Phil Starr

It's probably worth knowing that Celestion measure their Xmax differently to most other manufacturers so the 'true' level of excursion for comparison with other brands will be something comfortably above the 4mm they quote. Xmax isn't a hard limit, the speaker will go on producing an output without endangering itself beyond Xmax. The magnetic field extends beyond the magnet but weakens as you get further from the magnet. For the speaker this mens that the speaker begins to struggle to follow the signal accurately and it starts to distort. Xmax is an attempt to put a number on the linear part of the speaker's output. At the frequencies we are talking about, below 100Hz, you probably wouldn't notice 10% distortion. The old fashioned and harshest way of measuring Xmax is just to measure the voice coil and magnet gap; 16mm and 8mm in the case of this Celestion. That leaves 8mm of movement with the coil inside the complete 8mm gap and Xmax of 4mm each way. The other ways of quoting Xmax are to measure it at a specified distortion (often 10%) or to do a 'predicted' Xmax based upon the speaker's geometry, adding a bit of extra Xmax to allow for the extra bit of magnetic field beyond the gap. Both these figures will add excursion and to be fair will reflect real world behaviour. You can guess that marketing prefer a bigger figure. 18-Sound don't have details of how their Xmax is measured on the data sheet but I supect it is calculated with a bit extra and more generously rated than Celestions.

Well Santa brought me the set of ‘Holmer’ nut files. I’ll practice on some suitable material before risking my real nuts😅

Welcome to BassChat

happy christmas to you too Rob

I'm really sorry to hear that too. I hope she recovers quickly.

We will have to agree to differ on this Bill. You can see the frequency irregularities around the crossover point in pretty much any frequency plot I've ever seen or measured and the irregularities are audible as well as measurable. Without wanting to get into a philosophical debate about the nature of reality IMO if something is measurable and audible it is real. There's no doubt the software is getting better and will get better still but again IMO it isn't there yet. The measured plots still don't exactly match the computer's modelled responses and most of the big designers are going to use the software to get in the ball park and still use iterative measurements to check and hone their designs. Again I suppose it depends upon what you mean by "an exact result" it may be good enough but I'm a scientist not an engineer so exact is a very big statement.

It's looking really good Rob, don't do yourself down. There are two issues with the grille. You need to keep it flat and not let it rattle and then it needs to be removable for any maintenance. If the back is stuck on then the speaker hole is the only way back in. It's fairly conventional to use either velcro or the little neo magnets to hold something to the speaker. I'm not sure how well the magnets would work to just hold the grille rigidly in place. If you drop the speakr or somebody kicks it then it will get pushed towards the speaker it is meant to be protecting. I cerainly don't think just magnets in the corners would be enough, and would it rattle at higher sound levels. I'd probably want the grille screwed to a frame which supports the grille along the whole length. Use the magnets to fix the frame to the baffle. Or you could print some sort of peg system to lock the frame to the baffle.

You cut up the car! 🤣 Try using a bit of timber or hard plastic to concentrate the forces. You can then concentrate on the actually bent bits.

That's a great question and what your measurements have uncovered is one of the problems that have cursed speaker design generally and crossover design in particular since they were invented. If not really a secret it is something buried inside any speaker with different speakers handling different parts of the frequency spectrum. It's approaching impossible to have the two speakers in phase at the crossover point and it is practically extremely difficult with a passive crossover to get the roll off of each speaker to be symmetrical. Inductors and capacitors induce their own phase shifts too so time alignment of two speakers at crossover is still really difficult. https://www.learnabout-electronics.org/ac_theory/ac_ccts_51.php Bill has pointed you to the Altec "Voice Of The Theatre" A7 which is a really interesting design. Large installed systems of the time frequently had horns mounted on a sled. Active crossovers and DSP allow delay to be brought into the crossovers hence the rash of 'Firphase' into PA speakers, this is the problem it is addressing. Practically then you can pretty much always see a frequency anomoly at crossover. the designer has the choice of a blip or a dip and crossover distortion is audible in critical listening tests. Usually it shows up quite clearly and affects female voices in particular. We are really sensitive to any change in voices and the frequency of crssover is usually well within the range of female voices. Practically a dip at crossover is usually less obtrusive than a blip. Try reversing the wiring and you should see a change in your measurements and hear differences in voices and instruments in that area. Acoustic guitar often changes too but I use classical music for testing this sort of thing. Even tiny changes in a crossover can make an instrument pop out in a recording. You can choose your distortion mostly I prefer the dip. @stevie is the expert in this and the detail he goes into with his crossovers is extraordinary. It's really worth getting hold of a copy of the Loudspeaker Design Cookbook by Vance Dickason if you want to read up on all this, and much more.

Not very helpfully it took around 15mins, so I'm not sure what is going wrong. I've bashed a lot of metal in the past when I welded my own car bodywork amnd I've dressed a lot of lead when working on the roof so I'm probably fairly efficient. You kind of settle into a nice steady rhythm keeping the work moving. I'm trying hard to picture what you might be doing. I look not for the high spots but the bits where the metal is most bent. Are you perhaps going for the middle. The bent bit is around the cicumference around 2-3cm in from the edge (roughly, I dont have it in front of me). Knock these flat and the centre will start to lay flat. Push the middle down without taking out the edges will just mean it springs back.

Sorry I should probably have offered a bit more advice. If you aren't used to metal bashing steel is really forgiving but you can stretch it and even bruise it if you are too rough. use a soft faced hammer in this instance and keep the hammer square to the work. Swinging in an arc will mean you are beating with the edge of the hammer concentrating the force on the edge instead of spreading it. Find a flat rigid surface supported so it wont move or bend. The idea is to 'push' the metal down onto this surface. I found that I could simply push the sheet flat with my hands to start with. That takes out the convex curve but leaves the edges which have a more distinct fold in them. Turn the grille upside down and start to adress that. Use gentle blows with a soft hammer and remember you are trying to push it into shape so you can go gently and use repeated blows rather than trying to knock the hell out of it. Work steadily around the piece. You'll now find bits that were up are now down so you'll need to flip the grille again and try from the other side and it will gradually get flatter. The other thing to note is that beating the metal heats it slightly so lots of repeated blows will soften it, slowly working your way around means you'll find the metal starts doing what you want. I find it quite therapeutic That makes it all sound a bit complicated, it isn't. Just work steadily and it's a job of a few minutes

Flatten it first, and go gently as the paint flakes really easily

It is a really good fit and possible to get the thing cut so that the grille is centred nicely. I made a picture frame, screwed the grille to that and used velcro to fix that to the cab. I did cut the plastic covering strip to fit the edges but decided it looked better plain and I deburred the edges with a bench grinder. If I did it again I'd probably fold the edges and trim them down to 10mm. I practiced this with off-cuts and it was easier than I thought, clamping the metal between two bits of timber and beating it down. Folding it this way stiffened up the structure really nicely.

I may be able to help with the lift Not literally of course, and you may not thank me. It involves another build. I enjoyed playing around with my 15 and the Bash coincided with a couple of people asking for a 15" design. So... I've quietly adapted the design I have to be an 'Easy Build' project and when the drawings are ready it will be good to go. It weighs in at around 14kg at the moment which for me is a one handed carry.

I ran sound at free festivals in my 20's. Turnarounds averaged 15mins but were very band dependant. You never knew what you were getting but the pro's were always easiest to work with. Steve Winwood turrned up with Traffic refused to go on stage 1 because of the sound and wandered across to our stage listened to the band I was mixing and asked to come on. Are you kidding me! . A total gentleman, no fuss: "I'll play and you make it sound as good as you can" The easiest band I ever had to mix for. They knew what they were doing, the onstage sound was great and all I really had to do was make it loud out front. They even said "thanks" at the end. Sometimes it isn't the sound guy.