stevie

Paul bought a bridge from me and the transaction was as smooth as a baby's bottom.

[quote name='alexclaber' timestamp='1377861910' post='2192870'] That's a sketch-up of a pre-production model, we use it to check all the panel & brace dimensions are correct before cutting the first example. [/quote] Well it's a good job I asked. I thought you said "Here's an example of how we do bracing". [quote name='alexclaber' timestamp='1377861910' post='2192870'] But increasing panel stiffness through added thickness still leaves you with a single vibrating membrane with a fundamental frequency and a series of overtones - adding braces breaks that panel up into multiple segments. Spacing the bracing cleverly can make all the fundamentals occur at different frequencies. That makes a big difference. Now in an ideal world we don't want any panel resonances but I've yet to come across any PA or bass cab which has panels so rigid they have absolutely zero vibration at high SPL. [/quote] Yes, agreed. Bracing is good and should be a feature of all properly built cabinets - but bracing is not unique to lightweight cabinets. Offet bracing is something every amateur speaker builder knows about and hardly worth mentioning. [quote name='alexclaber' timestamp='1377861910' post='2192870'] Something that's been missed in this discussion, which makes a huge and under-appreciated difference to tone, is that complex internal bracing like this gives the backwave lots of different surfaces to to reflect off, of varying sizes, spacings and angles from the source - not a million miles dissimilar to how a Stealth F117 reduces its radar signature. The bracing also makes it easy to suspend the damping material away from the walls of the cab, which makes it much more effective at damping mids by catching the wave where the velocity is higher and pressure lower (like how you damp the right harmonics in a transmission line). The reflected backwaves that do return to the cone are thus much lower in amplitude and much more widely distributed in frequency so you don't suffer unwanted peaks and notches in the mids. [/quote] Firstly, you can suspend damping material away from the walls of any cab that has a front-to-back brace. There's nothing unusual in that. Secondly, I find it very difficult to believe that a few extra braces will have any affect on the backwave, never mind make a "huge and under-appreciated difference in tone". While we're discussing the backwave, the biggest problem with a thinwall/lightweight cabinet is transmission through the cabinet walls because the mass law equation says that 6dB more of the backwave will escape through a cabinet with 9mm walls than one with 18mm walls - even more with a lightweight panel. So, if the goal is to contain the backwave, reducing wall thickness is exactly the wrong thing to do.

My Yamaha BB has a very slim neck and does a perfect imitation of a P-Bass (P-Bass pickup). There's a thread on them in this section that shouldn't be too hard to find.

[quote name='alexclaber' timestamp='1377775016' post='2191573'] For someone with such strong opinions on this you're remarkably good at getting the facts wrong! [/quote] Always glad to back up my views with an explanation or evidence and always happy to take advice when I post incorrect information. [quote name='alexclaber' timestamp='1377775016' post='2191573'] Some points to consider: 1. Panel stiffness is proportional to the square of the thickness, not the cube. [/quote] Here are some links: [url="http://www.netcomposites.com/guide/core-materials/44"]http://www.netcompos...re-materials/44[/url] "Engineering theory shows that the flexural stiffness of any panel is proportional to the cube of its thickness." [url="http://www.plastemart.com/upload/literature/Core_Materials.asp"]http://www.plastemar...e_Materials.asp[/url] "But because a plate's bending stiffness scales as its thickness cubed...." From Experimental and Applied Mechanics by Proulx "Stiffness of a plate or beam is proportional to the cube of thickness." From Handbook of Troubleshooting Plastics Processes by Wagner "Since stiffness is proportional to the cube of thickness..." From Specialized Molding Techniques by Heim "The overall stiffness of a box can be quantified by its "EI" product. The higher this product the stiffer the housing. Thinning the housing wall dramatically decreases its moment of inertia which is proportional to the cube of wall thickness." [quote name='alexclaber' timestamp='1377775016' post='2191573'] 2. The resonant frequency of a panel is proportional to its stiffness divided by its mass - halve the mass and you double the resonant frequency [/quote] Well, physics really isn't my thing but I always thought the ratio wasn't direct and that there was a square root ratio involved here. You know, if you halve the mass, the resonant frequency doesn't double - it increases by 1.4 times. I checked this one out in the literature too. In the Handbook of Adhesion Technology (da Silva) I read: "In vibrating systems, the resonant frequency is proportional to the square root of the ratio of the stiffness to the mass" Mechanics of Poroelastic Media by Selvadurai "The shifting is due to the fact that the frequency is in general proportional to the square root of the stiffness to mass ratio..." Electromechanical Design "The resonant frequency of a vibrating member is proportional to the square root of the quotient of the spring constant (stiffness) divided by the mass." [quote name='alexclaber' timestamp='1377775016' post='2191573'] Here's an example of how we do bracing: [/quote] Credit where credit's due, that's very good - certainly far ahead of anything your competition has done. You should have a word with Mr Foxen - he thinks a cross brace is all you need. Nevertheless, if you're using 9mm ply for that cab, elegant though it is, it will not be as inert as a cab made of 18mm birch with a circular brace and a back brace. I am assuming, as is everyone else, that this is a Sketchup of a production cab.

[quote name='JTUK' timestamp='1377721073' post='2190953'] We looked into this very carefully when buying P.A ...and there was no doubt that a wooden cab beat the composite cab hands down on sound. Try the QSC K12 and K12W. for example. [/quote] Indeed. If you actually use your ears rather than swallowing every marketing claim on the web, the benefits of a stiff, non-resonant cabinet are plain to hear. Given the amount of engineering expertise available at QSC, you'd expect them to have maximised the performance of their plastic cab. (Bracing is a doddle with moulded cabs - you can build in as much stiffening as you like.) They don't say so, but it looks like they have even used the DSP to correct a cabinet resonance. Even so, there is a broad consensus that the wooden K12 sounds better than the plastic one. Whether they are worth the difference in price and in weight is, of course, purely subjective. I'm currently finishing off the design of a pair of passive PA speakers using the same drive units as the K12. They're working out very well indeed.

[quote name='Mr. Foxen' timestamp='1377720598' post='2190937'] additionally all the stiffness stuff doesn't work, half the panel thickness and brace across two opposing panels will happily give most of the stiffness of an unbraced full thickness one, along with massively reduced resonance/raised resonant frequency. [/quote] That's wishful thinking, as anyone who has ever braced a speaker cabinet will tell you. If it were as simple as this, nobody would make cabinets with thick walls. They would simply use material that is half as thick and stick in a cross brace. Job done! A cross brace adds stiffness at the point of contact but that stiffness quickly declines as you move away from that point. There is a formula for maintaining stiffness by bracing as you reduce panel thickness but, as I have tried to explain, you cannot keep making materials thinner ad infinitum. With a 12mm panel (birch) there is some weight saving to be made compared with a thicker unbraced panel, but if you try to make a 9mm panel as stiff as an 18mm one, you are likely to end up with something that is as heavy as an 18mm one. Quite where the point of diminishing returns lies is up to the designer, I suppose. Personally, I wouldn't want to go below 18mm to start with, not for bass anyway. But that is my opinion and I use a trolly. Comparing a braced thinwall cabinet with an unbraced cabinet made of 18mm material is all well and good. But this assumes that companies using 18mm ply for their boxes never brace them - which is not the case. [quote name='Mr. Foxen' timestamp='1377723525' post='2191037'] Changing ply composition is how you achieve the desired properties, there are a wide variety of different woods you can make ply form, as well as other materials that can be laminated into it. Kind of figured that was common knowledge. [/quote] If it's common knowledge, then do tell us what this material is and who is using it.

Wharefdale in its original incarnation made some hi-fi cabinets with foam cores in the 1970s using formica, I think, as the outer panels. They worked really well but were difficult to make because they had to be epoxied, which was problematic at the time. That is the only reason they discontinued them.

[quote name='redstriper' timestamp='1377720809' post='2190944'] I'm sure they shouldn't sound so good on paper, but they work for me and all those who have used them - that's not to say they would suit everyone. [/quote] If they are a plywood sandwich with foam in the middle, that does sound good on paper.

[quote name='Mr. Foxen' timestamp='1377720598' post='2190937'] All of this doesn't ring true. You absolutely can compensate for the reduced self damping of thinner materials, changing the ply composition can do that. additionally all the stiffness stuff doesn't work, half the panel thickness and brace across two opposing panels will happily give most of the stiffness of an unbraced full thickness one, along with massively reduced resonance/raised resonant frequency. Observing any engineered structure shows specific reinforcement is employed over plain mass, any large concrete structure isn't just made of masses of concrete, it is made of reinforced concrete, with an interior framework giving strength with concrete over it. Bridges are not made of a solid mass, but a framework of supports. [/quote] You know what doesn't ring true? The idea that you can keep making material thinner and thinner and by adding bracing maintain the same properties as thicker material. It's b*llocks. Otherwise, we'd all be using cabs made out of 3mm MDF. What does changing the ply composition mean? And yes, bridges are designed for rigidity. But they are made from strong materials to begin with, i.e. reinforced concrete or steel. You don't design a bridge by making a structure that is as light as possible and then reinforcing it with bracing, now do you? By the way, have you any idea how stiff a church bell is?

[quote name='Phil Starr' timestamp='1377403675' post='2187129']Like Stevie says there are two ways to save weight. Mass is one of the things that affects panel resonances, stiffness another and there are more, there is nothing new in making thinner speaker panels and bracing and damping them or even looking at alternate materials with different mass/flexibility/internal damping. Bracing any panel will reduce it's resonance and shift the frequencies involved. The lightweight cabs may use 12mm panels rather than 18mm saving 1/3 of the mass. Then use some of this saving to create the bracing, they may even then be more rigid and less resonant than a conventional cab.[/quote] Whilst theoretically possible in an ideal world, getting a 12mm panel as stiff as an 18mm panel requires some serious bracing – enough to make the box nearly as heavy as making it out of 18mm in the first place. The way I remember it, if you reduce the thickness of plywood by 3mm you reduce its stiffness by 50%. So a 15mm panel is half as stiff as an 18mm panel and a 12mm panel is half as stiff again. An 18mm panel is actually 8 times as stiff as a 9mm panel – and it's a lot worse if you're also using lighter materials like softwood ply. I don't think any of the companies making lightweight bass cabs are using a really sophisticated bracing system like the B&W Matrix because that would add too much weight. These companies are building lightweight cabs – so they use as much or as little bracing as they need to get the job done. The only company that provides any information on their cabinet bracing is Trace Elliot and they seem to be using a single circular brace. Good, but not belt and braces, although to be fair they don't make any special claims for their poplar ply cabinet anyway – except that it's light. Although in theory you can brace the bejaysus out of a thin panel to make it stiff, what you can't do is compensate for the reduced self-damping of thinner materials. Damping is what stops a material from ringing - and ringing is what makes cabinets sound bad. There's also the problem that the thinner material is less acoustically transparent and will allow sound to escape through the cabinet more easily at certain frequencies. So while I'm all in favour of neo drivers, I am not a fan of thinwall cabinets for bass. You can easily check the inertness of your cabinet (and thus how much spurious noise it is producing) by playing a low E on your bass whilst placing your (other) hand against the back panel.

As long as it fits in the hole, there is very little that could go wrong. Use some foam gasket round the Celestion when you fit it. Be VERY careful with the screwdriver - they have a tendency to slip off the screws and through the cone.

[quote name='RandomBass' timestamp='1377344307' post='2186485'] I wonder if BF are available on the NHS? [/quote] We should all lobby our MPs!

You can buy an Ashdown 1 x 15 for about £150 on here. Even brand new they're ony £350. What's a Compact - £700?

[quote name='alexclaber' timestamp='1377267953' post='2185624'] Yes you are! [/quote] Well I was only guessing but you certainly appear to have a lot of customers with bad backs.

There are two ways that the manufacturers of these lightweight cabs make them lighter. The first is by using drivers with neodymium magnets. These are equivalent in every way to the more familiar, heavier drivers and the weight saving is made possible because of a technological advance. Neo drivers are roughly half the weight of their ceramic equivalents. They are also twice the price. The second way of reducing overall weight is by reducing the weight of the cabinet. Because there is no technological advance involved here (such as carbon fibre panels, for example), manufacturers make the cabinet walls out of thinner or less dense materials to get the weight savings. It's fairly obvious that this is not ideal, because the cabinet is going to 'talk' when pounded with bass frequencies, but many people are prepared to accept the compromise to get the light weight. I may be wrong, but I have the distinct impression that most of the people buying these have bad backs. The PA industry went throught a similar kind of thing several decades ago when the plastic PA cab was introduced. Most of them make the singer sound like he is singing in a bucket, but they remain popular - especially with female vocalists. It interesting to note, however, that PA bass bins are almost without exception made from 3/4" stock because that is what has proven over many years to be optimum. A good, modern conventional cab will always outperform a modern 'lightweight' cab of equivalent price - especially at volume.

The 1.5V NiMH batteries I have only charge to a maximum of about 1.3V in practice but I get up to 10V from my 9V ones when they are fully charged.

And what would you think of a builder who insisted that you paid in full a month before he started the job? I'd tell him where to put his spirit level and I'd find someone else.

[quote name='alexclaber' timestamp='1376572694' post='2176245'] Regarding finish related things, if we switched to 1/2" ply then the front corners would be easier to fit because of the larger edge radius but the bare enclosures would then be about 1/2" bigger in all directions and would weigh 30% more without sounding any better. [/quote] Just out of interest, are you using 8mm akoume all round?

I suspect that Chris is concerned you might overload your cab - which you might. However, if you find it perfect apart from the very low end, it's certainly worth trying a couple of dBs lift around 50 to 60 Hz with a suitable equalizer - one that gives you a relatively narrow boost. I'd be wary if it needs much more than that.

What's wrong with the wheels - are they too small? I have 3" castors on my cab, and they're not great.

Mr Foxen's right, but why can't you eq some more bottom-end welly in?

Whilst that's true, sk, I don't think the handles are actually failing, are they? That would be quite serious.