Bear with with on this. My back is giving me grief at present so I have given up trying to get to sleep. Years ago, perhaps 15 years, I was in LA on short break (no not Luton Airport!). Fed up with the awful FM radio channels I went into the local Virgin Megastore to stock up on some CDs for the car. Whilst there I came across an inexpensive ($30) portable speaker package which used exciter panels to power a pair of folding cardboard pyramid speakers. Sounds awful, but they had one on display and I bought one immediately. It turns out that the package was made by Sonic Impact and included one of the first amps to employ the Tripath chip. It could be powered by either AA batteries or a 12v power supply. That amp became extremely popular and rightly so. I still have mine today. However, equally impressive were the SoundPax speakers. Once unfolded they were about the size of a traffic cone. Of course they had no real bass output to speak of, but what they did have was superb clarity and an ability to reproduce fine detail within a recording. They were perfect for travel and hotel listening, weighed next to nothing and took up little space in the bottom of a Samsonite suitcase! Unfortunately, mine met with a premature end when the air conditioner in my Dhaka apartment started leaking. I returned home to find my cardboard speakers resembling a partially deflated Zeppelin in a soggy pool of water. RIP (rest in puddle)!

I recalled those speakers when I recently decided to acquire a new pair of speakers for my home office. I am sure I am not alone in seeking a specific kind of sound from an office speaker. I like to listen to music when I am working (on non demanding tasks anyway) but do not want to listen at the volume levels I set when listening to music seriously. Nor do I want a deeply extended bass as I find that highly distracting. What I want is a clear, detailed sound that sounds good at low to medium listening levels. I also felt guilty at having relegated my 40 year old Rogers LS35a speakers to such a lowly task. I decided I would try to build my own speakers and found that Tectonic Elements had inexpensive exciter panels for sale on the RS website. However, whilst browsing I found they also had a number of inexpensive balanced mode radiator speakers for sale which use technology I understand is partly derived from research on exciters. Twenty odd quid and 24 hours later and I had two full range BMR drivers on my desk.

These are designed for an 86mm opening so out came the hole saw which I used to make suitable openings in a couple of pieces of hardboard, each 50cm by 70cm. Immediately, the sound was close to what I was looking for. The bass was limited but (Goldilocks like) was just right for me. Of course the hardboard was just a starter and was too resonant to be a permanent solution. My next step was to try a couple of cardboard tubes (poster tubes in fact). These were exactly the right diameter to accommodate the drivers and proved a good fit with some double sided tape. Again, results were good with a somewhat deeper bass output, no doubt due to the length of the tube. However, the cardboard tube was quite flimsy and resonated when the volume was cranked up. The experiment has progressed to some heavy duty fibre board courtesy of a now redundant Ikea cabinet. This dealt with the resonance problem and in truth I could live with the sound. The bass output was less than that of the original larger hardboard panels but better in every other regard. Out of curiousity I rested two board against the sides of my DIY panel to form wings and that restored the bass output. (see image below).

I have several options now to finish my DIY project. Stick with the small panel and properly attach wings to provide a better bass response. Alternatively, place the drivers in an MDF panel of the same dimensions as the original hardboard (i.e. 50cm by 70cm). Another route is to try and get a couple of heavy duty cardboard tubes from the local carpet shop. One advantage of the tube approach is that I can affix the tubes to the rear legs of my office table and have the drivers firing upwards. Whichever path I take I will have a pair of speakers that meet my particular needs for a specific environment. I would encourage other forum members looking for a similar type of speaker to try this out for themselves. What have you got to lose? OK £30 quid or less!

Geoff


BMR DIY.jpg