Acoustic Vector-Corrected Impedance Meter

Abstract

We describe the development of a novel instrument intended for the measurement of the acoustical reflection coefficient of materials. The instrument effectively implements a one-port vector-corrected network analyzer in the acoustic, rather than the electromagnetic, domain. Employing the welldocumented methods of error correction familiar to microwave engineers, this instrument permits automated measurement of an acoustic impedance presented to a waveguide port. A dualdirectional coupler allows a working frequency range of well over an octave. In principle, a set of six couplers would allow measurement from 100 to 50 000 Hz.

Full Two-Port Vector-Corrected Network Analyzer in the Acoustic Domain

“…Even in the electromagnetic domain, there are very few objects whose true impedance or transmission characteristics can be determined independently. For example, a single-port calibration to find the error coefficients demands measurement of three known, different loads, which can be especially demanding in the acoustic domain [10]. Readers interested in the mathematical approach and solution flow can find an initial tutorial in the appendix of [10].…”

“…For example, a single-port calibration to find the error coefficients demands measurement of three known, different loads, which can be especially demanding in the acoustic domain [10]. Readers interested in the mathematical approach and solution flow can find an initial tutorial in the appendix of [10].…”

“…An acoustic vector network analyzer (AVNA) is a device for measuring traveling waves, much like an impedance tube, but it has speed and accuracy advantages over the impedance tube [9][10][11]. An AVNA is capable of swept measurements at ≈200 points per minute.…”

“…Directional couplers are the key waveguide structure capable of separating forward and reverse-traveling waves and enabling vector network analysis. Acoustic directional couplers were first described by Lagasse [13] and used as a reflectometer by Pennington [10].…”

Waveguide Joint Design and Validation for use in Acoustic Vector-corrected Network Analysers

“…The design of the directional couplers permits a working bandwidth of a little more than one octave. For example, in in [5], [11] a range of 750Hz-2.2kHz, or just over 1.5 octaves, was demonstrated for the coupler design taken from [12]. Anticipating that an instrument covering the whole acoustic bandwidth will require several test sets, we have fabricated two examples covering nominal frequency ranges of 1-2kHz and 10-20kHz.…”