Trying to optimize a small wooden speaker box for voice (speech) frequency range. It's a 5 X 3.5 X 1.5 box made out of 1/8 inch basswood with a 2.5 inch speaker inside. I drilled thirteen 1/4 holes in front of the speaker which sounds reasonable with the backside open, but the bass response is too high with a solid back on the box.
I figure a few air holes drilled in the backside of the box are needed to improve the midrange response. Should the holes be placed directly behind the speaker, or offset toward the side for best speech response? Are a few large holes better than many more smaller holes?
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
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Yes, those are good ideas, but there isn't room in the box for a front air hole, so I'm limited to holes, or slots in the backside. I played around with a solid back and found I can tune the sound somewhat by slowly opening the back to some small angle allowing a regulated amount of air to escape. Or, I can slide the back so a small slot is exposed, but the sound quality seems best with half the backside or more uncovered. So, it appears to need a lot of holes. There is a circuit board on the backside of the speaker which forces most of the air to travel to the edges of the box and around the board before escaping to the rear.
I'm going to do more experimenting with a cardboard back with holes that can be moved around to see what happens.
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