Designer:

Bill S. (4thtry)

Project Category:

Tower Speakers

Project Level:

Advanced

Project Time:

20+ Hours

Project Cost:

Over $1,000

Project Description:

This speaker is my attempt to build a budget-conscious high end loudspeaker that would be comparable to some of the very best sounding loudspeakers available today. It is an active/passive 3 way using separate upper and lower cabinet sections. The upper cabinet section consists of 5 individual cabinets; one for the tweeter & passive crossover, and one for each of the 4 midrange drivers. The lower cabinet section houses an active pair of subwoofers which are driven by 250 watt plate amplifiers.

Design Goals:

My main design goal was to create a smooth, continuous tonal balance from top to bottom. I selected the tweeter and midrange drivers based on an analysis of individual driver directivity throughout the critical 1 to 5kHz region. In addition, individual cabinets were built for each driver so that I could time align the acoustic centers. This made it possible to place my crossover frequencies and slopes in the best possible design positions instead of having to develop compromised crossovers to solve directivity mismatch or time alignment problems. 

Another design goal was to include a very low 100-200Hz crossover point between the subwoofer and midrange drivers. I wanted most of the central midrange and upper bass frequencies to be covered by drivers with less moving mass. The same thing applied to the tweeter/midrange crossover point. I wanted to use a very low 1000-1500Hz crossover point so that most of the critical mid to upper treble region was being reproduced by a driver with less moving mass. A very low tweeter to midrange crossover point also helps to reduce vertical lobing issues.

Driver Selection:

For the tweeter, I selected the Bozhen CQ76B ribbon because it uses a very low mass ribbon element, yet has the ability to crossover as low as 1kHz, per the manufacturer. The ribbon element is attached to a voice coil, which makes this possible. I ended up crossing it at 1.3kHz, based on harmonic distortion and listening tests (see attached HD plot). 

For the midrange, I selected four Tectonic Elements TEBM46C20N-4 3″ full range drivers. These drivers are balanced mode radiator (BMR) types. They transition from pistonic radiation at low frequencies to “bending wave” type radiation at higher frequencies. The result is a very wide and uniform horizontal polar response that is very similar to the ribbon tweeter used. I used 4 of them per channel so that I could push the lower crossover frequency down to my target of 175Hz. My testing at fairly high SPL levels showed that harmonic distortion was still quite low at this frequency, consisting mainly of even order 2nd harmonics.

For the woofers, I selected two Tang Band W6-1139SIF subwoofers per channel. The individual 6.5″ diameter of each woofer is fairly small for a subwoofer application, but when you use two of them per channel they are equivalent to a 9.2″ diameter woofer with an xmax of 11.5mm. And due to the large, powerful magnets, two of these drivers work perfectly in a relatively small 2 cubic foot box tuned to 26Hz. F3/6/10 modeled out at an incredible 25/22/20Hz. Two separate Dayton SPA-250 plate amplifiers are more than enough to drive them to full output.

Enclosure Design:

The cabinets are somewhat unusual in appearance. They were designed for time alignment and for the smoothest downward sloping power response. The idea was to create separate cabinets for each driver so that I could move them around during the design process using screws, washers, and slotted brackets of various lengths (see photos). During the prototype development stages, I took measurements of several spacing and alignment configurations and then compared the data in the VituixCAD software package. The final and best design was a cabinet configuration with time aligned drivers and very close vertical driver spacing distances.

Enclosure Assembly:

Midrange: Each of the inner four midrange cabinets are sealed and are constructed of 3/4″ particle board material. They measure 3.5″H x 5″W x 9.5″D externally and are lightly stuffed with denim. The upper midrange cabinets are also sealed using the same material. They measure slightly larger at 4″ x 5″ x 9.5″ externally. The lower midrange cabinets are also sealed using the same material. They measure slightly larger, again, at 4.75″ x 5″ x 9.5″ externally. I varied the size of the outer midrange enclosures to spread the resonant frequencies out just a bit over the 100-200Hz region. Also, each midrange driver was flush mounted using a system of custom router templates.

Wedges: The upper and lower midrange cabinets are time aligned with wedges and bolted to the inner cabinets using four 1/4-20 bolts, nuts, and washers (see pics). Ken Rhodes printed the wedges for me using PETG black plastic. Since the wedges are black and partially hidden between the cabs, they give the speakers a somewhat unique looking”floating cabinet” appearance when viewed from the side (see pic). 

Tweeter: The tweeter cabinet measures 5″ x 5″ x 18.5″ externally. The Bozhen tweeter is flush mounted to the front. The crossover board is placed behind the tweeter and slides in and out for easy servicing or modification (see pic).

Upper cabinet base: I built a “curvy” base for the midrange/tweeter cabinet assembly. It consists of three parts: A 1.5″ x 10″ x 15″ base that is attached to a 2.25″ x 13.5″ curvy mid section, which, in turn, is attached to a 0.75″ x 5″ x 5.5″ top mounting plate (see pic). This places the tweeter height at about 43″ from the floor, which is about right for the average person seated in a standard folding chair. 
(Note that the upper cabinet assembly drawing that I attached shows a view of the cabinet before I decided to change over to an improved looking curvy type base.)

Woofers: The woofer cabinets measure 12.5″ x 16.5″ x 25″ externally and are constructed from 3/4″ MDF with an outer skin of 1/4″ oak plywood. They weigh 62 lbs each. The bass reflex port is mounted on the rear using 3″ID PVC pipe of approximately 13 to 14 inches with a single bend to keep from hitting internal obstructions. The Dayton SPA-250 plate amplifiers are mounted in external boxes that sit next to each woofer on the floor.

Crossover Design:

The crossovers were designed using the VituixCAD software package and a full 360 degree spinorama data set (CTA-2034). I measured each upper cabinet assembly in 15 degree increments from 0 to 180 degrees horizontally. Then I flipped the cabinets 90 degrees and measured them again from 0 to 180 degrees vertically. VituixCAD then mirrors the missing angles to produce a 360 degree ballon picture of the loudspeaker’s radiation pattern. The four midrange drivers were treated in my main model as a single coaxial driver, so this does introduce a small amount of convergence error in the model because all measurements were made at 1 meter to gate out room reflections. The model corrects for baffle stepping loss by merging the midrange far field spinorama data with near field measurements and a diffraction model data at 300Hz. The subwoofers were modeled separately and merged at approximately 100Hz into the main model. Crossover components were selected to produce a flat on axis and smooth, downward sloping power response and PIR curves. The design axis was defined as 20 degrees off axis to produce a smooth, downward sloping power response curve all the way out to 20kHz. VituixCAD optimization was selectively used to obtain the highest possible preference score rating.

Conclusion:

This project took me about 8 months to complete from start to finish. It is one of the best sounding speakers that I have completed to date. High frequencies are airy and smooth from the Bozhen ribbon tweeters. They consistently produce an even tonal balance from top to bottom either in my own listening room at home or when I take them to events such as InDIYana in Ft. Wayne, IN or the SDC in Springboro, Ohio. There is absolutely no tendency to sound bright or harsh in the 3 to 5kHz region. And they produce deep, powerful bass going all the way down to 25Hz with no sense of strain. I am very happy with the results.

Tips & Tricks:

This project was a huge learning experience for me, taking a deep dive into loudspeaker directivity matching using the VituixCAD software package and 360 degree spinorama data. This is now my 7th build using this system and I am now just beginning to understand the concepts and complex interrelationships involved. Most of my builds prior to using spinorama suffered from directivity mismatch problems. And the sad thing is that I kept misinterpreting the problems as something else. But I am now beginning to understand where I went wrong on previous builds. I would strongly encourage those that have not used the spinorama system to give it a try. I think you will be glad you did.

About the Designer:

I have been building speakers and amplifiers as a hobby since the mid 1970s. In my spare time, I enjoy football, fishing, camping, landscape photography, building amplifiers, and, of course, building speakers. I also enjoy attending the many DIY events in the Midwest to see what other speaker builders are up to. I do not have a formal degree in electronics or engineering. Instead, most of my skills have come from reading (and starting) detailed construction threads posted on the Parts Express Tech Talk forum, techtalk.parts-express.com.

Project Parts List: