Time to tackle the undesired resonance around 400 Hz which shows itself in both the impedance plot, the port output, and even the driver responses.  While reading Testing Loudspeakers by Joseph D’appolito, he described two projects where an imperfection in measurements similar to Speaker III’s was caused by cabinet bracing that formed double resonance chambers.  In both cases the bracing piece had small holes in them to allow air passage.  However, the holes ended up being acoustic masses because of their small size, creating resonances.  The solution in all cases was to enlarge the holes so that they did not act as acoustic masses.

The Dayton cabinets have a vertical brace which has two perfectly round holes.  The holes are large, nearly the width of the cabinet, so I was skeptical that a double resonance chamber could be the issue.  Then I measured the port output.  Time to perform some cabinet surgery with the saws-all.  I cut an additional hole in the middle of the brace.

Next step, repeat all of the measurements.  First up is the impedance measurement.  Interesting, the impedance artifact is at a different frequency – 450 Hz.

Here is the good news – before (green) and after (blue) port measurements.  Notice the 400 Hz peak is gone, replaced with a slight wavering in the port output between 400 – 500 Hz.  Yes!  Also notice a few other changes… the port tuning has shifted up several Hz, and the pipe organ resonance is stronger than before.  The cause in both cases is the material removed from the brace was also in front of the port tube, increasing its effective length.

Driver’s mounted – check.  Cabinet port cut – check.  Cross-over – parts on order.

So I drug out some Dayton Audio bi-amplifiers.  What is a bi-amplifier?  Two amplifiers, one each for the tweeter and woofer, along with an electronic cross-over circuit.

These were the second series Dayton Audio produced.  The high- and low-pass sections have separately adjustable cross-over points.  Frequencies are limited to 2.2 kHz, 3.2 kHz, 3.8 kHz, 4.2 kHz, and 5 kHz.  Order is 4th Linkwitz-Riley.  (In the first series, the cross-over was fixed to 3.0 kHz.)  The electronic crossover also has a +4 dB bass lift circuit.  Presumably the bass-lift is for bass extension.  Looking at the lift cut-out frequency, the circuit actually has the appearance of a diffraction compensation.

Results – sounded OK with the lift compensation enabled.  Woofers are crossed over at 2.2 kHz, the tweeter at 3.2 kHz.  Just sounded a little bright, maybe 3 dB to hot on the tweeters.

After 3 years I finally found some time to work on Speaker III.  Getting back to the cabinets, what they need are holes for the port tube and egg crate acoustic treatment.  A couple of bare spots, one each on the top and bottom, are for the crossover boards.  One board for the low-pass, the other for the high-pass, to separate the inductors as much as possible.

The next crossover design. The target for this crossover is a 4th order LR at 2.5 kHz. The tweeter circuit is electrically 2nd order at 3.6 kHz which combines with the natural 2nd order low-frequency roll-off of the tweeter at 1.0 kHz. Two tweeter circuits are shown… the top is a basic 2nd order with attenuator. The bottom circuit also includes a phase shift circuit. The mid-range circuit is truly 4th order electrical, although it is not a electrically LR.

Here is the crossover prototype boards. Sub-circuits are on separate boards so inductors can be separated in the cabinet – tweeter crossover, tweeter delay and attenuation, and midrange crossover.  The alligator clip jumper wires will be explained below…

Testing determined two necessary adjustments. First, the tweeter was “hot” by about +2 dB.  The attenuator resistors were immediately adjusted from -3.5 dB to -5.5 dB of tweeter attenuation.  Second, a suck-out at 3-4 kHz can be seen in the response graph below (lower trace).  The suck-out was verified to NOT be a polarity issue.  An appropriate deep dip was observed at 2.5 kHz when the tweeter polarity was reversed.

The dip was mitigated by increasing the tweeter output in the 3 kHz region by lowering the crossover frequency and increasing the filter Q.  Adding a 4.7 uF in series (via the jumper wires shown above) for a total of 13.7 uF did the trick (upper curve).  Note the curves are measured under identical test conditions – a 10 dB offset is added for ease of visualization.

These results emphasize the importance of having some type of acoustic measurement capability when designing loudspeakers!

It has been a long time since I have worked on a speaker project.  I have lots of parts sitting in boxes, just longing to become a full, functional speaker!  So this holiday season I decided to get back onto the horse and finish up Speaker III.  The first step is to figure out where I left off.

The first problem is the shielded tweeter.  I only bought two, and they are not available anymore.  This is not a bad problem – LCDs are the norm today and magnetic shielding isn’t as desirable as when I started Speaker III in 1998.  I found a substitution in the way of the Vifa D27TG-05-06.  It is a silk dome with ferrofluid like the D27SG-05-06.  Efficiency is similar between both units at 91/92 dB/2.83v, as is the resonance frequency of 1,000 Hz.

It is a lot of work to make your own cabinet with the number of small internal pieces.  So I decided to use a pre-fabricated MTM (mid-range / tweeter / mid-range) cabinet from Dayton Audio.  Internal volume is 0.75 cubic feet. The front baffle is removable and ready for your driver cut-outs.  Here is the Front Drawing with tweeter and mid-range locations and cut-outs.  Note how the lower mid-range is further away from the tweeter than in Prototype #3 (I will show why this is not good later in testing).

Well, I didn’t save myself that much work.  I wanted to do a round-over on the mid-range cut-outs based on my research into cleaning up the tweeter response.  I couldn’t find a router bit that would let me do that – not at a price < $100 anyways.  The solution was to make my own baffles out of two pieces of half-inch particle board and glue them together after making all the routes and holes.

Baffles drying after application of epoxy

Where did the work come in?  Trying to finish the baffles as nicely as the factory baffles.  To achieve a smooth finish I coated the baffles with an epoxy product.  Which I then sanded.  And then applied another coat.  And sanded.  And again.  And again.  Then I finally sprayed on a black laquer.

The design of the first cross-over is done. The target is an acoustic 4th order Linkwitz-Riley at 2500 Hz. My simulated target responses using measurement data taken using a ground plane at 72″ is on the top (the data is not measured with crossover network, it is measured then a crossover simulated).  The bottom plot is based on a simulation of the drivers + crossover (which is why it is so smooth).

The electrical network is shown below.  Electrically the tweeter slope is 2nd order, and the mid-range unit crossover 3rd order transitioning to 2nd order (that is what the 1.5 ohm resistor does connected in series with the 18 uF cap).  Zobels are used on the mid-ranges.  Zobels are even on the mid-ranges before and after the 1/2-way cross-over inductor.  (Note shown: the capacitor value is 45 uF in series with 7 ohms for the Zobel to the left of the 1.8 mH inductor.)  The Vifa D27SG-05-06 has a faster high frequency roll-off than similar tweeters.  To compensate, the attenuation resistor is by-passed with a 15 uF capacitor.