The Less-Compromised Horn Project by Andrew Pearce (Drew P) (That great woodworking pastime of the somewhat insane.) "OK, so I've built a regular small two way, a D'Appollito Vifa floor-standing two way, a couple of Audax carbon and HDA based subwoofers so what now? I know, I'll design and build a horn from scratch!" No doubt some of you folks know this story or one similar. The saner of you out there will have found a copy of someone else's plans for a design that had straight internal panels and given a couple of weeks effort had a completed pair of functional horns using a Fostex FE83 or similar. Wise of you. Let me tell you my story...... Firstly, I did the usual hifi thing and gradually upgraded from my parents crappy 3 in 1 to a medium fi Monitor Audio, Sugden Optima, Arcam Alpha 5, VdH system. Of course, the next step up from this starts to get into silly money and like many of you I’d heard cheap systems offering elusive glimpses of fantastic sound and systems worth more than my house that left me completely unmoved. Hey, I’m sorry but when I walk into a room where U.S.$50K system is playing I feel ripped off if my jaw doesn’t hit the floor and I want to stay lost listening for a week or so. This being the case, I started out on my DIY career. Project one (and my wife’s favourite to date) a pair of open baffle 20mm thick mahogany cabinets (more backless than open baffle I guess) using a single Audax 100ZO HDA 90mmcone. 300 Hz to 6K, some of the sweetest and fastest midrange you are ever likely to hear, great imaging. Aint exactly fullrange but if it sounds nice… Projects 2 – 7 were for friends who found someone who would build subs and speakers for them and are of no interest to us fullrangers really. Just setting the record straight that I’ve been guilty of multi-way dalliances in the past. Bringing me to today (or if you ask my wife and friends about 2 years ago). Having built and loved the immediacy of the single driver Audax’s I was looking for more of the same. I’ve listened to big floor-standers, small micros, panels, ribbons and pretty much anything in between except for backloaded single driver horns and electrostatics. People have raved about the realism, image, dynamics and so on of horns ad nauseum so I won’t bore you all repeating it. Let’s just say that I was interested enough to want to give it a go. Research and Theory So we look at all the literature we can find from the Radiotron Designers Handbook and original Voigt Patents through to Ketil Parow’s Big Fun horn. Stop and admire the Bravura Virtuosos, Lowthers and Living Voices Air Partners along the way. My reading told me that: Everyone wants low bass to 35-40 Hz Everyone wants a smallish cabinet To achieve 1 and 2, Everyone sticks their horn in the corner at 1/8 of calculated mouth area and truncates the bugger as much as theoretically allowable (and usually a bit more) Almost everyone uses flat panels inside, constant width, aspect ratios of up to 20:1 and 3 or 4 180 degree beds in their horn path. Most people get a reasonable sound but lumpy restrained bass I decided to approach the problem from a different direction Horn to be floor not corner loaded (1/4 of calculated mouth) Fc 60Hz, rather have higher cutoff than lumpy bloated bass Tractrix contour 90 degree bends only Shaped internal corners with curves not angled plates Aspect ratio of 3.75:1 maximum and 2:1 or less for majority of horn OK, here I come clean and say I did still truncate my horn design and chopped off the last 40 cm of mouth. My horn area reduction method was also a little different to the standard. Most people calculate the horn at full area (mouth radius 90cm or 160 x 160cm for 60Hz cutoff!) then divide that area by 4 for floor loading giving a mouth 80cm x 80cm. They then run back along the calculations till they reach this area, discard the rest of the distance to the mouth and keep 80 x 80 back to the size of the throat. Usually in an attempt to reduce mouth area further they’ll chop off more of the mouth back beyond 80 x 80 anyhow. No No No!!! In my opinion, this results in a long, thin horn with a mouth that is no where near opening out to a full termination, you’re lucky if it is opening out at 30 degrees. My design method is to take the entire list of calculations, divide all the areas by 4 then discard the stuff that’s too small to fit your desired throat. Result, smaller area as per the standard method but shorter horn and a mouth that’s still at 90 degree termination. So then I was bad and did that 40cm of mouth end chopping, wrote down an list of areas, grabbed a ream of photocopy paper and disappeared for about 6 months drawing obscure line drawings that my wife and friends could look blankly at, nod and smile. Next step: Purchase your drive units. In my case the best I could afford was a pair of Fostex FE168 Sigmas. The graphs for upper response looked flat, they didn’t need equalisation like the 208’s and they were 95dB for 1 watt with a smooth impedance curve. Now to get the wood. Hangs head in shame and admits to the use of MDF not Baltic birch ply in the construction of a horn speaker. So shoot me! Hard to find, hard to afford and as this is my first horn design I don’t want to fork out my life savings for something that might not work. Hooray!! At last he gets to… The Building Part I figured out exactly what size panels I wanted for baffles, tops, backs, sides, bottoms and to cut up to make the internal shaped sections. Except for the laminated curve at the mouth opening, the entire horn is constructed from 25mm MDF. The curve is laminated from 5 sheets of 3mm thick MDF. PVA wood adhesive was used throughout and the internal cabling was VdH SCS16. (Sure CAT5 may be better, I can always play with cables later.) To date building has taken about 2 months of occasional evenings and weekends. I’m now at he point where I can listen to the finished product. The bases still need to be made, the cabinet corners routed to a curve and the surfaces sanded smooth. Currently these babies are as ugly as sin. Given that there are 2 full sheets of 1200 x 2400 x 25mm MDF in these speakers I’ll leave you all to guess what they weigh. My local hardware shop where I got most of the panels cut thinks I’m totally insane. For those who care, final cabinet dimensions exterior are 100cm tall, 60cm deep and 25cm wide. The base plates are not yet fitted but will be 67.5 deep and 35cm wide to lend the speakers some feline stability. Note the restricting passage of less than cabinet width as you approach the driver chamber. This keeps the aspect ratio closer to unity (1:1 as it would be for a square cross section, which is close to the ideal of a circular cross section). Most other rear-loaded horns have very narrow and long slots at the throat with very high aspect ratios. The deflector at the top corner back end is a 100mm radius. The horn path in that section of the box is obscured by the enclosure walls taking the width down to 100mm instead of 200mm but instead of a 45 degree deflector as is commonly used I went the whole hog and shaped the outside of the corner in a curve. like the rest of the interior profiles, it's a laminated stack of 25mm MDF sheets cut to shape (same as the curve at the bottom corner just smaller radius and 4 sheets instead of 8). More vital statistics Mouth 75cm x 20cm Throat 10cm x 5.5cm Horn length from throat to mouth 162.5cm Length from driver front to mouth 185cm Cavity volume at present is approx. 4.5 litres Listening tests Bass: The test tones say there’s precious little below 70Hz. Fortunately I spend about 20 minutes a decade listening to test tones so I don’t particularly care. I don’t listen to organ music. Yes, orchestral timpanis are restrained as are the lowest of kick drums. Bass guitar sounds great, techno and dance bass sounds great, jazz double bass sounds great, grand piano sounds like a grand piano for the first time in ages. (Listen to the Tori Amos cover of Leonard Cohen’s "Famous Blue Raincoat" of the "Tower of Song" tributes album.) There are panel resonances in the low mid upper bass which will reduce once the base plates are fitted. At present the bottom front edge of the side panels is free to flap in the breeze and does so. I may install a 25mm dowel across the centre of the mouth later if the resonances are still bothering me. Treble: Fostex FE168 Sigma units are directional. They drop treble output substantially off axis. My wife complains that if you sit to one side it sounds like you have cotton wool in your ears. True. Yes, the highest treble is restrained and is not as shimmeringly stunning as it could be. Imaging is fabulous. The single driver Fostex units image pinsharp and the bass horn seems to lend piano drumskin hits and bass a sense of palpable scale that I just haven't heard in standard box speakers. I’d say these give a pinpoint precise but live scale image. They can pull apart the different singing voices in a harmony and give a sense of stage depth like no other speaker I’ve heard I can now understand what people say about the "They are here" rather than the "You are there" aspect of horns. Dynamics are great, real drum skin smack and guitar attack. One thing I have noticed is that these backhorns are astoundingly level dependant. They drive the room so well that if you play too loud it’s just a bloated overkill, too quiet and things just don’t sound right. When you get the level right WOW! It sounds sad but I’m seriously considering taking to my CD collection with a felt tip pen and marking down the right level for each track. In the past music has been too loud or too quiet. Now it’s just right or wrong. I’m very happy with the result for my first attempt, no doubt once things are run in, the cavity volume is tweaked, the panel resonances reduced and the cosmetics completed I’ll be even happier. These beasties still have much work to do. I wrote my initial appraisal (above) with the glow of a listenable project clouding my vision a little. I have a fair number of colourations to kill. The enclosed volume that isn't part of the horn contour (the bit forward of the rear contour but behind the mouth curves) is singing along and I think could do with sand filling. (What!! More weight!! At least no-one is goingto steal these as happened with my Monitor Audios!) I need to glue on the base plates which should stop the sidewalls from flapping as much. I think the 25mm dowell across the centre of the mouth height about 150mm back might help too. Ahh, tweaking, tweaking, tweaking. Stop reading this! Go design and build your own pair of horns! Andrew Pearce (Drew P) Top