A small device uses a spider web-like architecture to detect sound waves; this approach could lead to chip-sized microphones less sensitive to thermal noise.
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When a sound wave passes, the human ear senses pressure changes in the surrounding atmosphere. Engineers have used pressure-sensitive diaphragms to create microphones based on similar concepts since Alexander Graham Bell's time.
Miles, an engineer from Binghamton University, now offers an alternative. Inspired by how spiders hear through the vibrations of their silky webs, Miles invented a new type of acoustic microphone that detects microscopic winds (viscous air flows) produced by sound waves.
Presented at the 186th meeting of the Acoustical Society of America in Ottawa, Canada, earlier this month, the device’s airflow-sensitive design, while currently less sensitive than pressure-based microphones, could enhance signal processing in compact, chip-based microphones.
Today's studio-quality microphones capture even the smallest details. Unfortunately, they are also sensitive to thermal noise, resulting from the random movement of air molecules vibrating the pressure-sensitive diaphragm. Small silicon-based microphones found in phones, laptops, and other smart devices are particularly vulnerable to this noise issue. According to Miles, "Reducing the sensing area also brings a noise penalty."
Motivated to improve tiny microphones, Miles explored the animal kingdom for alternatives to pressure-sensitive eardrums found in mammals and other creatures. Miles and colleagues previously studied how some insects like fruit flies and mosquitoes sense sound through hairs on their bodies and how spiders react to sound by vibrating the silk threads surrounding them.
Both hairs and web threads respond to airflow generated by sound rather than pressure changes caused by extremely weak sound waves, as they are too thin to be affected by such waves. Miles developed a theoretical model for sound wave-associated air movement, determining the effect of sound waves on spider silk.
The oscillation of air molecules caused by sound waves is known as sound-driven airflow. Similar to drag on an airplane wing, viscous forces are applied by the air as it moves through the thread. Calculations show that sound waves in a wide frequency range from 1 to 50 kHz cause the silk thread to oscillate at the same speed as the air. "Studies on spider silk have shown that airflow velocity is an excellent tool for detecting sound," Miles explains.
Using "spider senses," Miles and his team successfully built a flow-detecting microphone. Since working with fine threads is challenging, the team started by creating more manageable structures, micro-scale cantilever beams.
Scientists fabricated silicon nitride microbeams 0.5 µm thick, in various lengths and widths. These beams were placed around a central hole on a silicon chip. The microbeam array was then placed in an anechoic chamber, designed to block sounds and vibrations above 80 Hz.
Initially, microbeam displacement in response to thermal noise alone was assessed using laser motion tracking techniques. Then, displacement was measured in response to pure sound waves ranging from 100 to 1000 Hz. Regardless of length or width, the speed recorded for each beam exactly matched the sound wave. "Ultimately, we discovered that if you design a microphone to detect airflow instead of pressure, it can be made smaller without sacrificing performance," Miles adds.
Miles points out that sound-driven airflows occur over very small distances and at small speeds, which helps explain the success. According to Miles, these small-scale flows don't affect large objects like telephone poles but do impact "fine-scale things like dust floating in the air." Small objects have very low Reynolds numbers in fluid dynamics, indicating that viscous forces dominate over other fluid-based forces, including random thermal forces that generate noise in pressure-based microphones.
The sound-sensitive microbeam device is just a proof of concept. Researchers suggest such a fully functional microphone would likely have a sensitivity of 50-60 dBA, where dBA is a measure of sound level weighted to human hearing. Future designs could convert microbeam speed into an electronic signal. In contrast, high-quality studio microphones can measure sound levels down to a noise floor of 0 dBA; chip-based microphones can measure up to 20-30 dBA. However, Miles notes, "pressure microphones have been in use for 150 years, so give us a chance." Researchers have largely overlooked the idea of detecting sound through airflow, but principles suggest it’s worth considering.
Federico Bosia, a materials scientist at the Polytechnic University of Turin in Italy specializing in bio-inspired metamaterials, believes there is potential in moving away from traditional pressure-sensitive microphone design and sees numerous potential applications. "Given the abundance of hair-like flow sensing elements found in nature," he believes other bio-inspired microphone designs could be created.