The Cymbal as an Instructional Device for Materials Education
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The Cymbal as an Instructional Device for Materials Education Mary Anne White and Peter MacMillan Department of Chemistry and Institute for Research in Materials, Dalhousie University, Halifax, Nova Scotia B3H 4J3, Canada ABSTRACT The materials chemistry behind the production of a cymbal provides an opportunity for exploration of a number of materials topics, which could well resonate with students.
INTRODUCTION The cymbal can be traced back more than 4500 years, to an area that is now Pakistan, although the first record of the use of a cymbal by an orchestra dates to an opera in Hamburg, Germany in 1680 [1]. When hit, a cymbal emits sound associated with vibrations bouncing back and forth between the central cup and the outer edge [2]. A wide variety of non-melodic frequencies is emitted [3], and the physics of this phenomenon has been observed by techniques such as holographic interferography [4]. The study of sound has been previously suggested as a teaching tool in physics [5]. Since the sound quality of the cymbal is closely related to the intricacies of the materials and their processing, we propose the use of the cymbal as an instructional device to illustrate the relationships between and among the structure, properties, processing and performance of a material – the very heart of materials science [6].
MATERIALS The best cymbals are made from high-tin-content bronze. The tin content can be as high as 20 wt %, with the remainder copper. Tin-rich bronze is also used for musical bells for its desirable acoustical properties. Indeed, sometimes this bronze is referred to as BB, for “bell bronze.” Other names are B20 bronze or Cu-20Sn (both named to indicate 20 % Sn). The cheaper B8 bronze also is used for cymbals, but B20 has superior acoustic qualities. Brasses (CuZn alloys) are also sometimes used for cymbals, but they are inferior to bronzes for reasons that follow. The atomic diameter of Sn is only about 15% greater than that of Cu, making Sn quite soluble in Cu [7] (maximum solubility 15 wt % at 550 oC). Furthermore, since Sn is larger than Cu, a significant solid-solution strengthening effect is observed [7]. Cu-Sn bronzes are harder than Cu-Zn brasses; for the same wt % of Sn or Zn added to Cu, Cu-Sn bronzes are harder than Cu-Zn brasses [7]. Although Zn is more closely matched in size to Cu than is Sn (Zn is only 4% larger than Cu [7]) and therefore has a higher solubility in Cu (maximum solubility of Zn in Cu is 39 wt % [8]), Cu-Sn bronzes are harder than Cu-Zn brasses. The hardness of Cu-20Sn is in the range 130-160 HB, compared with 65-85 HB for gear bronze (89 % Cu, 11 % Sn) and 40 HB for pure copper or yellow brass (64 % Cu, 1 % Sn, 2 % Pb, 33 % Zn) [7, 9]. The high values of
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hardness, and concomitant high yield strength of Cu-Sn bronze, both play a critical role in the acoustical properties of the cymbal.
COPPER-TIN PHASE DIAGRAM As the copper-tin phase diagram is particularly pertinent to the processing of the bronze for a cymbal, it is presented in Figure 1 [10]. The equili
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