Use of forward pressure level to minimize the influence of acoustic standing waves during probe-microphone hearing-aid verification

Probe-microphone measurements are a reliable method of verifying hearing-aid sound pressure level (SPL) in the ear canal for frequencies between 0.25 and 4 kHz. However, standing waves in the ear canal reduce the accuracy of these measurements above 4 kHz. Recent data suggest that speech information at frequencies up to 10 kHz may enhance speech perception, particularly for children. Incident and reflected components of a stimulus in the ear canal can be separated, allowing the use of forward (incident) pressure as a measure of stimulus level. Two experiments were conducted to determine if hearing-aid output in forward pressure provides valid estimates of in-situ sound level in the ear canal. In experiment 1, SPL measurements were obtained at the tympanic membrane and the medial end of an earmold in ten adults. While within-subject test-retest reliability was acceptable, measures near the tympanic membrane reduced the influence of standing waves for two of the ten participants. In experiment 2, forward pressure measurements were found to be unaffected by standing waves in the ear canal for frequencies up to 10 kHz. Implications for clinical assessment of amplification are discussed.