I'm not sure I can help solve your problem, but I'll take a stab at
why it *is* a problem:
The bass response of a conventional speaker system falls off
rapidly below its resonant frequency. The resonant
frequency is controlled by the moving mass and spring stiffness.
If the speaker is mounted so the back side faces a sealed
box, then the air in the box adds to the spring stiffness and
raises the resonant frequency. A speaker designed for
such a box typically has a very soft suspension spring, to
allow for the extra spring effect of the air. I also may have
more moving mass, which also lower the resonance.
However, more mass reduces efficiency.
If you put holes in the back of the box, you relieve the
spring pressure, but you also allow the sound from the
back of the driver to wrap around to the front, where it
can cancel the front sound (since the back of the speaker
is moving the opposite direction). This phenomenon gets
worse at lower frequencies, where the sound travel time
is a smaller fraction of the wavelength. That's why you
never see open-back speaker enclosures in hi-fi applications.
One popular alternative to a sealed box is a vented
or "bass reflex" enclosure. The vent or "port" is typically
on the front of the box, so it's output can reinforce the
output from the speaker cone. Their is still the same
issue with cancellation, since the port output is still
coming from the back of the speaker. The difference is
that now the port acts as a second resonant mass
in the system. (There is usually a tube behind the port,
which holds air that acts as the mass, plus the enclosure
air.) By careful tuning of the port length and diameter,
and box dimensions, the combined system has a lower
resonance that is lower than for the sealed box. (Their
is also an upper resonance that is higher, but is not a
problem.)
There are still more exotic approaches, such as horns
or transmission lines, but, as with the bass reflex, they tend to be
larger than the sealed box.
So the simple thing for you is to keep the box sealed
but use a speaker with a lower resonant frequency,
or a bigger box, or both. Note that the box should
typically be "dead": The sides should not flex or
have any resonance of their own. 1/8 basswood
would thus not be the best choice; speaker builders
typically use particle board (Medium Density Fiberboard
or MDF) for woofer enclosures, often times with
internal cross-bracing.
If you want to mess around with the system, you
can try adding mass to the speaker cone to lower
the resonance (and efficiency). Try carefully pressing
some "Mortite" or similar always-tacky window caulk
around the center of the cone. If you find a mass that
seems to improve things, and you can tolerate the
loss of output, you can replace the added mass with
something more permanent like silicone sealer. Just
remember that it needs to be confined to the little
ring around the center that has the voice coil
behind it. Personally, I think you won't find this to
be worth the effort on such a small speaker, but it
might be educational.
One last note: All else being equal, a more powerful
magnet is what gives more efficiency. Efficiency
is given as dB output at 1 meter for 1 watt input...
more is better.
Best regards,
Bob Masta
D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Signal Generator
Science with your sound card!
