Interferometric Sound Recording with a Glass Window

This application note explores the use of SmarAct’s PICOSCALE Interferometer for capturing audio by measuring displacements on a glass surface, providing a non-contact method for sound recording. The interferometer’s ability to detect minute vibrations on a glass substrate is leveraged to record sound with high precision, demonstrating the PICOSCALE’s adaptability for innovative applications in audio and vibration analysis.

Problem: Recording sound through non-traditional methods, such as interferometry, presents challenges in detecting and accurately capturing subtle displacements caused by sound waves, especially on low-reflectivity surfaces like glass. Traditional microphones capture sound through direct air pressure variations, but non-contact optical methods require sensitive detection of surface motion caused by sound-induced vibrations.

Solution: The PICOSCALE Interferometer, using the Michelson interferometer principle, enables displacement measurements on glass surfaces without needing high reflectivity. By positioning a PICOSCALE focusing sensor head toward a microscope slide, the system captures displacements generated by acoustic waves, converting these into an audio signal.

Implementation:

1. Setup: A PICOSCALE focusing sensor head (PS-SH-F01-10) was directed at a 1mm-thick microscope slide.
2. Data Capture: Displacement data from the glass surface was recorded at a 39 kHz frame rate, capturing vibrations from sounds emitted by nearby laptop speakers.
3. Signal Processing: A Python program was used to process the data with high-pass (400 Hz) and low-pass (4 kHz) Butterworth filters to isolate relevant sound frequencies.
4. Audio Reconstruction: The processed data was converted into a .wav file, enabling playback of the recorded sound.

Results: The interferometric setup successfully captured and reproduced an audible audio signal from a glass surface’s displacements in response to sound waves. This experiment demonstrates the PICOSCALE’s high sensitivity and effectiveness in capturing audio through non-contact interferometric measurement, opening potential applications in vibration-sensitive environments or for unconventional recording setups.