Compound Microphone

   Along with a renewed focus on craftsmanship, from this point onward the nature of my projects will largely return to electronic and electromechanical devices. In particular, I intend to develop simple analog equivalents for many of the tasks currently handled by computers. Of these tasks, communication is the most fundamental, and the starting point in this direction is the carbon microphone. After further experimentation with my earlier prototype, I found a way to improve both the simplicity of the mechanism as well as the quality of the sound.

   The compound microphone, seen at the top of the page, uses multiple carbon contacts to provide a cleaner audio signal, to be contrasted with a simple microphone which only uses one. Specifically, I used two contacts in parallel by two contacts in series; the parallel contacts provide alternate paths for current (increasing the audio fidelity), and the series contacts distribute the supply voltage across multiple junctions (reducing cracks and pops due to arcing). Furthermore, the carbon rods used to make these contacts are held captive between two tapered sockets (the rods fitting loosely inside), which I found to be a more portable mechanism than the hanging rod used previously.

   The body of the microphone is made of red oak, which I have found to be suitable for durable projects due to its hardness. The diaphragm (which absorbs and transmits sound) is made of white pine, and I found this to be of critical importance, as most other woods had very poor acoustic qualities. Interestingly, this configuration (a hardwood frame with a conifer diaphragm) is identical to that of a violin, and I believe this similarity warrants further investigation; compared to string instruments, carbon microphones are still very new (and un-optimized) technology.

   The assembly and finishing processes were a significant improvement over my earlier efforts. The red oak was first darkened by the process of ammonia fuming, in which the oak components were placed in a closed bucket alongside a dish of 10% ammonia solution for 24 hours. My wood stain was inapplicable here, as it turns oak (or any other wood high in tannins) jet-black. The frame, diaphragm, and central carbon rod were then glued together with gelatine, which was then diluted and used as sizing, to prevent subsequent coats of varnish from soaking the thin diaphragm. Three thin coats of varnish were then wiped on with a rag, and once dry the microphone was polished with paste wax. The metal components were made exclusively from free-machining (leaded) brass, soldered when needed with zinc chloride flux, filed and sanded smooth, and finally polished with jeweller's rouge (iron oxide in paraffin wax). The carbon components were machined from graphite rods and buffed with a rag, with no further finishing beyond that.

   In terms of performance, this microphone has significantly better sound quality than its predecessor, with approximately the same electrical characteristics. The power output (in terms of audio modulation) is about 50% less, however, and in general this trade-off appears to be the nature of carbon microphones. A larger diaphragm may be able to compensate for this, however, and I am by no means done with my experiments in this regard. I have included a video below, which demonstrates the features of the microphone and includes an audio sample. As the first of my new style of projects, I am entirely happy with the results.