A laser micromachined polymer microphone

The Braunmuhl-Weber microphone design, patented in 1935, is often used for studio microphones since directionality can be changed electrically. Materials, construction and machining methods have changed little since the patent date. The backplate is drilled, limiting how small each hole can be made. This leads to microphone noise problems through increased acoustic resistance from the remaining surfaces and the loss of electrode area. Tensioned diaphragms decrease the available compliance and microphone sensitivity. The goal of this work is to develop a new condenser studio microphone that exceeded the performance of the best commercial studio microphones used as benchmarks. Electroacoustic modelling aided the understanding of microphone behaviour and allowed more efficient optimization of microphone performance. Model performance was optimized for sensitivity, self-noise, and directivity (the sound pick-up pattern). Excimer laser micromachining extends the performance of the microphone by forming smaller and denser backplate holes that lower acoustic impedance and microphone self-noise. Direct write lithography allows patterning of the diaphragm electrode for lower parasitic capacitance. The physical format of the microphone would have been impossible to produce using conventional microphone manufacturing methods. By using Mylar® instead of the more traditional brass, it is shown microphone parts can be micromachined less expensively and more easily. Microphone performance was further improved by introducing design changes: the symmetric microphone arrangement, operation of the diaphragm in plate mode and elimination of the backplate blind-holes by careful optimization. Diaphragm compliance and microphone sensitivity increased when external diaphragm tension was removed to allow the diaphragm to behave as a plate. A symmetric microphone design doubles the available capsule capacitance, shunting more preamplifier input-current noise to create a quieter microphone. Symmetrical outerplates also balance the diaphragm electrostatic collapsing force to allow the use of a zero-tension diaphragm. A comparison of measured performance for the laser micromachined prototype and benchmark commercial studio microphones showed the superior sensitivity of the prototype performance, 55mV/Pa, compared to 18mV/Pa for benchmark microphones. Measured self-noise and directivity were as good as the studio microphones (Neumann TLM103) with 7 to 9 dBA SPL equivalent noise referred to input. The prototype performance greatly exceeded that of silicon micromachined microphones. The prototype high frequency response fell short at 15 kHz limited by excessive diaphragm radius. Symmetric microphone outerplates were prone to moisture ingress which was solved using a capsule heater and protective covers. Australian and U.S. patents were granted for the microphone design developed in this work.