Schumakubins Measurements
After gathering proper equipment over time to build my test system
and using a semi-broken microphone for a year (it finally died after the
measurements became more and more inconsistent) I was finally able to make some
good speaker measurements using TrueRTA on my laptop. I took my sound panels down and placed them
near the speaker under test to eliminate any reflections from the mid
frequencies and up. The bass response still interacts with the room for the measurements.
My pyramid speaker was actually behind the testing area.
This first plot was taken using the Quick Sweep in TrueRTA with 1 watt at 1 meter. Keep in mind that the room’s effects are present below 300Hz. Smoothing was applied to all the plots to aid in visualization.
In-room Frequency Response (1 meter)
In the next plot the near field measurement was added (orange curve) to eliminate the room’s effects. The green curve is the same as the sweep above. You should look at the near field measurement (orange curve) up to 300 Hz and then follow the green curve from 300 Hz and up which gives a fairly accurate response of the Schumakubins.
Nearfield Frequency Response
The best I could do to truncate and merge these curves to give a better picture was to airbrush the curves out in Paint. It is a bit crude but I think it helps in seeing the plot better. You can now see the shallow roll off as a result of the sealed box design. As mentioned before a ported version could be substituted.
Combined Nearfield Frequency Response
Here are the plots generated from the crossover design program for comparison.
Here is an impedance plot of the drivers from the crossover program.
One measurement that has some importance that you don’t see too often is the off axis response. To repeat what was mentioned earlier for convenience. The off axis measurement tells whether the side reflections of the speaker should be absorbed or if a reflective surface is okay. If the off axis response has the same shape as the on axis response then it is okay to have the first incident sound waves reflected otherwise they should be absorbed or diffused. I did these measurements at 15º, 30º, 45º and 60º off axis. As you can see each curve has the same basic shape as the high frequencies fall off.
15-60 Deg On / Off Axis Frequency Response
With the prices of copper these days, keeping the price of a passive crossover low almost mandates the use of iron/steel/ferrite as an inductance booster to lower the amount of copper required to achieve the desired result. The lower in frequency you go, the more it matters since the part values become quite large and expensive. Price out a 6mH air core inductor of 15ga or lower and you'll get a real shock.
The kit comes with many options, including one with no crossover, allowing the builder to simply use the accompanying schematic to construct the crossover with the parts they like the best. Low gauge air core inductors, boutique caps and metal film resistors may, or may not help in the overall production of sound, but it's a sure bet that the wallet will be a lot lighter.
The intention of the Schumakubins is bang for the buck, and a big sound in a small footprint at a low price. On that account, they really do shine. Crank them up and you'll really get a shock when you realize that those smallish 7" woofers are producing that full range sound.
wadeh911;411033The common belief is that air core inductors are better than any other kind for speaker crossovers is generally true, but not always. In 3-way speakers, or in this case a 2.5-way speaker, air core inductors are usually used everywhere in the crossover, except for the low-pass filter of the woofer, without significantly impacting the overall sound quality.
Surprised to see iron core inductors still used, what was the crossover designer's explanation for this rather than air core?
Iron core inductors use less copper wire, and as a result have much less resistance than equivalent air core inductors. For low-pass crossovers used with bass drivers, the lower resistance of an iron core inductor is worth it. The higher resistance of a large air core inductor would have a greater negative impact on the sound of a woofer that is probably already less sensitive than the other drivers in the speaker.