Developing Low-Frequency Ultrasonic Pulse Through-Transmission Absorption Spectroscopy in Liquids

Acoustic absorption spectroscopy can reveal properties of a liquid that inform processes and products for chemical and pharmaceutical industries. For many relevant chemicals, molecular relaxations occur at ultrasonic frequencies and extend down to kilohertz. The only commercial acoustic spectrometer covers a frequency range of 3 MHz to 100 MHz, missing essential absorption mechanisms at lower frequencies. To fill this frequency gap, I employed the variable path through-transmission method to build a low-frequency spectrometer that covers the 600 kHz to 6 MHz bandwidth. I used thirty-one amplitude measurements at varying distances. These multiple measurements significantly improved measurement accuracy when compared to conventional through-transmission and pulse-echo techniques that rely on two measurements. I measured water to correct the data for diffraction effects and used repeated measurements of water to propagate the uncertainty. To validate the spectrometer, I measured solutions of cellulose, salts, and polyvinyl alcohols. The data was compared to results obtained from a commercial spectrometer, and they were well within the margin of error for both spectrometers. The most notable relaxation peak I observed was near 1 MHz in scandium sulfate, which was only reported once before with the use of a resonator method. The data and analysis presented here serve as a foundation for developing acoustic absorption spectroscopy as a robust tool for chemical analysis in various industries.