After further listening and testing, the tweeter appear to be just a little too bright. Above is a 1/6 octave smoothed measurement at 18″ (this distance compares well to measurements at 36″, only with better signal-to-noise). Both left and right speakers are measured. Below is the left speaker and the tweeter, smoothed to 1/3 octave. It looks like about 2 dB more tweeter attenuation should even things out, which brings the total tweeter attenuation to -5.5 dB.
The final result, smoothed to 1/6 octave:
So I have noticed during testing that, when I add a second order time delay network between the tweeter and the cross-over, the response above 10 kHz changes. That doesn’t make any sense at all – the time delay network should only change the phase.
The blue line below is the starting point of this experiment, and the yellow line represents the “solution” to this dilemma.
The solution? Adding a Zobel to the tweeter to bring down the inductive rise in impedance. How much of an impedance rise? Almost nothing!
Maybe there is something else going on… I measured this multiple times and obtained the same result. Measurement conditions? The mic is 6″ away, output level set to -31 which isn’t too loud, no indication of overload on the mic pre-amp.
For this test the woofer cross-over is fixed. The lower mid-woofer has a 2.0 mH inductor in series, the whole connected in parallel with the upper mid-woofer. Then an electrical second order cross-over composed of a 1.1 mH inductor and 15 uF capacitor, the cap is in parallel with the mid-woofers, the inductor is in series. Comparing to the target of 2.5 kHz (in figure below) shows that the x-over frequency is just a little low. So change the inductor to 1.0 mH. (Measurement distance is at 36″.)
For the tweeter a 2nd order electrical crossover also is appropriate to reach a target 2.5 kHz LR4 crossover. The series capacitor value is 8.2 uF with a 0.2 mH inductor in parallel with the tweeter. Notice that the match to the target is excellent to about 1 kHz, and then below that the tweeter level slope appears to change from 4th to 3rd order.
The final integrated result. The tweeter is -5.5 dB, set by a parallel resistor of 7 ohms + series resistor of 1.5 ohms. (Measurement distance is 36″.)
Ok, major change in direction. The Vifa mid-woofers have not held up well while in storage. Resonance and Qts have increased, implying that the suspension has undergone major changes. Into the trash they go. Sigh.
Welcome aboard the Zaph Audio ZA14W08 available at Madisound !!! Not quite the same form factor, so new cut-outs are required. And while I am at it, lets change to a matching metal dome tweeter, the Seas Prestige 22TAF/G (H1283).
The cross-over design will be a 2.5-way. Shown below is the 2 mH inductor on the lower woofer. The two woofers are then wired in parallel. You can also see an initial tweeter circuit (more details in the sequel).
Some measurements. Blue line is the raw drivers, green is with first prototype cross-over. The cyan line is the cross-over target of 2.5 kHz LR4. Notice the metal cone resonance exactly where its suppose to be.
Today’s lesson is in INPUT SATURATION. Yep, during the close mic testing I was saturating the A/D input. This is what caused the strange differences in lower vs. upper mid-woofer
Below is a close-mic test without saturation. The output level on the signal generator are separated by 10 dB. The cyan line is the difference, showing that the difference is +10 db above 100 Hz. Below 100 Hz the ambient noise on the lower volume recording interferes with the test.
Now raise the signal level by 10 dB as shown below. No need to take a difference, the blue curve is clearly not 10 dB greater than the green line.
Multiple close mic measurements on the mid-ranges have showed the same phenomenon over and over again: the lower mid-range has greater low-frequency output than the upper mid-range. About 2 dB more output, which is significant! I would expect less output from the lower mid-range given that it has a 2 mH inductor with a significant DC resistance.
The first test is comparing each mid-range’s TS parameters. The results are the upper mid-range efficiency is +1 dB over the lower mid-range! A close mic of the two mid-ranges with the 2.0 mH inductor removed show they are within 0.5 dB of each other. Hmmm. Retest with the 2.0 mH inductor on the lower mid-range and voila! The inductor is causing the increase in output. How? My speculation is it increases the mid-range’s Q, which would increase low-frequency output.
This result is shown above. The lower mid-range frequency response is flat up to the limit of close mic testing. With the 2.0 mH inductor in series with the lower mid-range (blue line) the appropriate roll-off occurs and a +1 dB gain at low frequencies. (The upper curve is the difference between the two curves with and without the 2.0 mH inductor.) The data is from close mic with 1/6 octave smoothing applied. The drivers were outside of an enclosure mounted on Prototype #4’s baffle. Below is the same plot with the baffle mounted on Prototype #4’s enclosure. Note that now the gain at low-frequencies is +2.0 dB – mystery solved !!!
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!