Musical spectrum and transfer function measurements: Techniques, pitfalls, interpretation
A. H. Benade · The Journal of the Acoustical Society of America · 1980
Sound spectrum measurements are difficult. The priorities follow: Instrument and player mutually comfortable. Player's task familiar and specified in the terms of his profession. Adequate averaging by microphone and source motion (net displacement ≫ λmax, distance from walls and floor > λmax). Care that microphone is truly in the reverberant field despite source directionalities. Signals from several microphortes summed or averaged only after analysis (otherwise results fluctuate as do 1-mike data). Angular distribution and frequency response measurements differ widely in very-nearfield (kR <<< 1), nearfield 1 < kR, ka < 6, and farfield R/a ≪ 1, kR ≪ 1. Here a = instrument size and R = microphone distance. Choice of transfer impedance, admittance, or simple ratio is governed by musical function. Excitation source impedance must be measured via response of known inertance, elastance, or transmission line. Similar (small) perturbers at driving point aid verification that Zs does not alter significant modes of the driven system. Parallel criteria exist for detectors. Care is required to prevent mechanical, acoustical, or electrical leakage from exciter to detector. [Work supported by NSF.]