It has been shown that the vibrations in the bell section of the trumpet can influence the sound produced by the instrument. Recent research has indicated that these effects may be traced the axial vibrations of the bell. We report experimental results that relate the input impedance of the trumpet to the phase of externally induced longitudinal motion. These results indicate that the phase difference between the driver and the bell motion can significantly affect the input impedance. [Work supported by NSF Grant #PHY-160749.]