Stereo Speaker Design
Project Background
Growing up, my parents had a set of dark stained oak speakers powered by a stereo receiver in our living room. I learned in around 2010 that my uncle had built the speaker pair for my mom and gifted them to her. Once I found out that high-quality speakers can be designed and built at home and constructed from the same base audio components found in store-bought speakers, I knew that I wanted to build my own stereo set.
At first, I wanted to follow a similar design to the one that my uncle chose for my mom’s speaker set but changed my mind later on as I discovered how the speaker components and dimensions worked together to produce sound. Based on the idea that, with diminishing returns, a larger internal volume would aid in better production of lower frequencies of sound, I decided to build mine much larger than the set my uncle had built. For the components that he selected, the volume of the speakers that he built was selected to balance performance with the package envelope, minimizing speaker size when possible without largely sacrificing performance. Since I had no problem sacrificing space for performance, I selected more components to divide the sound production.
The entire process of designing them, selecting components, solving for the cabinets’ dimensions, deciding component locations, purchasing materials, shaping, attaching and finishing the wood, and finally assembling and soldering everything into place took place over 14 and a half months. I currently drive my speakers through a 100-watt-per-channel Denon DRA-685 stereo receiver which nicely compliments the power output that my speakers’ crossover circuits can handle. A lot of work was involved in bringing these speakers into existence, but the quality of sound that they produce has been well worth it, rivaling that of far more expensive, professionally built speakers. I learned very much from my research involving design equations pertaining to my selected components, resonance tuning, optimal volume calculations and the physics and wiring of the crossover circuit and sound-producing components.
Detailed Description
As a result of my uncle and I splitting the work on the speakers, since he had the tools and experience necessary for precisely manufacturing the cabinets, each step in constructing the speaker pair took a long time to complete. I began the process by doing a small amount of research into the components that most speaker systems utilize and learned about two-way and three-way component setups. Each speaker, in its simplest form, contains what is called a crossover, containing capacitor and inductor circuits designed to handle the expected current loads and filter out frequencies passed to each of its three channels.
In a three-way system, which is what I ended up selecting due to its superiority over two-way in the way of consistent frequency response, the crossover divides the signal into three groups to be sent to the different audio components. One channel is dedicated to high-frequency signals, and therefore has a high-pass filter in the form of a capacitor, welcoming abrupt changes in voltage and current while impeding changes that take longer, i.e. low frequencies. These frequencies are directed to the tweeter which tends to be either a piezoelectric crystal tweeter or dome tweeter in conventional speakers. The signal is then converted into mechanical motion of a diaphragm through flexing of the piezo crystal or through a magnetic voice coil.
The second channel employs the use of inductors which act as a low-pass filter, impeding abrupt changes in current and allowing longer changes to pass through. These low-frequency signals are sent to the woofer component(s) on the speaker, which converts them through a large voice coil and fixed magnet into mechanical motion of a large cone. The third, mid channel, is dedicated to mid-range-frequency sounds produced by a small cone driver. This signal channel uses a combination of high-pass capacitors and low-pass inductors to filter out sounds outside of the mid-range. This is done in an attempt to expose the driver only to frequencies that it can produce efficiently, and therefore to better take advantage of its ability to resonate with signals received in that range.
A two-way speaker generally only has a low channel and high channel, relying on a full range cone and a tweeter to produce the whole range of sound frequencies. I selected a high-capacity three-way crossover component because it would allow me to spread the frequency range across more specialized components and, hopefully that way, produce better sound. To improve said quality even more, I opted to send the high-frequency range between two tweeters as well, to be produced by both a dome and a piezo style tweeter.
Piezoelectric tweeters are better suited for extremely high-frequency sound production, so I had one piezoelectric wired in parallel with my dome tweeter, which conveniently has nearly no effect on the impedance of that circuit. The impedance values of the selected components in a single circuit loop are expected to match the impedance that the receiver is built for which, in my case, is eight ohms. My dome tweeter is an eight-ohm tweeter, matching my receiver’s expected impedance, which is all that matters since the piezo tweeter has little to no effect on that impedance rating.
I selected two four-ohm woofers for each speaker which, when wired in series, combine to be eight ohms. My midrange drivers were trickier, as I desired to have four components stationed in each speaker to share the load. I had to wire them in what is called series-parallel configuration, with two sets of series wired drivers combining to be 16 ohms each and wiring those pairs together in parallel, bringing the total back down to eight ohms.
After selecting all of the components and ensuring that they would be compatible with each other in my final setup, I purchased them all online for around 150 dollars and awaited their arrival. Meanwhile, my uncle cut the oak plywood sheets that I had purchased into sections which were to be assembled into the speakers’ cabinets. Once cut, he returned the front panels to me so that I could cut holes out to allow each component to fit into them before they were permanently added to the speaker frames. I first used a cardboard representation of the front panels to carry over the designs from my computer model that I’d drawn up to store important dimensions, then transferred the markings to a scrap piece of plywood and cut the holes into it with a jigsaw.
In order to ensure that the components would all fit into the sections that I had cut out, I performed a mock-fit and adjusted the hole sizes, finding that the tweeter horn bulb wouldn’t fit through my original cutout. Using the scrap plywood section as a template for the final two oak front sections, I lightly transferred the holes over in pencil, padded my jigsaw to prevent it from marring the wood surface, and then cut out the final holes for the components. The next step was to fit every component into its hole and pre-drill pilot holes for the wood screws that I had purchased. I then returned the front face sections of the speakers to my uncle so that he could assemble all of the oak cabinet components using a combination of 2x2 wood framing in internal joints and wood screws and wood glue to ensure air-sealed cabinets.
He has many clamps at his disposal, so after he cut the 2x2 framing into appropriately-sized lengths, he would glue each into its final spot on the oak plywood, screw it into place and leave it clamped for a few days for the glue to fully set. This ensured an extremely rigid connection between the components of the speakers, ultimately eliminating undesirable internal vibrations and sealing everything together to prevent air from whistling through small cracks and holes between wood sections.
Once both cabinets were assembled, he applied a stain called English Chestnut to the outer surfaces and then coated them in a polyurethane layer to protect the wood and add an aesthetic sheen. At this point, the cabinets were ready for the sound drivers and crossover to be installed, so he returned the cabinets to me. I had already pre-drilled pilot holes for mounting the components, making it easy to drop each into place and screw it down flush with the wood.
Before connecting the last woofer in each speaker, I ran speaker wire from each component to the crossover board and soldered the wires to their designated contacts, then mounted the crossover to the inside of the speaker at the base. After I mounted the last woofers, as an aesthetic touch, I painted the screw heads black to blend better with the speaker rims, so they wouldn’t stick out so clearly. I was able to use these speakers for years, but I had to leave them at home when I left for college due to their size. Near the end of 2019, they will return to Iowa City and be used in my condo.
A few years after my speakers were completed, a friend of mine desired to follow suit and build her own speakers from scratch. It took about 7 months to complete her set, including both design and construction, but since I had experience this time and was able to essentially go at my own pace, the process went more quickly. Building speakers is a fun and rewarding project. I would love to build another set in the future, this time entirely with my own resources and tools.
