of geometry, the primary structure, meaning that the frame concept would be the secondary consideration and ‘fitted’ to the cab after – the other ways round are that the steel frame is the primary structure or the cab and frame is an integrated combination in the design. Whichever way it is, it is a completely different philosophy of design, manufacture and bespoke assembly method. Other considerations are: How do they fix to the frame on all sides, is the frame clear of the outside panels or is it a precision frame that can be fixed to direct, does it have implications for the edge panel-joints of the wooden sides parts of the cab. If it’s not a frame, is it just reinforcing joint angle brackets in the corners (as per JBL paper) – screws, glue or through bolting – if it’s through-bolting, where are all the bolt heads on the outsides in commercial designs, if it’s screws then they aren’t as strong as glue, and if it’s glue, we’re back to square one with a cab that could fall apart. I think your original point with CAVE suggested it was an Australian national standard that prescribed the internal steel frame – and this is what design standards unfortunately sometimes do – they mandate a certain/specific approach, that is inflexible in its implementation. The point of SQEP persons involvement in the development of standards is to mandate where necessary, but otherwise give leeway to technical substantiation of integrity with an appropriate amount of scientific rigor.

A controllable and repeatable assembly process in BFM terms is: acceptable joint area and closure (gap / stand-off) by appropriate joint preparation, high PU glue strength, squeeze-out as a main indicator of nominal representative joint strength on each sample of cab that you make, and monitoring of any other factors considered important in the process, etc. It would be nigh on impossible with BF designs to insert a full steel frame - there’d be cuts and notches all over the place and it would make assembling a sealed cab difficult and commercially demanding. The way I read it, the JBL paper implies manufacturers are intentional and selective about the reinforcement they put into boxes.

Here’s my pragmatic take on one mounting method that I looked at whilst reading up on this. Page 13 of http://www.dbaudio.com/fileadmin/docbas ... Manual.PDF shows a (MAN UK flying systems) D ring flying stud and cabinet fixing. When you look at the cabinet, the cabinet fixing is flush with the outside of the cab, so the countersunk fixing holes in the cabinet fixing are screwed into something inside the cab, but those screws are then ‘blind’ to the outside of the cab. My gut feel is that the cabinet fixing is screwed into a block/plate piece of plywood that is glued on the inside back of the side piece (for all the issues with metal frames, screws, through bolts, corner brackets, etc. listed above) and the functional interface of the cabinet fixing sticks through a hole in the side, as shown, making the tear-out strength of the fixing very high, but implying nothing about the connectivity of sides to bottom and top of cab of course. I’d wager that there isn’t, but if there is a steel frame in that cab fixed to every side, I’d very much like to see it.

I feel I have to make a stab, as a mechanical engineer, at what I believe is a suitable testing process for flown cab mounting points. If there is actual industry knowledge on here I’ll gladly accept correction and guidance. If a manufacturer believes they have a controllable and repeatable assembly process with the necessary mechanical integrity then that is a starting point – screwed on front mesh – no velcro. The d&b manual quotes their cabs as compliant with the highest German standard VBG70 load safety ratio of 12:1, replaced by BGV C1 (http://www.movecat.de/engl/fakten-pdf/B ... nglish.pdf) also with 12:1 load safety ratio. The 12 times load is generated by shock loading – see Understanding shock loads (http://tdt.usitt.org/GetPDF.aspx?PDF=49-2shockloads). If suspended from catenaries (wires or chains) the loads can only be imparted in the direction that the wire/chain supports – this can make things easier or harder. But, testing houses use shaker tables with accelerometers for feedback. Assuming a pair of mounts on each side of a cab, geometrically opposite one another, and generally designed just to accept shear loading, not bending. If you want to shock load all elements/parts of a cab assembly with 12 times their own weight you’d need to accelerate the whole thing at 12G by holding it at its pair of mounts. The cab would need to be rigidly held if the C of G was not on the line between the pair of mounts – in such case torsional load would be put on the mounts as well due to an offset C of G – if rigidly holding the mounts is not representative of how the system attaches to the mounts, i.e. bending can be imparted, then this would need consideration. How many cycles of loading? That’s the tricky part where there may / may not be a standard for it –maybe 20 to 50, but enough to know that in the original manufactured state nothing is going to shake off due to a 12G shock load in whichever plane it may be applied. That would be for a single cab. The testing process would be written up as a technical proposal document and it’s adequacy agreed before implementation. If I was a manufacturer, I’d be able to sleep at night with a testing method along these lines. …Personally I’d really like to know if the big players do their testing by shaking or static pull-out. IMO, static pull out isn’t enough because you’re not subjecting every part to the shock load, and if anything fell off a cab, and under cross examination you were asked, “did you subject your product to shock loading under test conditions?” one would not be able to say “yes” if one did not.

The analysis of an assembly of cabs would look at the connectivity on a static stress calcs basis between the cabs, same with the top frame – the factor of safety would have to be at least the 12:1 for catastrophic failure, but ideally just on yielding (ultimate tensile failure and yielding are different). Chains/wires suspension for a tall line array is good, because any sideways seismic load istaken out of the equation by sway of the chains. Rigid mounting a line array from one end is a recipe for disaster from a seismic point of view – it’s a great big cantilever, with an offset combined C of G, ready to lever itself off the mount. But each rigid or non-rigid mounting approach is done on a case by case basis.

There should be clear indication in documentation and on the cab as to whether a cab is rated for flying or not via the use of integrated mount points that may look suitable. I’m not convinced there is a formal process of ‘certification’ however, which would imply someone with ‘official status’ signs off the cabinet. Official processes and technical SQEP’ness are 2 different things –the latter may be integrated into the former, but for that to be the case, the official person would have to be SQEP to review the docs, and would have to have reviewed all documentation relating to the claims of the manufacturer to sign it off. I think, normally, the due diligence would be on the manufacturer of the product, so it is their ‘word’ that is the certification or approval in the case of flying rated speakers. That ‘word’ ought to include mechanical validation processes they’ve gone through with certified testing houses.

In relation to your point about who is responsible, my understanding of the way things work are that once a flying installation is complete, everyone walks away, presuming all has been done satisfactorily, until there’s a serious incident. Then expert witnesses (SQEP practitioners in the appropriate discipline) help the legal system decide who’s to blame after the fact. Open communication and asking the appropriate people to be involved at the appropriate time is the approach to getting things right from the start.

Charles,

The subject is deeper than I thought initially. The depth of your knowledge is way deeper than I could imagine but just like Bill, I'm also an acoustical engineer who implemented a design I thought was credible that paid off eventually. I agree with your submissions which in the final analysis, would remove that element of blame on the manufacturer ie. BFM based builders and beyond. I hope this thread gets all the attention and support it deserves because, this is what will narrow the gabs between BFM designs and the competition. I will urge Bill to support this thread highly.

Charles Jenkinson wrote:It would be nigh on impossible with BF designs to insert a full steel frame -

Nor should it be considered. The safe way to fly these is with an external metal frame, so that the weight is supported by the frame, not the speakers. But as far as endorsing a particular frame here, that's not going to happen. As soon as I say that a frame is suitable then if one fails, for whatever reason, I become ensnared in the legal issues that would follow. That's a position that I'm not willing to find myself in.

my understanding of the way things work are that once a flying installation is complete, everyone walks away, presuming all has been done satisfactorily, until there’s a serious incident. Then expert witnesses (SQEP practitioners in the appropriate discipline) help the legal system decide who’s to blame after the fact.

Your understanding isn't correct. It may be different outside the US, but here when a liability suit is filed everybody and anybody involved in any way gets dragged into the muck and mire, and must bear the costs of defending themselves. One can find themselves bankrupt even if they are found totally innocent, while the process can drag out in the courts for years, and in the end the only ones to benefit are the lawyers.

http://www.penn-elcom.com/default.asp?PN=R1738
http://www.penn-elcom.com/default.asp?PN=AAY0327
Used with
http://www.penn-elcom.com/default.asp?PN=R1737
Open up the commercial boxes and these are what you find. Certified when installed in 3/4 plywood. It still doesn't certify the box itself, but if installed correctly the box itself carries virtually no weight.

With
http://www.penn-elcom.com/default.asp?PN=R1438 these you can get custom lengths and tie multiple cabs together.

I don't suggest any of these solutions for mobile work. That needs a full frame rigging system. For permanent installs these solutions have been used for years.

Some great points included in the posts since mine.

I tried to stay away from any particular country or thread on purpose, and some of what I know is based on experiences of mates working in countries other than my own.

Lets go back to commercial cabs for a bit.
Basically there's 2 types.
You have your injection molded plastic/ABS type cabs.
These usually have M10 nuts integrated into cab material.
No doubt engineers will sign these off because:
- the manfacturing process is repeatable.
- information is abundant (even if only to the manufacturer) about the structural characteristics of the cab based on testing.

Wooden based cabs.
Same again, M10 nuts (or the products Leland linked) integrated. Cabs tested etc.
If the balances and checks are in place with a given manufacturer, there's no reason why these cabs are also totally repeatable in the manufacturing process.

Bill's designs:
Herein the difference lies.
Bill provides the plans, we all build the cabs.
So, on a cab to cab basis, because build skill levels vary, not every cab will be built the same.
On a cab to cab basis, we improve as we go to the next cab (authorised builders is the exception here, though, in the early days, that situation would have been the same for them).
Meaning that repeatability is perhaps questionable, even though Charles pointed out some excellent indicators of build integrity.
In this scenario, one would have to have each cab in their rig individually certified.
To that end, if one was intending to fly their system once built, it would probably be a smart move to include observation for the build(s) by the certifying engineer (despite the cost).

I believe the system Bill suggested is the way to go.

In cave's case, I suggested the internal steel because his idea/system did not include wholly supporting the cab's weight, and relies on the cab's integrity to stay in the air. A steel to steel connection would at least avoid pull out on the internal side was my thinking.
I wouldn't do it that way, and I'm not having a go at cave for the method he employed.
Cave has hopefully at least had an engineer look at his flying method, if for no other reason than safety, much less potential litigation.

I think one will find, generally, that no manufacturer will imply that the existence of flying attachment points in their cabs means that the cabs are certified to be flown.
This is because the manufacturer has no way of knowing how or what system will be employed to fly the cabs.
Yes, flying points are stated in sales pitches, promo's etc.

Litigation:
Australia is (perhaps) an odd case. It's oldest law(s) comes from the Old Dart, but in litigation it cloesly follows (but is about 5 years behind) the USA, though that gap narrows as time progresses.
At least here and in the US it seems, all parties are liable.
Even if it's not that case in any given country, it pays to consider the results of litigation from Oz and the USA, because litigation does lead to change and upgrades methods of best practice.

Rigging:
I have a great mate, who, through education and improvement as a rigger, has basically done himself out of a job. Due to his experience and qualifications, he can no longer consider "small" jobs because his quote to do that job is cost prohibitive due to the insurance aspect of his quote.
He has told me he hates doing permanent installs.
If he rigs it, he is held responsible until the day that install is removed.

Charles, I certainly err to you as a qualified engineer. I'm not.
I still maintain, for all those here in the forum, who intend to fly any of Bill's designs, that the onus is on them to thoroughly investigate applicable laws in their own country, and implement all and every recommendation and requirement.
It will cost a packet of money to do it right.

I have to say though, that my reasoning is not so one doesn't face litigation, but more that some poor bastard doesn't need to die (or be temporarily or permanently injured) just because they wanted to go out for a night of music...

I texted a friend in the week who works at MIRA (Motor Industry Research Association) about this integrity testing. He's just phoned me this afternoon. He says they can do shaker table type vibration testing at MIRA, but I "couldn't afford it". He knows just how cheap I am. He concluded that shaker table testing is probably not necessary and so we talked a bit more. He said that if there's no speaker vibration anchor/mount integrity design standards, that to find out how other manufacturers do it is one way and copy what they do. Or do a Failure Mode and Effects Analysis (FMEA) to determine what a sensible test regime would actually be. I then talked about the 12G drop load shock from the German standard, rigging safety load reserves etc., and he said "well, drop load shock testing is easy and relatively cheap to do. You just drop it." With an accelerometer mounted on it and a data capture card and software.

My feeling is that drop load shock testing is probably not beyond the bounds of BFM builders, since as long as the accelerometer is calibrated and the data capture card, with the method and results written up, one could verify what one had done. An FMEA could also be worked up oneself to be fairly conclusive. Which just leaves the rigging and structural mounts in the building. I think support frames are only particularly load tested when they are fabrications, i.e. involve welding, so there's a bit of grey area there, or particularly if one uses basic strip (steel) product. If the grade is known and there's a C of C, but no welding involved, it may not need load testing - I'm not sure - LOLER would say - It would need stress calc's though.

Best regards to all BF line array manufacturers,

Charles

Further to the above, here’s a sensibly priced ‘usb accelerometer’ that looks capable of capturing a speaker drop load shock test event with sufficient accuracy as to prove cabinet mounting/anchor point integrity. If the ‘design’ shock limit (say 15g) is greater than the maximum permissible ‘operating’ shock load (say 12g) then the margin of safety would absorb any inaccuracy involved, sufficient to be able to meet the 12 times safety load of the German rigging standard. Note that there should be a calculable/deterministic margin of safety anyway, in any mechanical design. …It may not be expensive or with calibration back to a national standard (at values other than 1g, for which a procedure is given), but if one is serious about going down the road of proving mounting integrity / safety under known shock loading conditions, then it’s an extremely cost effective starting point.

http://www.gcdataconcepts.com/xlr8r-1.html

Given the lack of speaker mounting design standards, my take is the that rigging limits are a sensible and pragmatic design specification to work back from.

I'll run it past my mate who works at the Motor test house, and see what he says, in terms of its rigor as a measurement device to support drop load test for substantiation of selling a commercial product, i.e. the APPROACH, and report back.

Good job Charles. I would want to know how it is used in testing

Nice.

This is where I'm up to with this, below. The concluding question appears to be satisfactorily answered to me, however, I don't really have the time, money or effort to continue with this at the moment from a practical point of view, which IMO is setting up the tests on an individual cab using the fixing approach cave has used, but also doing stress calcs on the assembled configuration of steel strip, at specific kelping configuration/s and g loads. Once again, any positive critical feedback welcome.

Assessing the performance of a ‘usb accelerometer’ for testing of speaker cabinet mounting / anchor points

- Plus a discussion of test variables/parameters for drop shock testing of the same,

Or rather: we’ve found a cheap accelerometer / data logger – will it do what we want?

The problem: There is little known information about specific design standards applying to flown Speaker cabinet anchor / mount points, however, there are many different rigging safety load limits, where the rigging capacity has to be greater than simply the ‘mass x gravitational constant’ (weight) of the product supported, to satisfy shock loads that may arise. It is therefore necessary to design flown speaker cabinet mounting points to satisfy shock loading arising from the most arduous rigging safetyload limits.

Safety load ratios are specified from as low as 2:1, up to 12:1 in Germany. For a speaker cabinet to survive a drop load whilst supported by wire or chain rigging, the integral cabinet mount must also sensibly survive the shock load.

The practical test set up is not of consideration, just yet. We are seeing if a specific accelerometer will do the job, and discussing the parameters of the test that can be changed – we are ‘designing’ the test and measurement method.

Principles of a shock event - ‘Shock’ or ‘Acceleration’ loading?: Generally speaking, shock is a relatively quick event and relatively cheap to test. Acceleration is a slow event, and relatively more damaging to a product, and more expensive to test. A shock force event is defined by its pulse width duration, in time units (…and also a pulse shape). The pulse width is shorter or longer due to the mutual combined effect of how quick the shock/accelerating force is applied and how quick the energy is absorbed. Short pulse widths generate higher shock (g) loads, but less damaging energy is input to the structure than for wider pulse widths.

The following paper discusses the above:

https://www.endevco.com/news/archivedne ... /tp321.pdf

Note: The paper indicates that pulse width durations for 12g are of the order of 25msec and longer (for a 6" drop height) –this is important wrt sampling frequency of the accelerometer. (NB: My guess is 6" is unlikely to generate 12g dropped onto supporting rope strops, but one doesn't know, so start with 3" drop and work upwards. I'd use fishing line to elevate the load and then cut with sharp scissors / secateurs)

[If it’s still a bit confusing: An analogy for short and wide pulse width is: imagine hitting a car with a sledge hammer (with handbrake off), and expecting it to roll forward. If the car doesn’t roll forward, then no useful work has been done. The converse is hitting a 6” nail into wood with the same sledge hammer (where pushing it concertedly by hand would not drive it in). As a crude analogy (i.e. not equating total hammer kinetic energy to hand pushing energy), moving the car needs wide pulse width energy, and driving the nail needs short pulse width energy.]

Therefore, to err on the conservative side, the required acceleration (g) should be put into the cabinet mounts with as wide a pulse width as practicable, i.e. dropping onto relatively stretchy rope (see practical test considerations below). The real life mitigating factor is that a stiff rigging chain or wire will create a shorter pulse duration than say a polyester strap or rope, the latter which is not permitted for rigging duties. The inputted /damaging acceleration energy (giving rise to stress and failure) will therefore be greater under test conditions, than the same peak acceleration event under real conditions.

The suggested transducer is: http://www.gcdataconcepts.com/x16-4.html

…based on the following conversation between CJ and gulf cost data concepts:

--------------------------CJ to GCDC: I’m interested in the X16-1D, and measuring up to the 16G capability by the method of drop load ‘shock’ testing. I say shock, because the testing involves acquiring the load through dropping a product mounted on rigging wire, chain or straps. I know pulse width (duration) and associated sample rate are important but I’m not time-served in vibration /shock analysis so the answer is a little elusive. Please can you advise if the 800Hz is enough for the peak acceleration to be adequately and accurately described or pickedup?

GCDC response to CJ: The X16 series loggers have an undocumented feature that allows sampling rates of 1600 and 3200 Hz. The logger deactivates the oversampling algorithm so the sensitivity is reduced but the response time is improved. The 3200Hz sample rate and micro-resolution option will provide the best configuration for capturing the peak impact event.

Impact peaks are easy to underestimate. Dropping a logger 1m can exceed 50g if the impact point is rigid, such as a concrete floor. The shock test must be a balance between impact energy (drop height) and impact surface (energy absorption) to keep the peak acceleration within the range of the 16g sensor. A soft impact surface, such as foam rubber, will spread the energy over a longer period of time and make it easier to capture the peak.

I don't recommend the X16-1D for impact testing because the alkaline AA battery tends to shift upon impact. The X16-4, X16-mini, or X250-2 loggers use a lithium-polymer battery that is more resilient to impacts.

---------------------------

Concluding question: For rigid body peak acceleration, can sufficient pulse width duration be achieved to be able to utilise the relatively low sampling frequency (up to 3200Hz), but not resulting in too soft a suspension system that the g won’t be generated and/or too high a drop height that the apparatus is unwieldy…? The two parameters that can be changed are: (a) stiffness of suspension system, i.e. swap between polyester rope, or steel wire/chain, and (b) drop height. The objectiveis to have as wide a pulse width as practicable to achieve inherent conservatism in the test method.

Other conclusion: Try not to drop the accelerometer on the floor, …but even then, for the above reasons, it probably won’t break, fingers crossed.

Further considerations for potential commercial manufacturers: i.e. Is this procedure good enough?: For product (cabinet fixing integrity) development and safety considerations; yes. To prove product quality and manufacturer due diligence in a court of law; probably not. Running calibrated accelerometer and data capture hardware on the same drop test side by side would prove the test method and accelerometer accuracy, and satisfy for the sample of one tested. Through-life fatigue accumulation for the product is a consideration about which the test (no of cycles or duty cycle) might be further designed/scoped to mitigate for. Other aspects of quality control are relevant for manufacture of more product to the same design and for the same purpose of being flown. i.e. quality control, measurements, manufacturing documentation, C of C, how to install it (specifying potential use of any other products), …and again not forgetting, product liability insurance.

The End.