Results of the masking of one sinusoid by brief pulses of another sinusoid are reported as a function of the phase of the masking pulse. In one case, the brief (10-msec) masker is presented as a sine pulse; in the other case, the masker is presented as a sine pulse. The sine pulse produces more masking than the cosine pulse at frequencies below the frequency of the masker and less masking at frequencies above the frequency of the masker. This inversion is explained on the basis of the power spectra of the two pulses. Details of the results are compared with a simple filter model and indicate that the ear's filter is at least a second-order system with a greater rate of attenuation above the center frequency of the filter than below.