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| By: cplkittle on Fri, 3 Oct 19:04:54 -0500
| | Sealed boxes are the simplest of all loudspeaker designs, consisting of an enclosed volume of air and loudspeaker or driver. There are two basic types of closed-box systems: Infinite Baffle, and Acoustic Suspension.
An infinite baffle speaker is an enclosure made large enough so that the compliance of air within the enclosure is greater than the compliance of the woofer’s own mechanical suspension. So in other words the woofer is using it's own suspension to control the cone. There are not many Infinite Baffle (IB) drivers around today because it seems manufacturers have taken the approach that most people want smaller boxes. Also IB type drivers tend to be higher Q drivers which will not give as good of transit response.
An acoustic suspension, or air suspension, has a volume of air within the enclosure that is smaller than the compliance of the woofers suspension. The springiness of the air in the enclosure has a much greater effect on the woofer than the woofer’s own mechanical suspension. So the woofer cone is essentially suspended by the enclosure’s air. Giving it a tighter, punchier response, but cutting out the lower frequency response of the speaker.
The point of fitting a woofer to a box is to control the response characteristics. The objective method is to measure and align the Q factor [The ratio of reactance to resistance in a series circuit, or the ratio of resistance to reactance in a parallel circuit].
Certain values of Qtc [The total Q of a driver and sealed enclosure] have specific response characteristics, which can be characterized as follows:
Qtc= 0.5 Critically dampened; Transient perfect.
Qtc= 0.707 Butterworth response-max amplitude response with minimum cutoff.
Qtc >0.707 Chebychev response-equal ripple response; max power and handling efficiency.
Qtc=0.5 is usually regarded as excessively taut and over dampened, however some standards still consider this value to be optimal.
Now that we have established the differences between the two types of boxes and the characteristics of the q factor, there is one more thing I would like to add when considering a design for a sealed enclosure. This is called the ‘Golden Ratio’. Using the golden ration will cut down on the boominess and panel vibration of your box. The ratio is the cubed root of the determined enclosure size in inches (x1), (x.7939), (x1.2599)
For example if your internal volume in cubic inches needs to be 2160^in or 1.25^ft.
Take the cubed root of 2160 (12.927) and multiply it by 1 to get side A
A=12.927
To get side B multiply side A by .7939
B=10.263
To get side C multiply side A by 1.2599
C=16.286
To double check multiply side AxBxC, and you should get within 1 of 2160.
If you decide to use one of the many programs available on the internet, this ratio is already calculated in, or is an optional feature within the program.
You should use only the best available materials in construction of your speaker box. ¾” MDF is recommended for almost all 10” and 12” subwoofer applications. For dual sealed or dual bandpass applications, 5/8” MDF is recommended. Internal bracing should be used on longer spans and should be subtracted from the internal volume. A high quality wood glue is needed on all joints, and should squeeze out when screwed together. Coarse thread drywall screws have proven less likely to strip out. Use a screw at least twice the length of the thickness of your material, and always predrill and countersink the holes for a professional look and durability. Silicone can be used on the interior corners to ensure an air tight box. If silicone, spray adhesive, or especially superglue (for upholstery) is used , allow at least 2 hours to dry and air out, as some of the chemicals will cloud the finish of your speakers or plexiglass.
Here are some recommended products that I have found very useful in speaker box construction:
A carpenter’s or hobbyist’s wood rasp (for rounding or finishing edges)
A round-wheel type upholstery razor blade
Loctite or Titebond wood glue
3M super 77 spray adhesive
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