There is a wide range of potential military applications in which ambiguity in bearing occurs with respect to sound. For example, autonomous unmanned aerial vehicles (UAVs) could employ a sensor to determine the bearing of an explosion and conduct battle damage assessment (BDA) on it. With existing sensors this is difficult to do because the explosion is too short in duration to use the Doppler effect to determine the bearing. Also, an autonomous underwater vehicle (AUV) acting as a quiet platform to tow a short, omni-directional hydrophone array must contend with bearing ambiguity.

Figure 1. This mechanical model represents a simplified version of The Fly’s Ear System. Ki, Ci are the spring and damping constants of the equivalent mechanical model of the fly’s ear system, respectively.
A directional microphone has been developed that eliminates the bearing ambiguity and was added to the towed array. The integrated system constitutes a valuable acoustic measurement tool.

Characterization of a directional sound sensor has been determined using micro-electromechanical systems (MEMS) technology based on the hearing system of a small fly (Ormia ochracea). The fly uses coupled bars hinged at the center to achieve the directional sound sensing by discriminating the vibration amplitude of each bar. The sensors used in this case were fabricated using Silicon on Insulator Multi-User MEMS Processes (SOIMUMPs) technology available through MEMSCAP.

Figure 2. The Fly’s Equivalent Mechanical Model is displaced by the incident sound wave at time t. Because the left and the right ears are physiologically identical, the spring and damping coefficients of them (Ks, Cs) are taken to be the same. Additionally, K3 and C3 of Figure 1 can be renamed Kt, Ct, which represent the parameters of the intertympanal bridge.
The wings of the sensor were made primarily of solid silicon plate in order to maximize the vibration amplitude when the substrate underneath them was removed. A set of sensors with perforated wings was also included in the design to gauge the sound coupling efficiency. The sound sensor was found to have two resonant vibrational modes (rocking and bending). The sensor was simulated using finite element analysis and tested by actuating the sensor using a sound stimulus. The purpose of the simulation was to obtain a valid representation of the sensor under study that could be used in future sensor development.

An analysis to describe the relationship between the sensor's amplitude of vibration and various parameters as the angle of incidence and the intensity of sound was conducted. Experiments, as well as simulation using finite element software, were conducted to assess the performance when two prototypes located on a single chip are tested under varying conditions.

The experimentally observed vibrational frequencies were found to be in good agreement with those of the simulated sensor. The amplitudes of vibration were found to be of the same order of magnitude compared with the simulated sensor and significantly larger than values reported in previous studies that employed sensors fabricated using the PolyMUMPs process. The amplitude of vibration was found to increase as the incident angle was increased and followed in good agreement the theoretical predictions.

Some differences between the two prototypes were found, especially as the frequency diverges from the resonant frequency (2,980 Hz) of the sensor. This analysis points out some disadvantages of the current setup of the physical experiment. Some changes regarding the position of the sensor and the absorbing material that was used were made to attain more reliable experimental units. The model developed in this work used as a response variable the natural logarithm of the vibration amplitude of the sensor. In order to find a goodness-of-fit measure that applies to the response variable directly, estimates on the logarithmic scale were converted back to the original units.

Because a logarithmic transformation was used, it is appropriate to consider a measure that expresses the explanatory power of the model in relative terms. The statistical model developed achieved an average relative error (ARE) of 3.80 percent, which implies that the model was capable of predicting the vibrational amplitude of the sensor with an error that averages 3.80 percent of the actual value. This suggests that the model provided an adequate representation of the behavior of the sensor.

At the last stage of the research, a second chip of a design identical to the chip under study was analyzed. A regression analysis was conducted in order to characterize similarities and differences between the two identical sensors located on the chip. This analysis revealed almost identical performance from the two sensors in a band of frequencies near the resonant frequency. It also showed that the vibrational amplitudes of the two wings of each sensor differ significantly, which is a key factor in the fabrication of a sensor with improved bearing determination. The ARE for this last model was 6.25 percent.

The research suggests that it is feasible to fabricate a MEMS-based sensor that mimics the fly's hearing organ. However, the successful development of an integrated system of sound sensors that resolves the bearing ambiguity problem requires additional research in sensor design. In particular, a broader range of response around the resonant frequencies must be achieved to enhance the coupling of the two vibrational modes. This enhanced coupling would increase the difference of the amplitudes associated with the two wings of the sensor, leading to improved bearing resolution.

The sensor fabricated using the SOIMUMPs process is found to provide greater amplitudes compared to the PolyMUMPs-based sensors, and is therefore better suited for resolving the problem of bearing ambiguity. However, sensors designed with either process that rely on damping due to air to produce large amplitude differences between the two wings may not be able to compensate for sharp responses near the resonant frequencies. Additional mechanisms such as squeezed-film damping are needed to increase overlapping near the two modes of vibration. However, the perforated holes on the wings needed to produce squeezed-film damping also severely reduces the amplitude of vibrations due to the leaking of sound pressure through the holes, which degrades performance of the sensor. The answer might be to put holes in a relatively small part of the wings and leave the substrate intact in those areas. Simulations reveal that such a configuration provides a sensor with a broader frequency response curve.

This work was done by Antonios Dritsas of the Naval Postgraduate School for the Naval Research Laboratory.

NRL-0033


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Characterization of a MEMS Directional Sound Sensor

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This article first appeared in the February, 2009 issue of Defense Tech Briefs Magazine.

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