Is it all about the BASS? Subwoofer Build

DESIGNING A SUB (Part I):

{Please note: this is a retrospective build thread.  The sub is finished and I am now documenting the design and build process.}

Hello again!  Sorry for the delay in starting this, life has been busy and it's taken a while to find time to begin.  

This is probably going to be a longish build thread because (as always!) I tend not to make things easy for myself!  I'll be taking you on a journey of designing (the cabinet) and building a 10" subwoofer with passive radiator and Hypex plate amp.  

My Subwoofer journey started about 4 years ago when I owned some Wilmslow Audio MTM (Midwoofer-Treble-Midwoofer) Gemini standmount loudspeakers.  These speakers were my first (semi) DIY build and I loved them.  I did, however, find them a little bright and lacking in decent bass extension -  they were good down to 40/45Hz, but not much below.  I ended up with 2 REL t7i subwoofers and theses were a revelation.  I didn't think subs were for real hifi and was previously convinced that they couldn't be integrated seamlessly into an 'audiophile's' world.  After weeks of experimentation and learning (and a lot of help from REL) I got everything dialed in well.  The bass was tighter, more extended and better controlled and more importantly I had virtually eliminated some pesky room modes which had previously dominated some tracks.  Was is all about the bass?  NO!  Much to my surprise the biggest improvement was in the soundstage and richness through the midrange and treble.  The soundstage was much wider and deeper and the mid/treble had way more richness and sounded fuller.  It was a complete revelation.  I loved how the new sub enhanced sound and couldn't bear to turn them off.  When the subs were switched off you could immediately hear the soundstage collapse.  The music became harsh and incoherent by comparison.  How had I spent so long without a sub in my life?!  

Move on a few years and the Geminis now have a new home with a mate who loves them.  I have built some Troels Gravesen designed Ekta MkIIs (build thread to follow sub) - these are medium-sized 3-way floorstanders and comfortably reach down into the 30-35Hz region.  The REL t7i's still added something to the Ekta's but the difference was subtle.  The RELs were simply not going deep enough to add much extension to the Ektas.  They still improved the bass a little and opened up the soundstage slightly, but were simply not deep enough anymore.  The REL's needed a new home and I needed a new sub or subs....

I love my Troels Gravesen designed speakers so decided to go for a Troels DIY sub build.  Troels is very well regarded in the DIY speaker build world so I chose his 10" Scanspeak Subwoofer.  I would build one first and plan to add another at a later date.  Here is the information on the Troels 10" sub:
www.troelsgravesen.dk/26W-4558T00.htm

I realise a 10" sub will not go really deep, but I couldn't justify (to myself or my wife) a massive box in the Music room.  I had to get a reasonable performance from a roughly 400mm cube. The sub comprises a Scanspeak 26W/4558T00 with a Scanspeak 26W/0-00-00 passive radiator and a Hypex FA251 plate amp.  I realise a sub can be sealed (with and without a passive radiator) or ported.  There are many different designs all with advantages and disadvantages.  I do not understand enough to choose between them, so chose to just copy a Troels design - albeit with a significant cabinet redesign.   The Troels cabinet is a 400mm cube.  I needed to maintain a very similar volume but wanted to reduce the frontal area slightly - this would soften the visual impact and massing of the sub.  I decided to go for a 390mm x 390mm front, which would result in a 430-440mm deep cabinet.

After some thought I decided to go for a super solid and rather time consuming laminated cabinet design!  I've always wanted to do a laminated cabinet and a sub would greatly benefit from a rock solid and tough construction.  I realise this is extremely time consuming, but I love building stuff (especially woodwork) and wanted a project that would challenge me and keep me busy for some time.  A conventional subwoofer 'cube' is 6 panels fitted together with bracing to minimise panel vibration and prevent the thing shaking itself to pieces.  Sub drivers are powerful and the cabinets can experience some pretty large vibration forces, I wanted a cabinet that was built like the proverbial brick outhouse!  A laminated construction would comprise many layers of plywood all glued together to create a solid block.  The sub would, of course, require an internal volume so the layers would need a considerable amount of material removed from their centres.  In addition, the layers would require integral bracing.  I planned to drastically deviate from the Troels suggested design, so needed to ensure I maintained some key parameters and didn't compromise the original ethos of the design.  My cabinet must maintain a similar volume (plus/minus a litre or two) and must comprise the main driver, passive radiator and Hypex plate amp.  The Passive Radiator cone weighed 400grams, so Troels advised that it should NOT be placed on the base of the cabinet - he was worried it would sag over time and compromise it's performance.  He placed the Pass Rad on the side, however, I wanted the sub to look balanced and symmetrical from the front, so would need to place the pass rad and Hypex on the rear.



My next aim was to make a sub looked pretty!  Subs are usually black cubes, which isn't ugly, but I wanted something that looked like a piece of furniture.  We are planning to renovate of house and the stereo would eventually (maybe 2023) relocate to a music room.  We have a 1960's house, so we plan to renovate in a mid-century style and we (my wife and I) want the music room and the stereo to look nice and fit the new style of the property.  This meant the sub needed to conform too - no black boxes allowed!   The trans laminated design would allow a large radius on the corners - this would soften the design and make the sub look smaller.  Birch plywood would be used and the layers would be visible over the sides, top and bottom.  As an added feature the front baffle would be inlaid with a veneer.  This would add interest and help the main driver blend into a darker background.  I may add a grille at some point, but am still undecided.

That's all for now, DESIGNING A SUB (Part II) will cover the design of the rear (passive radiator and Hypex placement) and bracing.