|So, you're considering the possibility of
building your own subwoofer enclosure, but you're not sure where to start? Perhaps the
following notes can help...
Low resonant frequency:
Drivers with a lower resonant frequency ( Fs less than
40 Hz) will make better subwoofer drivers in most instances.
Size Does Matter!
The lower the frequency, the more air the driver's cone has to move. Consider this - for
every octave that you go down in the audio spectrum, your speaker has to move 4 times as
much to provide the same output level! As a consequence of this, your subwoofer is
probably going to end up being the largest speaker in your living room.
Flimsy cones and driver baskets
are likely to produce distortion, resulting in a less than perfect subwoofer design. To
test the cone of a driver, push down one side of the cone near the surround. If the cone visibly distorts or doesn't move
straight down into the basket, choose another driver.
If you're planning to build a passive subwoofer (i.e. one that's driven by your
main amplifier), then its impedance has to be the same or greater than the amplifier's
minimum impedance rating. If you plan to build a powered subwoofer, then impedance
is less of a concern (most subwoofer amplifiers can easily handle 4 ohm impedances).
Please note that there is NO requirement for the
impedance of your subwoofer to match the impedance of your main speakers!
Car Audio Drivers
Typically, car audio drivers will not do well as drivers for a home subwoofer
system, as they are usually designed to produce a good response when a car's cabin gain is
taken into consideration. However, there are a few for which the T/S
parameters suggest that they can produce good results in the home. IOW, if you're
planning to use a car audio driver in your design, treat it as a home audio driver and
design a system based on its T/S parameters to get the best results.
Using an existing driver
If you've already purchased a driver that you
want to use in a subwoofer design, then chances are that the manufacturer has provided one
or more enclosure designs for the driver. For the least amount of worries, it's usually
safe to go with the manufacturer's recommendations. If a specification sheet was not
included with the driver, try contacting the manufacturer directly.
If the manufacturer has not provided recommendations for suitable enclosures for the
driver, then you'll need to use the Thiele/Small
parameters for the driver in order to determine the optimum enclosure
for it. Sometimes these parameters are provided along with the driver, but
if you can, you should measure them yourself in order to get the best
results with your subwoofer design. .
Do you want something small and unobtrusive or is something the size of a small
refrigerator Ok? Smaller subwoofers tend to have higher cutoff frequencies, but the SAF is higher as well. Subwoofer drivers typically
require big boxes to perform at their best.
Is the subwoofer going to be used as part of a small three-piece system that will be used
mostly for popular music, or is it going to be part of a large home theater system? For
the former example, a subwoofer that reaches below 45 Hz should be good enough, while the
latter may require a subwoofer that extends below 30 Hz.
In most rooms, the room itself provides additional gain at frequencies below approximately
30 Hz. There's a slight risk of creating a boomy system if you create a design that on
paper has a cutoff point much below 30 Hz. A commonly-recommended rule of
thumb is to aim for a design with a response that is -3dB at 30 Hz and -9dB
at 20 Hz, which should work well in most rooms.
Most cars provide an additional gain of approximately 12dB/octave below 80 Hz or so. Note
that sealed and 4th order bandpass systems have response characteristics that naturally
complement the natural bass boost provided by a car's interior. When designing a sealed
subwoofer for car audio use, aim for a cutoff point between 40 - 50 Hz to achieve the best
results. For other types of systems, you will need to adjust the design parameters a
bit to get the best results in-car.
Note: if you are using an existing driver, the driver's T/S
parameters will typically make the choice for you. Low Q (<0.3) drivers are generally
more suited to higher-order bandpass and horn-loaded systems (which are not covered here).
Drivers with Qts between 0.3 to 0.4 are usually best used in vented systems, and
drivers with higher Qts are usually best suited for sealed systems. 4th order bandpass
systems can usually work with drivers with any Qts between 0.3 to 0.6. In the case of car
audio systems, you will almost always get the best results with a simple sealed system