Further assessment of forward pressure level for in situ calibration
Schematic of probe tips. Panels: Source characteristics as a function of frequency. Similar characteristics were observed using the ER10-14 (dark gray) and modified ER10D (light gray) tips. The dotted black lines are the source characteristics obtained with the ER10D tip using four acoustic loads with overlapping resonant peaks at 17.5 kHz (see Fig. 2), and the solid black lines are the source characteristics obtained with the same tip using five acoustic loads with non-overlapping resonant peaks.
Load impedance magnitude (top) and phase (bottom) for the four cavity lengths (30, 40, 50, and 70 mm) used during source calibration with ER10D tip. Gray lines are the expected values (Keefe et al., 1992), and black lines are the calculated values for each tube. Impedance magnitude notches correspond to resonant peaks; the pair of lines with the lowest frequency notch corresponds to the longest tube length. The arrow at 17.5 kHz highlights the overlapping resonance at this frequency for all four cavity lengths. Low impedance at this frequency for all cavities caused errors in the calculation of source impedance and pressure around this frequency range (Fig. 1). This situation illustrates the importance of choosing cavity lengths with minimal overlap in resonant peak frequencies.
Comparison of differences between SPLterminal and IPLentrance across frequency for the three probe tips. Ideal characteristic impedance () was used for the calculation of IPLentrance. Each panel displays results from a single test cavity. Diameter of the test cavity is indicated in the top left corner. The dashed vertical line marks the quarter-wave null frequency associated with the diameter of the test cavity.
Comparison of differences (top) and absolute differences (bottom) between SPLterminal and IPLentrance across frequency for the three probe tips, collapsed across test cavities. For each probe tip, the shaded area encloses the minimum and maximum SPL-IPL differences across the test cavities. Differences between SPLterminal and IPLentrance are the smallest at frequencies below 4 kHz for all three probe tips. Below 10 kHz, differences are ≤2.5 dB. Above 14 kHz, differences are the largest for the standard ER10D tip (black shading).
Effects of using a single characteristic impedance estimate to calculate FPL in test cavities with various lengths and diameters. (top) SPL at the test cavity entrance. (bottom) Differences between SPLterminal and IPLentrance when was used to calculate IPLentrance. The y-scale is different for top and bottom panels. (left) Test cavities with equal lengths but variable diameters (Table I, column 1). Differences between SPLterminal and IPLentrance systematically increase around null frequencies, and the differences become greater as actual characteristic impedances deviate further from the estimate used (Table II, column 3). (right) Test cavities with equal diameters and variable lengths (Table I, column 2). Differences between SPLterminal and IPLentrance do not systematically change as a function of tube length.
Comparison of differences between SPLterminal and IPLentrance resulting from different values of characteristic impedance (, , and ). Each characteristic impedance value was used to determine FPL and RPL of the set of test cavities with equal lengths and variable diameters (Table I, column 1). Shaded areas enclose the minimum and maximum of differences between SPLterminal and IPLentrance across test cavities obtained for each characteristic impedance condition. When cavity-specific characteristic impedance was used ( or ), errors did not systematically increase with cavity diameter and were generally below 2.5 dB through 10 kHz and below 4 dB through 18 kHz.
Test cavity dimensions. Three sets of cavities were used in this study. The first column describes the cavity set with equal lengths and varying diameters. The second column describes the cavity set with equal diameters and varying lengths. The third column describes the cavity set with diameters and lengths chosen to co-vary in a manner similar to the human ear canal (Keefe et al., 1993).
Comparison of characteristic impedance estimates according to cavity diameter. The far right column displays expected quarter-wave null frequencies in the transverse direction.
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