For those who are interested, I am detailing the ideas, principles and build of my OB speaker project which many of you will have seen develop over the last few months in system pics.
The ‘project’ started as an experiment and a convenient way to run in a new pair of full range drivers with me fully expecting to use them in a closed cabinet design eventually. Anyway, despite obvious shortcomings (particularly the bass and sounding a bit coloured in the mids) the run-in baffles showed more potential than expected - the sound was free from the cabinets and what bass there was was very tuneful and quick.
I then decided to stiffen the baffles with a frame attached to the rear - this significantly reduced the mid-band colouration and adding the actively driven ‘bass modules’ made a significant change to the low-end, so much so that I thought I would research the principles behind OB speakers properly.
Many hours/days of research later I felt confident to tackle the project properly. Before that, some basics from my research:
Full range drivers have specific strengths in the critical mid-range. Basically, not employing a passive crossover in this region is a really good idea, unless you really know what you are doing! A passive crossover’s job is to:
a) divide frequencies going to different drivers - what those frequencies should be is determined by design goals, driver parameters, cabinet size/shape, driver spacing etc.
b) compensate for any phase anomalies resulting from sorting out a.
c) level match drivers as they all have different efficiencies - this is critical if you want a smooth frequency response. This involves padding down more efficient drivers in a system to match the least efficient drivers. This wastes amplifier power.
Crucially a decent FR driver eliminates the need for a passive crossover, significantly simplifying the build but also increasing efficiency and transparency by eliminating electrical components.
the principles of OB speakers:
i) easier build and no box calculations to make.
ii) low frequency roll-off is determined by baffle width - the narrower the baffle, the less bass response one gets.
iii) passive OB speaker designs require higher qts woofers to compensate for baffle loss - actively driving woofers below the baffle loss frequencies combats this and allows one to use lower qts woofers, which are more responsive.
iv) OB speakers need plenty of distance behind them to work properly.
v) OB bass interacts differently with a room - this can give a drier, leaner bass than sealed and especially ported cabinets.
Thanks to my chosen FR drivers being good to below 200hz in my initial baffle it soon became clear that actively crossing over and serperately driving woofers below that frequency was critical.
My next step was to start again with slightly smaller baffles made from much less resonant MDF with a vinyl floor finish at the front - for both aesthetic and acoustic reasons. These sounded clearer/less coloured than before. I then added 2 x 8 inch 8 ohm drivers per side to add bass below 200 Hz. This was very effective and made them into much more of a full range speaker.
More research then convinced me that I needed large OB drivers in larger rooms - cue me investing in a 15 inch OB bass driver. Building a baffle and a few tweaks later the 15 ‘sub’ has really fleshed out the lowest octaves yet it is still quick to respond.
Finally swapping out the little Chinese amps for a high SQ English made car amp (with it’s active crossovers and adjustable gains) gave the bass and ‘sub’ drivers a more full and textured sound to their lowest notes.
It’s been quite a project but all in all I’m really pleased with the outcome and I have produced a speaker set that easily fills a room, works well with lower powered amps, disappears acoustically and presents a lovely sense of space around instruments, which sound very textured and natural. The actively driven lower drivers require some electronic filtering and serperate amplification but that does free up the choice of main amplifiers with no punishing ohm loadings. With using the car amp and it’s supply I’ve freed up cabinet space so that everything can comfortably fit in the new IKEA Stockholm TV bench.
I hope some of you find it interesting and/or useful. Feel free to ask questions and comment.