GoodSoundClub - Romy the Cat's Audio Site

In the Forum: Horn-Loaded Speakers
In the Thread: Good midbass is complicated, if not unobtainable.

Post Subject: Choosing a Bass SolutionPosted by skushino on: 9/27/2013

I was rushing when I made the last post. Wanted to share a little more about my thought process and how I arrived at doing another large-scale masochistic audio project. Making a bass horn is a huge risk - the results can vary from exceeding expectations to ending up with something that doesn't work at all, or just partial success. One won't know the outcome before building and listening.

I'm currently using two channels for bass - tapped horns from the bottom up to around 100 hz, and a 142 hz tractrix horn w/ 4" throat covering around 115 hz - 800 hz. It's well known that tapped horns have benefits in form factor and packaging, but they sound progressively worse the higher they go. I can hear the benefits when I listen with the crossover set lower and lower. Naturally I can only set the crossover point so low before a hole opens up between the THs and the upper bass horn. I would like to end up with the THs limited to a narrow bandwidth, around 20 - 50 hz. In this range they work fine, pressurizing the room per expectations.

Another goal is to raise the 'center of gravity' of sound presentation. The horns are presently set-up with the upper bass horn physically lowest, with the mouth resting on the floor. The mid and treble horns are above the larger upper bass horn, around 36" high. This works ok, but I think there's an opportunity to improve the sound presentation by raising the overall height of the speakers, so that the apparent center of gravity (center of acoustic mass?) is taller. Re-arranging the horns so that the folded bass horn serves as a base for the other horns, located closest to the floor, and raising the 142 hz horn up high, closest to the ceiling, should move the entire sound stage up, for more life-like sense of scale and size.

Another consideration is how to power everything. I played around with different amps dedicated to single or double channels. I'm generally happier with one amp powering all. My amps are ML-2s. Operating in the SET world, the bass channels need to be 'SET friendly' - benign impedance and sensitivity congruent with an 18W tube amp. I considered building a bass tower with direct radiators in sealed or ported cabinets. I think these could be made to work acceptably well. But the horn topology seems more congruent with the other horn channels and the limitation of using a single channel SET power amp.

So a folded horn seemed to be an acceptable compromise for my application in my 16' x 18' listening room. The Dean bass horn is elegant in it's simplicity, and has an adequately sized mouth, throat, and path length for the freqs I wanted covered. There are no sharp bends in the path. The numbers for length and area are acceptable (but they too are compromises). And it passes the 'intuitive' test - the cabinet measurements are way too big for my standard sized listening room, which means it could be large enough to work as a proper horn over most of it's range. So the cabinet dimensions are adequate - how about the driver? The C-15W makes my Altec 515s look like toys. The magnet is massive. The frame is massive and strong. And the cone is all paper, including the stiffly-suspended surround. The TS numbers seem reasonable (although I haven't measured my pair, relying on second hand measurements).

I hired someone with wood and metal working skills to do the actual fabrication per my guidance. He's a quirky character, artsy and sort of a loner. He has a strong sense of aesthetics and seems to be as excited about the project as I am. I've positioned the project as a combination of wood and steel media, aesthetic proportions, textures, and colors, and sound and music combined in to a grand sound sculpture.

The pieces are in place to move forward. I'm committed to writing the check, and having a significant interruption in my life for 2 - 3 months, all without any guarantee of the results. It feels like that first step jumping out of an airplane with a parachute in to the void, or boosting a jump down a mountain on skis, deep out of bounds.

Here's to soft landings.

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