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Session 06 - Organs

CFD analysis of air jet deflection --- Comparison with Nolle's measurements
S Adachi
ATR Human Information Science Laboratories, Department 1, Kyoto, Japan

Motion of an air jet deflected by sound is the key to understanding sounding mechanism of air-jet driven instruments. The basic mechanism has successfully been explained by several hypothetical models of the jet deflection. Our understanding does not, however, reach such a level that the transition among the oscillation modes can precisely be predicted for a given organ flue pipe. It is probably due to conceptual approximations the models include. To overcome this situation, a more rigorous method based on the Navier-Stokes equations of fluid dynamics is needed. This paper uses computational fluid dynamics (CFD) to directly simulate wavy motion of a jet deflected by sound. For the jets having parabolic and tophat velocity profiles at the flue exit, deflection amplitude and phase lag behind the sound field are numerically obtained at various distances in the downstream region. The results are compared with those obtained in Nolle's hot-wire measurements.

Breathing modes and sound radiation of metallic organ pipes
S Bergweiler¹, A Bergner¹, T Görne², M Wegener³, R Gerhard-Multhaupt³
¹University of Potsdam / UP Transfer GmbH, Department of Physics, ACMP, Potsdam, Germany; ²Görne Akustik, Berlin, Germany; ³University of Potsdam, Department of Physics, ACMP, Potsdam, Germany

The tube resonator of an organ pipe has long been considered a perfectly stiff tube. Not the pipe material, but only the tube cross section and length were supposed to influence the sound, but recently this was shown to be wrong. It was found that the pipe-body surface is not motionless. Ellipsoidal dipole-like oscillations of the upper lip and the upper end of the pipe body were detected. Furthermore, the material does influence the pattern and strength of the pipe-body oscillations. The sound radiation of a small source will be strongly influenced by the volume flow, caused by its breathing monopole behavior. However, concerning organ pipes, this has not been studied in detail yet. We measured the breathing motion of the pipe body by means of a ring-shaped piezoelectric sensor film tensioned around the organ pipe. From the length variations of the sensor, we obtain an integrated electrical signal for each cross section along the pipe length if pipe-wall breathing occurs. We found a close relationship between the pressure distribution of the air column resonances inside the pipe and the change of cross section along the pipe. On pipes with thinner metal walls, a stronger breathing of the pipe body was found. The same sensor was used to record the attack phase of the pipe-body breathing. The results will be discussed with respect to the coupling between the air-column and pipe-body oscillations and to sound spectrum differences between pipes with thicker and thinner walls.

Particle image velocimetry (PIV) measurements of velocity fields at an organ pipe labium
E - L Johansson, L Benckert, P Gren
Luleå University of Technology, Division of Experimental Mechanics, Luleå, Sweden

A pilot experiment is presented that measures velocity fields in two planes at a blown organ pipe labium for a fundamental tone at 260 Hz. The sound pressure is also measured at the same time as the velocity fields are registered. This makes it possible to follow the change of the velocity field as the sound pressure varies with time. The methods used are stereoscopic particle image velocimetry (PIV) and two-dimensional particle image velocimetry. The difference between the methods is that the first one measures three velocity components by use of two CCD cameras and the second one measures two velocity components with one camera. A double pulsed Nd:YAG laser is used as illuminating source. It gives short light pulses (~13 ns) necessary to resolve the quite high air velocity (~10 m/s). Results show that it is possible to follow a travelling vortex at the organ pipe labium in time as the sound pressure changes. The stereoscopic measurements show that the velocity fields are three-dimensional. The measurements have shown to be repeatable.

Space and air economy in the organ low register
J Liljencrants
KTH, Speech, music and hearing, Stockholm, Sverige

The low register in an organ generally commands the space and air supply required. A number of classical and novel measures to economize with these are compared, with a sideglance at efficiency in sound level generation and musical usefulness. The comparison includes the source mechanisms of flue, beating and free reeds, pipe and cavity resonators, and particular measures like Haskell tubes and sharing resonators between notes. A design is presented where a flue pipe is detuned by one semitone by an additional cavity at its flow node. Another design is a half length free reed pipe where the shallot blocks the flow during half the fundamental cycle.

Free reed instruments: clues for a physical model
L Millot
ENS Louis Lumiere, Son, Noisy le Grand, France

Experiments have shown that free reed motion is sinusoidal and can then be described by the reed tip displacement. Taking into account the local rotation of transverse sections, an efficient expression for the section by which air escapes from the instrument will be derived; an escape area which does not predict any closure discontinuity. Using also a quasi-stationary assumption for the airflow through the reed (assumption of free jet formation), a minimal model for a free reed will be proposed. Details and application in the case of the diatonic harmonica, in which two different free reeds interact, will be given. This particular instrument requires a non-linear description of the vocal track which will also be described. Listening of time-domain simulations will illustrate that a virtual diatonic harmonica may be able to perform chromatical playing (normal notes, bends and overnotes) and follow temporal modifications of the vocal track configuration. Some clues for the extension of this model to other free reed instruments will finally be discussed.

Subjective evaluation of organ pipe timbre in the standard listener positions
V Syrovy, Z Otcenasek, J Stepanek
Music Faculty, AMU, Sound Studio, Prague, Czech Republic

The paper deals with the dependency of tone timbre sensation in the real room on the listener's position relative to the individual organ pipes. Unlike the other musical instruments, in the case of organ three incommutable standard listener positions exist. These positions are closely associated with the subjective tone timbre evaluation of the individual pipe and the whole pipe sounding ensemble too. The first one is the organmaker's position inside the instrument during the pipe voicing, the second one is the position of the playing organist by the console and the last one is the position of the listener inside the church or concert hall. The sound of individual pipes (for different stops) was recorded in the positions mentioned above using artifficial head, which simulates well the binaural sensation and the acoustic field deformation in the listener vicinity. The digital recording of a tone was carried out for several defined head shifts. Afterwards the head was replaced by the pair of omnidirectional microphones and the whole recording procedure was repeated. All recorded tones were subjectively compared using the headphones and by listening in the free field.

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