Design Goals
This design is intended as a replacement for the Bose "cube" speakers, only with decent sound.
I have owned Bose, and I can tell you that these sound light-years better,
although they are a bit larger. These speakers are intended to be used with a subwoofer,
and should cost around $100/pair to build. I have not designed a center channel yet
(too many distractions), but a 5th speaker is small enough to sit vertically on your TV without
being too ugly.
The front profile of the speaker is roughly the same size as a sheet of paper folded in half. As you can see, everything just barely fits on the baffle.
The front profile of the speaker is roughly the same size as a sheet of paper folded in half. As you can see, everything just barely fits on the baffle.
Drivers
This speakers uses MCM 55-1855 5" aluminum cone woofer (about $13.50
from MCM Electronics) and
the Audax TM020J3 3/4" tweeter (about $19 from
Madisound or Parts Express).
Both drivers were chose for their small size and decent reputations.
The Box
The box is sealed (no port) and is 8 1/2" x 5 3/4" x 6 1/2" (HxWxD) and made
from 1/2" MDF. There is no bracing. The box is fully stuffed with Poly-Fil.
The drivers are both centered on the baffle. The tweeter's center is 1 5/8" from the top. The woofer's center is 5 5/8" from the top. Both drivers are flush-mounted.
The drivers are both centered on the baffle. The tweeter's center is 1 5/8" from the top. The woofer's center is 5 5/8" from the top. Both drivers are flush-mounted.
The Crossover
The XO is 4th order acoustic LR at around 3kHz. The trap on the low-pass
is actually used as baffle-step compensation. Because the woofer has
a depression from about 2kHz-3kHz, normal baffle step compensation would render it
unusable above 2kHz.
Ignore the DCR of the coils. The XO is fairly insensitive to DCR, so you can use just about any coil you can put your hands on.
Ignore the DCR of the coils. The XO is fairly insensitive to DCR, so you can use just about any coil you can put your hands on.
Conclusions
I think these speakers sound pretty decent. There is some roughness in both the
tweeter and the woofer, which is to be expected from small, cheap drivers. They need
to be crossed-over to a subwoofer at 80-100Hz to avoid distortion on bass-heavy
passages. The F3 point is around 80Hz, but the small woofer is incapable of
driving a lot of bass below this point without distortion. This
is true of almost any 5" driver.
The impedance is fairly benign. It stays above and around 7 Ohm with a dip to 6 Ohm at 3kHz. The phase varies +/-40 degrees, but is well behaved. A commercial manufacturer would call these 8 Ohm speakers. The sensitivity is low, maybe 80dB/W/m at best, which is what you get from a woofer that works in a small box. Any 80W Dolby-Digital receiver should be able to drive it, so long as the bass duties are handled by a self-powered subwoofer. I was able to drive it to reasonable levels with an old 20W receiver.
Below are the measurements.
Very deep reverse null.
The impedance is fairly benign. It stays above and around 7 Ohm with a dip to 6 Ohm at 3kHz. The phase varies +/-40 degrees, but is well behaved. A commercial manufacturer would call these 8 Ohm speakers. The sensitivity is low, maybe 80dB/W/m at best, which is what you get from a woofer that works in a small box. Any 80W Dolby-Digital receiver should be able to drive it, so long as the bass duties are handled by a self-powered subwoofer. I was able to drive it to reasonable levels with an old 20W receiver.
Below are the measurements.
Very deep reverse null.