A spider-web-inspired structure can detect very faint sounds by directly tracking tiny movements in the air.
Sound is not merely something our ears perceive. It corresponds to the movement of particles present in the air. Conventional microphones mainly detect the pressure variations produced by this movement. The researchers took a different approach, inspired by certain spiders that can directly sense air movement with their webs.
The sensor therefore relies on an extremely lightweight structure resembling a small web. Its great flexibility allows it to follow the movements of air particles caused by a sound wave. This approach is particularly useful for detecting faint, low-frequency sounds, for which air movements can be difficult to measure with a rigid membrane.
The challenge is then to convert this tiny movement into usable information. To do so, the researchers use light traveling through an optical fiber. Vibrations in the structure alter the optical path inside the sensor. These variations are then converted into a signal representing the captured sound.
The device responds to frequencies as low as 1 Hz, far below the range of sounds audible to humans. It can also determine the direction of air movement. These characteristics stem from its highly flexible structure, designed to move easily with the surrounding medium.
The researchers notably tested it on heartbeats. These sounds are faint and concentrated in the low-frequency range. Yet they contain useful information about the functioning of the valves and blood circulation. Placed on the chest, the sensor recorded these signals with enough fidelity to distinguish their main components.
This method differs from a conventional electronic stethoscope, whose sensor generally relies on a membrane or a more rigid mechanical structure. Here, the sensitive component is instead designed to offer very little resistance to movement. Optical readout also avoids placing an electrical circuit directly on this lightweight structure.
The device remains a laboratory prototype, and the study does not yet demonstrate its use as a standard medical device. Its sensitivity will in particular need to remain reliable in a real-world environment, where body movements, voices, and external noise can interfere with the measurement.
The researchers are now considering this principle for highly sensitive acoustic sensors, particularly in physiological monitoring. The structure could also be useful for other measurements requiring the detection of very small air movements without adding weight to the part that captures them.