Acoustic measurements taken with several liners in a flow impedance tube are used to assess two waveguide methods, the single mode method (SMM) and the finite element method (FEM), for impedance eduction in the presence of uniform grazing flow. Both methods use complex acoustic pressure data acquired over the liner length to educe the liner impedance. The SMM is based on the assumption that the sound pressure level and phase decay rates of a single progressive mode can be extracted from the measured complex acoustic pressures. No a priori assumptions are made in the FEM regarding the measured data. For no-flow conditions, the accuracy of each method is demonstrated by the excellent agreement between no-flow impedances educed in a grazing incidence tube and those acquired in a normal incidence tube. For grazing flow conditions (Mach numbers up to 0.5), the relative accuracy of the two waveguide methods is demonstrated by comparing the impedances educed with the FEM to the corresponding results for the SMM. Significant discrepancies occur for both methods for tests conducted at 0.5 kHz. Possible explanations for these discrepancies are explored with, as yet, no clear answer. Above 0.5 kHz, the results indicate the SMM can be used when the acoustic pressure profile is dominated by a single progressive mode, whereas the FEM can be used for all cases. Nomenclature c, ρ = sound speed and ambient density in duct d, r eq = equivalent depth and radius of liner channel d s , l s = depth and length of ceramic tubular liner segment f, ω, t = frequency, angular frequency (= 2π f ) and time H, L = height and length of duct i = unit imaginary number (= √ −1) k, k x , k y = wavenumbers (free space, axial and transverse) L 1 , L 2 = location of leading and trailing edges of liner M = Mach number, averaged over duct cross section N = number of points used in statistical calculations p, p ref , p s = general, reference (= 20 µPa) and source-plane acoustic pressures S j , S = individual segment and total surface areas SPL, φ = sound pressure level and phase angle x, y, z = axial, vertical, and transverse coordinates x j = wall measurement location β j , β u = individual segment and effective uniform admittances ζ, ζ u , ζ exit = dimensionless wall, effective uniform, and exit-plane impedances (normalized by ρc) θ, χ = dimensionless impedance components: resistance and reactance ζ = θ + iχ µ, σ = error mean and standard deviation FEM, SMM = result achieved using the finite element and single mode methods M = 0.1 = result achieved at M = 0.1 (similar for M = 0.3 and 0.5) NIT = result measured in normal incidence tube

We have developed an automated procedure for aligning peaks in multiple TOF spectra that eliminates common timing errors and small variations in spectrometer output. Our method incorporates high-resolution peak detection, re-binning, and robust linear data fitting in the time domain. This procedure aligns label-free (uncalibrated) peaks to minimize the variation in each peak's location from one spectrum to the next, while maintaining a high number of degrees of freedom. We apply our method to replicate pooled-serum spectra from multiple laboratories and increase peak precision (t/sigma(t)) to values limited only by small random errors (with sigma(t) less than one time count in 89 out of 91 instances, 13 peaks in seven datasets). The resulting high precision allowed for an order of magnitude improvement in peak m/z reproducibility. We show that the CV for m/z is 0.01% (100 ppm) for 12 out of the 13 peaks that were observed in all datasets between 2995 and 9297 Da.

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Acoustic measurements taken in a flow impedance tube are used to assess the relative accuracy of two waveguide methods for impedance eduction in the presence of grazing flow. The aeroacoustic environment is assumed to contain forward and backward-traveling acoustic waves, consisting of multiple modes, and uniform mean flow. Both methods require a measurement of the complex acoustic pressure profile over the length of the test liner. The Single Mode Method assumes that the sound pressure level and phase decay rates of a single progressive mode can be extracted from this measured complex acoustic pressure profile. No a priori assumptions are made in the Finite Element Method regarding the modal or reflection content in the measured acoustic pressure profile. The integrity of each method is initially demonstrated by how well their no-flow impedances match those acquired in a normal incidence impedance tube. These tests were conducted using ceramic tubular and conventional perforate liners. Ceramic tubular liners were included because of their impedance insensitivity to mean flow effects. Conversely, the conventional perforate liner was included because its impedance is known to be sensitive to mean flow velocity effects. Excellent comparisons between impedance values educed with the two waveguide methods in the absence of mean flow and the corresponding values educed with the normal incident impedance tube