Build Your Own Loud Speakers

To answer some of the questions you may have concerning loudspeaker design, we have included this design primer. It takes you step by step through the design and construction of a simple two-way system. Please keep in mind there are many factors to consider when building a loudspeaker, and the following tutorial is intended to address the major concerns.

Design Goal

For our example, the design objective is to build a two-way system, with a 1" soft dome tweeter and a 6-1/2" woofer, without using any test equipment. We choose a moderate sized, sealed enclosure for the best combination of bass quality and extension. A -3 dB down point of at least 55 Hz is also a requirement. The crossover point will be around 2,500 Hz. This is a good compromise, since most quality 6-1/2" woofers should have no problem extending smoothly to this frequency, and a well designed 1" dome tweeter will perform well at this crossover frequency.

Loudspeaker Selection

The first step to any successful project is to select the loudspeaker drivers. Since we are using a two-way design, a 6-1/2" woofer is chosen because of its ability to provide a good quality midrange performance and give adequate bass response. Very few 8" woofers can be crossed over successfully above 2,500 Hz and even fewer 5-1/4" woofers provide bass below 65 Hz! A 1" soft dome tweeter will be selected for its ability to be crossed over at 2,500 Hz with low distortion.

Now we reference the Woofer Selection Guide and the listings for 6-1/2" woofers. We see that the Dayton Audio DC160-8 6-1/2" Classic Woofer (#295-305) meets our goal nicely. In a 0.34 ft.³ vented box it should have an F3 around 54 Hz. In the specs listed on the Parts Express website, we see this driver has an upper frequency range of 4,000 Hz, so a crossover point of 3,000 Hz will not be a problem.

On to the tweeter! Staying with the Dayton line, we decide on the Dayton Audio DC28F-8 1-1/8" Silk Dome Tweeter (#275-070) for its frequency range of 1,300 to 20,000 Hz and Fs of 834 Hz. When picking a tweeter, always cross it over 1 to 2 octaves above the Fs. In this case, Fs = 834 Hz, so a crossover point of 1,668 to 3,336 Hz will suffice.

Enclosure Design

We will now turn our attention to designing the enclosure. Previously, we determined that in 0.34 ft.³ vented enclosure, the Dayton woofer will give a -3 dB down point of 54 Hz. To absorb internal standing waves and to prevent reflections into the woofer cone, we must add some sort of acoustic dampening material. Adding this material to an enclosure will effectively increase the apparent internal volume. In this case all of the volumes suggested on the Parts Express site will already take that into account with a typical level of damping used for the High-qFidelity suggestions from BassBox 6 Pro. That is an average of a ½ lb per cubic foot. In our example, we will choose a .34 ft.³ box and fill it with a 0.17 lb. from a 1 lb. bag of Acousta-Stuf™ (#260-317) dampening material.

Standing waves in an enclosure are minimized somewhat by choosing proper ratios for box dimensions. The "Golden Ratio" is commonly used where the ratio of height/width/depth is given as 2.6/1.6/1. This effect is secondary to properly damping the enclosure with dampening material. Just avoid building the enclosure too narrow and tall as this could lead to strong pipe resonances inside the enclosure.

In our example, we chose internal dimensions of 14" high x 8.5" wide x 5.25" deep (14" x 8.5" x 5.25" = 624.75 cubic inches). 1 cubic foot equals 1728 cubic inches. So, 624.75 divided by 1728 = .361 which is reasonably close to our target of .34 cubic feet. We will then use 3/4" thick material to construct our enclosure, so our external dimensions will be 15.5" high x 10" wide x 6.75" deep.

Enclosure Construction

An often-overlooked aspect of loudspeaker performance is the construction of the enclosure. The quality of the finished cabinet is almost as important as the drivers themselves! A well-built cabinet will limit the amount of sound radiation caused by wall and baffle flexing and the frequency response aberrations it will cause. Also, if the box joints and seams are not properly sealed, low end performance will suffer. For this reason, we recommend using only high-quality materials such as MDF (medium density fiberboard) or high-density particleboard of at least 3/4" thickness. Preferably, the front baffle should be at least 1" thick, since it is being constantly pounded by the woofer and is structurally weakened by the woofer and tweeter cut outs. Each cabinet should include some sort of internal bracing to reduce any resonant vibration. Also, the drivers, particularly the tweeter, should be flush mounted to minimize diffraction effects. It's amazing how much smoother the tweeter's frequency response is by simply flush mounting it!

The two frequency response curves below are of the popular Vifa D25AG-35 aluminum dome tweeter and compare flush mounting versus conventional mounting. Without flush mounting, the tweeter's response from 1,500-20,000 Hz is +/- 4.5 dB (referenced to 91 dB). Note the 3.25 dB peak at 4,500 Hz followed by a 4.5 dB dip at 7,000 Hz; not a good result. When flush mounted, the same tweeter is now +/- 2.0 dB from 1,500-20,000 Hz and +/- 1 dB from 6,000 to 20,000 Hz! A superb performance for any tweeter.

Prior to enclosure assembly, each panel is cut to size, then woofer and tweeter holes are cut. A router is used to cut the countersinks. Next, the bottom, top, back, and side walls are glued together (drywall screws are used to hold the box in position and assure proper alignment). Then, a shelf brace is installed horizontally between the tweeter and woofer positions. A shelf brace is a solid piece of MDF or particle board that joins the front, back and sides of the box with 4 large squares cut out for air flow. When installing a brace, never position it exactly in the center of the enclosure. A rule of thumb is to install it about 1/3 of the way between top and bottom. Now the baffle is glued in and allowed to dry. Once dry, apply a silicone sealer to every internal seam to seal the box. Next, the terminal cup and internal wiring are installed.

Crossover Design

For our example, we decide to use a crossover with a 12 dB per octave slope set at 2,500 Hz. A network with only a 6 dB per octave slope may not attenuate enough to protect our tweeter from damaging low frequencies. Higher order filters introduce greater phase shift and are much more difficult to sum together correctly without using measurement equipment. A 12 dB per octave filter is, therefore, a good compromise.

To determine the correct values for our crossover network, we go to the Technical Resources for the Crossover Component Selection Guide. The instructions are followed for using the chart, and it is seen that for drivers with an 8 ohm impedance, our crossover values would be a 5.627 uF capacitor and a 0.720 mH inductor for both the tweeter and woofer circuits. The closest capacitor value we have is a 5.6 uF (C1 and C2) polypropylene, which is well within range. The inductor value we have is a 0.70 mH (L1 and L2) which will work fine as well. At this point, our crossover network will look like this:

Note that the tweeter should be connected with reverse polarity. This is normal for all 12 dB per octave filters. The two filters are also wired in parallel to one another at the signal input from the enclosure terminals.

The Crossover Component Selection Guide, as with all crossover formulas, assumes that the crossover network is terminated with a purely resistive load, but a loudspeaker is not even close to being purely resistive! Due to voice coil inductance, the impedance is constantly rising with frequency. To remedy this, a simple capacitor and resistor impedance equalizer called a Zobel network is placed across the positive and negative leads of the woofer. The Zobel is not usually necessary with a tweeter network, so we will design one for our Dayton woofer using this formula:

R=1.25 X ReM
C=Le/R2
C = Capacitor value in farads
R = Resistor value in ohms
Re = DC resistance of voice coil in ohms
Le = Voice coil inductance in henries