Which Tannoys, folks?

[Reffc]

Hi Paul

Have you considered Deflex Panels rather than all that wadding - I had good results with them years ago - they kill the box reverb without deadening the sound.

http://www.madisoundspeakerstore.com...tandard-panel/

I have Martin, but at £17 each (280 by 210mm) I would need 50 of them so way too expensive! What I have designed is tried, tested and cheaper. Bitumen sheeting helps damp resonance and also helps to pull resonant frequency down below the critical mid range. 12mm dense wool felt helps with acoustic damping, the foam panels help with absorption of mid frequency to high frequency reflections and the long lambswool helps with attenuation of LF. You want the cabinets to be as dead as possible resonance wise and internally very well damped so that there are no reflections colouring the information coming from the front of the driver. You can over do damping, but not as you may think. Add too much and the driver sees a virtual volume greater than the actual volume so that you can reduce SPL in bass output, but I'm with the school of thought that there's frequently not enough internal damping in loudspeaker boxes.

[Macca]

I have Martin, but at £17 each (280 by 210mm) I would need 50 of them so way too expensive! I'm with the school of thought that there's frequently not enough internal damping in loudspeaker boxes.

Fifty! You don't need to coat the whole interior with them! I guess we differ about damping - I think most speakers have too much. I alwyas used 3 quarter inch chipboard all round for cabs so pretty dead sounding anyway.

[Reffc]

There's various schools of thought but I want the panels to be as acoustically dead as possible and the construction is designed to be very dense and very stiff. I much prefer Birch Ply for cabinets to the chip board, but most timbers can be made to work with adequate bracing. I'll be using three internal cruciform braces. I know that the design works because there's been very similar and very successful designs carried out in the past using similar design/technique to that I'm using. Chipboard just doesn't have enough panel stiffness for my liking.

[walpurgis]

Your system damping ideas make sense to me Paul. It's good to hear somebody describing accurately what the various types of materials are useful for and how to use them. So many people cock that up.

[Reffc]

Thanks Geoff. I must admit, I've seen (heard?) many more under damped cabinets than over damped! Most problems seem to revolve around inadequate front baffle mass/stiffness. You need both high mass and stiffness to lower resonant panel frequency AND to limit panel excursion to minimise distortion. It always puzzles me to hear comments about over damping the inside, when it does require at least 70mm depth acoustic foam and wool all the way round, with twice that behind the driver!

[walpurgis]

Well I'm a firm believer in bracing everything I can sensibly do inside a cabinet, as well as the damping of course and I too like the front baffle to be as substantial as possible. I often brace the main driver to the rear panel as well.

[Reffc]

Some more progress. Came out of hospital this morning and told to rest, so naturally my time has since been spent in the workshop where the speaker side panels were busy "cooking" in the DIY laminating machine. This consists of a table top with a hinged alloy frame and inlets for vacuum pipes connected to an electrical motor and vacuum pump. It generates 14lbs/sqr inch pressure on whatever former is placed into it (there's a tough plastic cover attached to the hinged frame which pulls down over the former). Once the motor's started, the plastic pulls down evenly across whatever piece of work you want to put in there, and holds it for gluing/forming for as long as needed. Each of these panels was in for around 4 to 6 hours:


Machine in action with one of my speaker side panels being formed from 6nr 4mm Birch ply sheets glued with PVA:



Vacuum pump turned off and former lid removed:



One of the panel edges had rippled slightly but not as bad as it looks since the panel width is oversized to allow for this edge rippling which can sometimes occur. It'll be trimmed up and will be fine:



One of the laminated panels at rest for a day or so after gluing up to allow for any further movement before final trimming:



Each of these sides weighs approximately 15kg! You can imagine what each speaker will weigh with two of these, the front and rear panels (front panel is 30mm thick!), bracing, damping and drivers. It will be a two man lift per speaker for sure.

[Reffc]

A little more progress in between jobs...these are now taking shape!

First dry mock-up of first speaker cabinet today. Due to very slight deviations (a few mm) along the length of the curved side panels, making a template and using the same uniform shaped ribs didn't work, so each rib has had to be custom tweaked by a few millimetres to ensure an exact fit. the square panels and uprights are not part of the build; they're just there to aid stability of the mock up. This is just the beginning and there's loads of work left to do, but at least you can start to get a feeling for the height. These are about 4 feet tall:




Close up of one of the ribs:



More to come next week.

[walpurgis]

Cor! You don't hang about do you.

It's getting more interesting by the day.

(just seen a really nice looking pair of Mansfield cabinets on eBay by the way)

[Reffc]

Cor! You don't hang about do you.

It's getting more interesting by the day.

(just seen a really nice looking pair of Mansfield cabinets on eBay by the way)

Yes, they are my Mansfields. Someone bought them and asked for a cancellation because he thought it was for the speakers including drivers despite the block capitals stating it was for cabs only. He has not offered to pay my re-listing fees either, despite being invited to. Some people...Grrrrrrrrrrrr.

The speakers under construction still have I guess around 2 to 3 weeks left before they're finished. Once the panels are all cut, formed and tweaked for fit, they have to be veneered before final construction as they wont fit in the the vacuum former used for pressing the veneer whilst gluuing unless in individual panels. The veneer has first to be trimmed and bookend matched. The veneer is pretty stunning in the flesh.