Mechanical-acoustical analogy: From laboratory to home during the COVID-19 pandemic.

A clear comprehension of the oscillatory nature of sound for acoustics undergraduate students is of paramount importance. In this paper, two online experiments were implemented to aid teaching of the oscillatory nature of sound through the analogy between a mechanical mass-spring model and a Helmholtz resonator. The study was conducted among undergraduate students taking a science course in the Electronic and Electrical Engineering career curriculum. These in-class experiments were conducted during the COVID-19 pandemic via the Zoom platform. Students measured the Helmholtz resonant frequency of a plastic bottle with a smartphone application and compared its oscillatory behavior with that of a conventional harmonic oscillator under a professor-student collaborative environment. The results of this study suggest that, with careful experiment design, students can effectively benefit from the use of common technology tools, which, in turn, poses these methodologies as a rather satisfactory alternative to face-to-face laboratory sessions.

Evaluation of the bias error of transmission tube measurements of normal-incidence sound transmission loss using narrow tube reference elements.

The bias errors of transmission tube measurements are evaluated using an empty test tube condition, implying full sound transmission, normal-incidence sound transmission loss (nSTL) = 0, and two narrow tube elements presenting a theoretically known sound transmission loss (which includes modeling of thermo-viscous losses), varying in frequency around moderate non-zero nSTL levels, and approaching insulation levels more typically found in applications. Results show that the different reference conditions are virtually equivalent in presenting negligible amounts of bias error within the corresponding measurement uncertainties, typically within ±1 dB, in the full 6000 Hz measurement frequency range in this particular case. Slight differences are evident only in the upper third of this range, from 4000 to 6000 Hz, in which bias errors for the narrow tube elements are found very slightly beyond the measurement uncertainties, suggesting the existence of additional sound loss mechanisms. Measurements of a typical insulating test sample are also presented for illustration of the possible non-significance of bias errors in practical cases.

Modified acoustic transmission tube apparatus incorporating an active downstream termination.

Current techniques for measuring normal incidence sound transmission loss with a modified impedance tube, or transmission tube, require setting up two different absorbing termination loads at the end of the downstream tube [ASTM E2611-09, Standard Test Method for Measurement of Normal Incidence Sound Transmission of Acoustical Materials Based on the Transfer Matrix Method (American Society for Testing and Materials, West Conshohocken, 2009)]. The process of physically handling the two required passive absorbing loads is a possible source of measurement errors, which are mainly due to changes in sample test position, or in test setup re-assembly, between measurements. In this paper, a modified transmission tube apparatus is proposed for non-intrusively changing the downstream acoustic load by means of a combined passive-active termination. It provides a controlled variable sound absorption which simplifies the setup of standard two-load techniques, without the need of physically handling the apparatus during the tests. This virtually eliminates the risk of errors associated with the physical manipulation of the two passive terminations. Transmission loss measurements in some representative test conditions are reported, showing improvements over current implementations, in reducing by approximately 50% the measurement variations associated with the setup of the two required absorbing terminations. Measurement results agree within 0.4 dB (maximum difference in high resolution broadband), and 0.04 dB (mean difference in 1/3-octave bands), with those obtained using standard passive two-load methods.