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DIY AUDIO & VIDEO

TUTORIALS, FAQS, CALCULATORS AND EXAMPLES
FOR SPEAKER BOXES, CROSSOVERS, FILTERS,
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DESIGNING AND BUILDING A SPEAKER BOX EXAMPLE

This example of a designing a speaker box makes use of many of the calculators
found on this site. You should also review the Speaker Building Guide for
additional help.

For this example, I picked 3 ScanSpeak drivers for a 3-way speaker - the same 3
used on the Crossover Tutorial. This design is old and the characteristics of
these drivers have since changed and/or been discontinued. These 3 drivers might
be smaller than what is expected of a typical 3-way system. The mid is 4" and
the woofer is 6.5" in size, but this system is still capable of producing deep
frequencies at 35Hz. The drivers I chose (with specifications provided by the
manufacturer) are:

DriverModelSizeFreq. RangeImped.SensitivityXmax TweeterD2008/851220mm2k-30k Hz8
ohms90 db SPL  Mid13M/86364"200-4k Hz8 ohms88 db SPL1.5mm
Woofer18W/85436.5"35-3.2k Hz8 ohms89 db SPL6.5mm

DriverEquivalent Volume (Vas)Free Air Resonance (Fs)Total Q (Qts)Electrical Q
(Qes) Tweeter 1000Hz   Mid3 liters77Hz0.320.36 Woofer49 liters30Hz0.220.26

DriverModelSizeFreq. RangeImped.SensitivityXmaxEquivalent Volume (Vas)Free Air
Resonance (Fs)Total Q (Qts)Electrical Q (Qes) TweeterD2008/851220mm2k-30k Hz8
ohms90 db SPL  1000Hz   Mid13M/86364"200-4k Hz8 ohms88 db SPL1.5mm3
liters77Hz0.320.36 Woofer18W/85436.5"35-3.2k Hz8 ohms89 db SPL6.5mm49
liters30Hz0.220.26

We will first use the Sealed vs. Ported Speaker Box Calculator to determine if a
sealed or a ported enclosure should be used. The Sealed vs. Ported calculator
uses the formula below. If the EBP is less than 50, then a Sealed Enclosure is
recommended. Above 90~100 and a Ported Enclosure is recommended. Between 50 & 90
either type would work.

Efficiency Bandwidth Product (EBP) = Free Air Resonance (Fs) / Driver Electrical
"Q" (Qes)

The tweeter in our example is already sealed and doesn't require an enclosure.
The EBP of our midrange and woofer drivers are calculated as:

Mid EBP = 77 / .36 = 213

Woofer EBP = 30 / .26 = 115

Both values are well above 90 and call for Ported Enclosures. To keep this
example interesting, I am going to ignore these results and use one sealed
enclosure and one ported enclosure to show the differences between the two.

A sealed chamber will be used for the mid and a ported enclosure for the woofer.
Calculating box volume for a sealed box requires Vas, Fs, & Qts. We will use the
standard .707 value for Qtc for a smooth roll-off at the low end. Calculating
box volume and port size for a ported enclosure requires Vas, Fs, Qts, and the
cone diameter.

To calculate the speaker box volume, we will use the Speaker Box Designer. We
must remember to change to metric units in this calculator - all ScanSpeak
numbers are in mm & liters. The sealed box midrange driver gives us:



.03 cubic feet (0.77 liters) is an extremely small space for a speaker. Remember
we should have been using a ported enclosure for this driver. The speaker itself
is only 4" round. If we go with a minimum 4" x 4" box (the smallest the speaker
will allow), then the box can only be about 3" deep - But I am of course
ignoring the space taken up by the speaker itself and the internal bracing.
Also, these are internal dimensions. The thickness of the wood will make the box
larger.

ScanSpeak doesn't provide driver displacement information, so we will use the
Driver Displacement Calculator. We will use the diagrams provided by ScanSpeak
to enter numbers into our Displacement Calculator.



The Displacement Calculator is a best guess to the displacement of the speaker
driver. It is not exact, but the more information you provide the better it will
be. We will use 103mm as the cone diameter - not the 130mm diameter of the outer
metal edge of the driver. Mounting depth is 45.5mm and magnet diameter is 90mm.
Magnet height is not provided, so we measured a value of 30mm assuming this
drawing is to scale. If we had the drivers in hand, then we could measure them
to be sure. We also need to indicate that we will be flush mounting the driver
and using 3/4" thick lumber in our enclosure.



Now we are ready to use the Speaker Box Enclosure Volume Calculator to determine
the outer dimensions of the speaker box. Also be sure to read the Help for the
Speaker Box Calculator.

To use this calculator, we enter a speaker diameter of 4", the Box Volume (0.77
liters), Driver Displacement Volume (0.24 liters), Wood Thickness (3/4" - same
as above), and indicate we will be using 3/4" battens. Now it is up to the user
to adjust the width, height, and depth of the speaker box until the volume is
.77 liters. The width and height of the box must be at least 4" to make room for
the speaker itself. We are going to use 7" as the width here so that the same
box width will also work for our 6.5" woofer.



The Speaker Box Volume Calculator also provides the size of each panel of the
box. We will not use this information as we will be building a single enclosure
for all 3 drivers.

Now for the woofer. When using the ported speaker box calculator, the driver
size (Effective Cone Diameter - D) is only used to determine port width. Using
the Speaker Box Calculator for the ported box for the woofer gives us:



Our calculator gives us a .48ft3 enclosure with a 2" wide circular x 4.26" long
port. Note: The port is given in both standard and metric units. These numbers
are not the same. 2" = 5.08cm, not 5 cm exactly. The port length is calculated
twice. Once for a port exactly 2" in diameter, and one for a port exactly 5cm in
diameter. The assumption is that you will be purchasing an off the shelf port
that only comes in nominal sizes.

Again, we don't have the woofer driver displacement, so we will use our
calculator to estimate it. Here is the ScanSpeak diagram.



And here is the displacement calculator.



So we need a 13.64 liter box with a 2" diameter 4.26" long port and a 0.61 liter
driver displacement. We want to keep the driver width the same 7" as the
midrange speaker as above, so the additional volume will come from increasing
the height and depth.



The next step is to decide on the general size and shape of the speaker box.
Some possible options are shown below. The box volume for these drivers allow
for a bookshelf style speaker, but for this example a floor standing speaker
will be used instead. Note: It is not required to use the entire speaker box for
the driver enclosure, so the outer dimensions of the speaker itself can be as
large as desired.



From left to right:

 * A bookshelf speaker
 * A floor standing speaker
 * The side view of the same floor standing speaker. Note: the woofer will be
   ported in all of these design options, even when it isn't shown. Notice that
   the chamber for the midrange driver isn't the full depth of the speaker.
 * Side view of an alternate design where the speaker is angled back. In an
   ideal speaker configuration, the back of each speaker cone lines up
   vertically. When flush mounted to a vertical piece of wood, the woofer will
   be a couple of inches behind the mid & tweeter.
 * Another side view alternate configuration. In this design, only the front
   board is angled. The math in calculating chamber volume gets a little more
   complicated, but the build may get simpler with having only one angled side.
   There are also less balance problems. This design also leverages the fact
   that the top chambers are smaller (our midrange chamber needs to be small) so
   that the mid chamber can extend to the full depth of the speaker.
 * In the final configuration there are two 6.5" woofers. Two woofers could be
   used to help with the fact that the woofers have lower power handling than
   the other drivers in this system and two woofers will also help in the low
   end frequencies. Adding a second driver doesn't simply mean you should double
   the box and port sizes. It might be a good starting point, but
   experimentation would be required to achieve the desired result. Without
   adequate testing equipment, you would be better off sticking to a simpler
   system unless you were following the published design from someone else.
   Searching online for DIY ScanSpeak 18W/8543 reveals several published designs
   using 1 or 2 of these drivers.



Some other thinks to consider:

 * The box depth must be at least the port length + the port diameter - not
   really a problem for this system.
 * Stay away from perfect cubes when determining speaker dimensions. The width,
   height and depth should not be the same value. It should be OK if 2 of the
   numbers are near the same value.
 * Account for the size of the internal bracing and speaker driver displacement
   when determining the size of each chamber - again not a problem because our
   calculator does this for us.
 * Account for the size of the crossover when determining the size of each
   chamber. Since this speaker has an unused chamber at the bottom the crossover
   will go there. This allows for changing/repairing the crossover without
   taking the speaker apart.



For some tips on the box construction, see the Speaker Building Guide.

For help in designing the crossover, see the Crossover Tutorial and Crossover
Guide.


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