VITA 2010

  Vienna Talk 2010 on Music Acoustics
"Bridging the Gaps"
      September 19–21


Special topic: Wall vibrations and the sound of wind instruments (T. Moore, W. Kausel): Description

The burning question how and why wall vibrations can influence the sound of wind instruments and organ pipes will be discussed in this session. Experimental observations as well as attempts to explain the effects can be presented in this session.

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Dalmont; Jean-Pierre: 
(Invited) / O
Wall vibrations of a musical wind instrument can be clearly noticed and measured in playing configuration. However, the problem of quantifying its effect on the emitted sound remains a subject of debate. Wall vibrations can be generated by two mechanisms: first mechanically by the impacts of a reed or musicians lips on the mouthpiece and second acoustically by the sound field inside the instrument. In this paper, we present an investigation of the second mechanism using an experimental approach and a theoretical model of a generic simplified instrument i.e. a cylindrical vibrating shell, with a slightly distorted circular cross section. Analysis leads to the conclusion that in most situations vibroacoustic couplings are small and do not induce any audible contribution. However, the wall vibration can play a significant role for some particular choices of material and geometry, which lead to coincidences between structural and acoustical modes. In these configurations, model and experiments show that the input acoustic impedance is significantly perturbed by wall vibrations. Using a blowing machine, it is shown that these perturbations can induce changes in timber or even unstable oscillations. The vibrations of the pipe also radiate directly outside which may induce an audible contribution to the sound field. Experiments on a trombone bell suggest that this effect is very limited and probably not audible in most situations.
Kausel; Wilfried: 
(Invited) / P
The effect of vibrating walls on the radiated sound of wind instruments has often been claimed to be audible by musicians and instrument makers. Many scientists resisted such ideas because comprehensible explanations have been missing. Elliptic oscillation modes which are the most obvious vibration states of tube-like structures could be excluded as a reason for audible timbre differences except in certain cases where structural resonances and air column resonances coincide or where the perfect axial symmetry is broken. Direct radiation cannot explain all the observed effects either, although it is likely that it contributes to a certain extent.

In this paper a theory based on breathing modes is presented which predicts changes in input impedance and transfer function which are qualitatively comparable and in the same order of magnitude as those having been observed in experiments. Sound pressure induced breathing modes are modelled by a local loss of flow into the wall, a loss of energy dissipated by the wall material and additional pressure fluctuations due to the oscillating volume. These influences have been added to a typical transmission line model propagating complex pairs of sound pressure and flow through a sequence of cylindrical elements represented by their transmission matrices.
Moore; Thomas: 
(Invited) / O
The effects of wall vibrations on the sound of modern brass instruments has been a topic of discussion for decades. Recent theoretical work has indicated that damping the breathing modes of the flaring bell may be responsible for observed changes in the sound when wall vibrations are damped. We present measurements of the changes in the transmission function, input impedance, and acoustical signature of a modern trumpet when vibrations of the bell are heavily damped, and compare them to predictions from such a theory. The magnitude of the effect attributable to changing the air column is compared to the effect attributable to direct radiation of the vibrating bell. Measurements indicate that much, but not all, of the change in the sound of a brass wind instrument that results from damping bell vibrations can be attributed to changes in the air column. The magnitude of these changes compares well with the theory.
Pyle; Robert: 
(Invited) / O
Previous measurements of trombone tones (J. Acoust. Soc. Am., Vol. 125, No. 4, Pt. 2, April 2009, p.2597) showed considerable player-to-player variability in the degree to which the radiated spectrum changes with different bell alloys. This might arise in the following way. Consider two very different players, A and B. Player A is able to produce the desired timbre, independent of the bell alloy, by altering the embouchure as needed. Given a choice of instruments, player A will presumably pick the one that most easily produces that timbre. Player B, on the other hand, always blows each instrument as freely as possible, allowing the instrument to determine the tone quality, and then chooses an instrument with the desired timbre. It is plausible to think that the characteristics of the sound pressure in the mouthpiece cup will show greater variability (with changes of bell alloy) for player A than for player B due to the embouchure adjustments by player A. This hypothesis will be tested by simultaneous measurement of sound pressure internally within the mouthpiece and externally on the bell axis. The relationship between the internal and external sound pressures, to the extent that it is independent of the player, may reinforce (or, perhaps, contradict) common beliefs among players and instrument builders about the effect of alloy on tone color.
Smith; Richard: 
(Invited) / O
In the early 1970s, there were strongly opposed views from musicians and scientists regarding the musical effect of materials in the construction of brass instruments. But then, there was little research (without computers) and no supporting evidence for either view.
At that time, many players thought that the final finish of silver, gold, lacquer or nothing-at-all gave the extra quality they were looking for.
As chief designer at Boosey & Hawkes (and Smith-Watkins from 1985), Richard Smith, using a range of specially prepared trombone bells, set out to measure the metal vibrations, the degree to which the vibrations affect the radiated sound and whether professional players can detect these differences through the ear or response at the lips. Knowledge of these results has given him a framework for the design of brass instruments to this present day.
Zum Einfluss der Wandschwingungen bei Metallblasinstrumenten
Gunter Ziegenhals
IfM Institut für Musikinstrumentenbau e.V. an der Technischen Universität Dresden

Die Wand (der Korpus) der Metallblasinstrumente wird beim Spielen über die im Inneren schwingende Luftsäule zu eigenen Schwingungen angeregt. Untersuchungen führten zu dem Ergebnis, dass für das Aufrechterhalten der unter normalen Spielbedingungen festgestellten Betriebsschwingungen des Korpus zwischen 1% und im Extremfall 20% der Leistung benötigt wird, die im abgestrahlten Schall steckt. Diese Leistung geht entweder dem Ton verloren oder muss vom Spieler zusätzlich aufgebracht werden. Die Unterschiede der Wandschwingungen werden vom Spieler über den Tastsinn eindeutig wahrgenommen. Die Pegeländerungen liegen im Bereich < 0,5dB und sind deshalb nur bedingt hörbar. Man kann die Verlustleistung der Wandschwingung durch Minimierung der Schwingwege vermindern. Dies realisieren offensichtlich steifer gebaute Instrumente, die aber nur von einem Teil der Musiker bevorzugt werden. Es ist wahrscheinlich, dass der Musiker als Reaktion auf unterschiedliche Energieaufnahme der Wand seinen Ansatz und damit die Klangfarbe des Instrumentes verändert, also ein indirekter Wandeinfluss auf den Klang vorliegt.
Effekte der Wandschwingung werden anhand extremer Beispiele diskutiert. Diese Beispiele gehen über die normale Spielsituation bzw. üblich verwendete Materialien hinaus. Die Effekte der Wand sollten hier extrem deutlich hervortreten. Da dies nicht der Fall ist, muss der Einfluss der Wandschwingungen als sehr gering eingestuft werden.

To the influence of the wall oscillations at brass instruments
Gunter Ziegenhals
IfM - Institute for research and development of musical instruments, associated Institute at the Dresden University of Technology

The wall (the corpus) of the metal wind instruments becomes lively when playing over inside swinging air column own oscillations. Investigations led to the result that for maintaining the operating oscillations of the corpus between 1% and in extreme cases 20% of the power, determined on normal play conditions, one needs, which is in the radiated sound. This power goes either to the clay/tone lost or must be applied by the player additionally. The differences of the wall oscillations are clearly noticed by the player over the sense of touch. The changes in level lie in the range < 0,5dB and are therefore only conditionally audible. One can decrease the energy dissipation of the wall oscillation by minimization of the oscillation ways. Apparently more rigidly built instruments, which are preferred however only by a part of the musicians, realize this. It is that the musician changes his beginning and thus the tone quality of the instrument as reaction to different power consumption of the wall, thus an indirect wall influence on the sound is probably present. Effects of the wall oscillation are discussed on the basis extreme examples. These examples go beyond the normal game situation and/or usually used materials. The effects of the wall should step out here extremely clearly. Since this is not the case, the influence of the wall oscillations must be classified as very small.
Banner Pictures: (c) PID/Schaub-Walzer