Using microphones to estimate droplet size

Viral particles, perfume, and mist. Liquid aerosols have numerous applications, and knowing their droplet size distribution is crucial for understanding the dynamics of these processes. However, current methods for measuring droplet size, such as laser diffraction, are expensive and complex.

In an unconventional use of microphones, Avshalom Offner developed an inexpensive method to indirectly estimate the size of aerosol droplets.

An electret condenser microphone acts like a parallel plate capacitor. Using a precise pipette, Offner dripped water droplets of varying sizes onto the microphone surface. Upon impact, each droplet vibrated the top plate at a distinct resonant frequency, resulting in a voltage change on the microphone.

Each droplet’s distinct resonant frequency is determined by its liquid properties — density, viscosity, and surface tension — and its interaction with the surface material. For the same liquid, Offner found larger droplets had lower resonant frequencies that took more time to decay.

The measured signals were used to train a neural network to estimate droplet radius from an input of resonant frequency; the model had a relative error of less than 10%.

Offner’s method is easily scalable and can operate despite background noise. Preliminary parallel measurement tests simultaneously recorded multiple droplet impacts. He hopes to expand the methodology to measure particle size distributions of droplet clouds and refine the neural network with additional data.

“It’s not trying to compete with very, very fine lasers. But if you want a coarser analysis, if you’re looking at the size distribution, it’s what you need,” said Offner.

Source: “Mic drop”: On estimating the size of sub-mm droplets using a simple condenser microphone,” by Avshalom Offner, Applied Physics Letters (2025). The article can be accessed at https://doi.org/10.1063/5.0286709 .