Search-Tone Measurements in Blown Wind Instruments

Abstract

A variable-frequency search tone is injected into an instrument's air column, and the response is detected by a probe microphone and narrow-band voltmeter tuned to the search-tone frequency. Accurate response curves are obtained at all frequencies except for narrow regions surrounding each of the strong (discrete) spectrum lines produced by the instrument as it plays. The search tone can be weak, or strong enough to produce nonlinear (heterodyne and synchronism) effects in the instrument's regeneration mechanism. The “blown” resonance peak for a bottle rises above the “unblown” natural frequency as the exciting air-jet velocity increases, but the playing frequency rises at a different rate. The width of the resonance is also somewhat sensitive to jet velocity. The first-mode resonance of a diapason organ pipe behaves similarly, but the higher “blown” modes show little change. There is considerable nonlinearity found in the regeneration mechanism of the organ pipe, but little in that of the bottle.