Prototyping

To validate independent aspects of the design concurrently, we will work on two separate prototypes in parallel. The “works like” model (or in this case, “sounds like”) will focus on the functional aspects of the speaker, while the “looks like” model will explore options for the look and feel of the final product. They are not completely independent, though. While building the “works like” we will keep in mind size, weight, and cost limitations. The “looks like” prototype will be similarly constrained once we have an idea of the physical dimensions of the components (speaker driver, battery, ideal enclosure volume, etc.)

Given the overall project goals, we came up with a decent starting point for the speaker’s characteristics. The main driver will be a (sub)woofer targeting 5 inches in diameter, with maximum power of 40 - 70 watts, and free air resonance (Fs) around 50-60Hz (there are other important parameters which we will discuss in a later post). Depending on the final choice of driver, the frequency response will be cut off below 20Hz and above 500-1000Hz. Two approximately 15 watt full range drivers will ensure highs are sufficiently represented. We feel this is a good starting point for balancing portability, sound quality, and bass response.

Works Like

Development of the functional prototype will consist of testing different speaker drivers, calculating optimal enclosure parameters, and experimenting with different driver/enclosure combinations. For reasons which will be explained in the next log, the enclosure will be a passive radiator system. In order to iterate quickly the following general procedure will be used:

• Order sample drivers with stated parameters that best match our criteria (via Parts Express or directly from driver manufacturers)
• Use BassBox Pro and hand-calculations to determine ideal enclosure size
• Build an enclosure from particle board (using off the shelf rubber and ladle stands from our local Daiso for the passive radiators)
• Measure the frequency response and adjust by adding clay to the ladle stands on the passive radiators
• Using what we’ve learned, repeat until satisfied

We will use off the shelf components in the initial prototype for most of the electronics, which will allow us to focus most of our efforts on the enclosure. A few RC car batteries in series should work well for powering the system, and a 100W Class D amplifier board from Amazon will suit our purposes. The amplifier board uses a TDA7498 IC from STMicroelectronics, which is a good candidate for the amplifier circuit once we move to production. Since the board already has RCA input, not much is required for basic playback except an RCA/Aux adapter.

For wireless connectivity we will use a custom PCBA with a CSR8760 chip, which was developed for another ongoing project of ours called Jack. CSR8670 modules can also be found on Amazon, eBay, Aliexpress, etc. In addition to streaming Bluetooth audio, the chip has a fairly advanced DSP that can handle crossover for the system. It also has touch sensor and GPIO inputs for the external buttons. Unfortunately, a development license for CSR (now owned by Qualcomm) chips is about $3000 USD, so it’s probably a no go for one off projects. A better solution for DIY projects would be to use a separate crossover board and Bluetooth module, which are widely available on Amazon, parts-express.com, and many other sites.

Looks like

Finding the right aesthetic and mechanical features is very important, and can be worked on separately from the functional prototype. Some aspects of the final mechanical design are dependent on the result of the functional prototype, such as enclosure volume and external interfaces (buttons, chargers, wired audio connectors, etc.). We chose a spherical enclosure to highlight the passive radiators, to encourage omnidirectional sound, and for its unique look.

The first step is to come up with a preliminary design for the housing without any internal structure. We use SolidWorks, though any 3D CAD software would suffice. The design will have a handle for portability, an aesthetically pleasing shape, and a creative way of integrating the passive radiators and driver. Once we’re confident enough in the CAD, the next step is to build the prototype. Normally we would begin with 3D printing but we were concerned the build quality might not hold up to the internal pressures, so we are jumping to the higher cost of CNC and painting it.

That process will be repeated until we’re happy with the result. Once the functional prototype is in good shape and the BOM is more or less finalized, we can begin DFM.

Manufacturing

Moving from prototype to a manufacturable product is never easy, and always takes longer than expected. To some extent this is unique to every project, but there are procedures we can follow to alleviate risk and ensure quality. The first step is to develop a relationship with the parties involved, including critical component suppliers (e.g. the driver manufacturer, Li-Ion battery supplier, plastics/metal supplier) and CM. We have already begun reaching out to a few driver manufacturers to obtain samples and review capabilities, and luckily have plastics/metal and battery contacts from previous projects.

Once we begin seriously preparing for a manufacturing run, we’ll write a log detailing our experiences and progress.