Speaker Box Design Example

  Note, this sample speaker box makes use of many of the calculators found on the menu on the left. You should also review the Speaker Building FAQ for help with this example.

  For this example, I picked 3 ScanSpeak drivers for a 3-way speaker - the same 3 used on the Crossover Example. Note: This example old and the characteristics of these drivers have since changed. 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 for this example are:

Driver	Model	Frequency Range	Imped	Sensitivity	Equivalent Volume (Vas)	Free Air Resonance (Fs)	Total Q (Qts)
Tweeter	D2008/8512	2k-30k Hz	8 ohms	90 db SPL		1000Hz
Mid	13M/8636	200-4k Hz	8 ohms	88 db SPL	3 liters	77Hz	0.32
Woofer	18W/8543	35-3.2k Hz	8 ohms	89 db SPL	49 liters	30Hz	0.22

  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, & Qtc. The drivers above don't specify Qtc, so the standard .707 value will be used. Calculating box volume and port size for a ported enclosure requires Vas, Fs, Qts, D, & optionally Fb which is not provided.. The tweeter itself is sealed and doesn't need an enlosure.

  To calculate the speaker box volumn, we will use the Speaker Box Calculator. Remember to change to metric units - all ScanSpeak nubmbers are in mm & liters. on the sealed box midrange driver gives us:

  .03 cubic feet is an extremely small space for a speaker. 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 - 4in x 4in x 3in = 4/12ft x 4/12ft x 3/12ft = .027 ft^3. But I am of course ignoring the space taken up by the speaker itself. This driver is small and only half of it will stick out the back of the front baffle, but we will do the calculation anyway. ScanSpeak doesn't provide a specific number for the volumn of the driver, so we will approximate it using some of their drawings.


  The 51.5mm depth and 90mm diameter look like the best numbers to use. Converting to inches. 51.5mm / 25.4 = 2.03" & 90mm / 25.4 = 3.54". To calculate the volume of a cylinder, we need height * pi * r^2. Our cylinder diameter = 3.54", so the radius is half that = 1.77". Our cylinder height = 2.03". So 2.03 * 3.14 * 1.77 * 1.77 = 20 cubic inches. The metal basket housing on the speaker isn't solid like the magnet, so to be safe we will assume 80% of this volume is accurate. 20in^3 * 0.8 = 16in^3.

  We will be flush mounting this driver with the front of our speaker box, so not all of this 16in^3 is inside the chamber. Assuming 3/4" MDF is used, we need to calculate the size of another cylinder. .75" * 3.14 * 1.77 * 1.77 = 7.38in^3. With this small driver, almost half if its volume is in the wooden baffle. 16in^3 - 7.38in^3 = 8.62in^3. This is the number that we use to increase the size of our enclosure.

   Converting cubic inches to cubic feet, 8.62in^3 / (12*12*12) = .005ft^3. Not a very large number, but this was a small enclosure to begin with. 03ft^3 (original calculated enclosure size) + .005ft^3 = .035ft^3 which is the size of our midrange driver enclosure.

  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 .23ft^3 enclosure with a 1" wide circular x 2.15" long port (5.47cm / 2.54 = 2.15"). Again, we don't have the driver displacement. This time we will skip the exercise of trying to calculate it ourselves and simply assume that an additional .03ft^3 box volume needs to be added to the .23ft^3 from our box calculator for a total of 0.26ft^3. Instead of calculating these values, you can usually contact the manufacturer through email or their web page, or check online for projects using similar drivers and see what box sizes they used.

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. Googling 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.
• Account for the size of the internal bracing when determining the size of each chamber.
• 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 FAQ.
For help in designing the crossover, see the Crossover Tutorial and Crossover FAQ.