ï~~Adding Vortex Noise to Wind Instrument Physical Models
Chris Chafe
Center for Computer Research in Music and Acoustics, Stanford University
cc ccrma.stanford.edu
ABSTRACT: Flutes and other switching air jet instruments do not exhibit period-synchronous
pulsed noise. However, period-synchronous spectral changes were detected in a flute tone. A synthetic flute model is described which imitates turbulent qualities of breath noise with a vortex noise
generatorcircuit.
1 Introduction
Noise in bowed stringed instruments and many other musical oscillators exhibits distinct pitchsynchronous amplitude pulses (Chafe, C.). Synthesis quality has been improved by coupling a
pulsed noise mechanism to the basic physical model of these instruments. Edge-tone instruments
such as flutes and organs are an exception. In the present study, their breath noise is shown to have
pitch-synchronous spectral features and a more constant amplitude contour.
Turbulent noise caused by the switching air jet of edge-tone type instruments includes vortex formation and shedding in three dimensions (Verge, M.). As the player excites the air column into
oscillation by blowing across the edge, the air jet at the mouthpiece begins to rapidly alternate
direction in time with the column's vibration. The process elicits both frictional noise at the constriction where air enters the pipe and air puffs that roll away as the returning pressure wave impedes
the entering flow. The formation of vortices and modulation of fricative noise is regulated by the
oscillation of the air column. When the instrument starts to "speak," these are timed by its pitch.
The hypothesis that prompted the present investigation is that though breath noise appears fairly
constant in amplitude, periodic vortex shedding suggests a source for spectral modulation.
2 Analysis of Pitch-Synchronous Spectral Change
Recordings were made from a plastic "research" flute with no tone holes. Breath noise was extracted
from audibly stable portions of tones and examined for pitch-synchronous features. The extraction
method used an adaptive linear pitch predictor which allowed the predicted (periodic) signal to be
removed (Cook, P. 1993). Spectral fluctuations in the residual (breath noise) signal were detected
via transformation to medium-width frequency bands. These fluctuations were projected against
the original waveform to reveal pitch-synchronous and longer-term features.
Pitch-synchronous noise extracted from a bowed cello tone is pulsed in comparison with the flute in
Figure 1. The latter was further analyzed for the possibility of fast time-varying spectral changes.
The noise residual was transformed with a Hamming window and 64-point Fast Hartley Transform.
The analyzed tone had a pitch of 204 Hz. and was recorded at a sampling rate of 44100 kHz.,
resulting in a frequency resolution of approximately 690 Hz. per bin and a temporal resolution
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