On Turbulent Coherent Structures in Transient Processes of Aerophones - Principles of Sound Generation

Abstract

The present work has its investigative focus on transient processes of aerophones and the question how sound is created in this class of musical instruments. In order to do justice to the complexity of the research question, the author has carefully structured his approach. The work is divided into three parts. Part one starts with a short introduction into the topic in chapter 1, followed by an overview of historical milestones and the current state of the art of research on sound generation in aerophones in chapter 2. The second part provides the necessary mathematical, physical and numerical tools that are used in this work. This is particularly addressed to students of musicology. The preliminary remarks of chapter 3 should make it easier to get started with the complex topic of sound generation in aerophones and at the same time provide some basic concepts of mathematics, fluid mechanics and aeroacoustics. The interested reader with appropriate mathematical and physical knowledge can skip this part. It’s in the nature of the research subject that for certain passages of the work, extended physical knowledge in fluid mechanics and in nonlinear dynamics is required. This will be carefully motivated and executed as far as the scope of the work allows. In cases where this framework is no longer sufficient, reference is made to the relevant literature. In order to understand the considerations in which numerical simulations are used, it is necessary to convey the basics and the methodical procedure for numerical calculations. This will be explained in the chapter 4. In the third part a broad range of aspects of sound generation will be discussed by acoustic measurements and by utilizing numerical simulations in chapter 5. We start with the class of brass instruments, and in particular discuss trans-sonic phenomena in the initial transient of the piccolo trumpet using acoustic measurements on the propagation speed of pressure waves in the initial transient in section 5.1. The results are discussed in detail. Subsequently, analogue acoustic measurements are carried out on other brass instruments such as on the trumpet, the trombone, the French horn, and on the tuba. Then we return to our role model, the piccolo trumpet. With the help of numerical simulations we look at the blowing process inside the instrument. In section 5.2 we also focus on velocity measurements and trans-sonic phenomena in the initial transient, but change the instrument class by examining the recorder and recorder like instruments. After this we change the focus on optimization of sound generation. By looking at the reconstruction of a bone flute we introduce an interdisciplinary project where we show how different scientific and technical disciplines work together to make the fragment of a bone flute playable again. The next instrument we look at is the organ pipe in section 5.3. We focus on turbulent, coherent structures and on the role of shock waves in the genesis of the sound field inside the instrument. For this we use the methods of numerical simulation, advanced visualization techniques as well as the method of coarse-graining. In particular we discuss the jet of the organ pipe, its fluid mechanical properties and its stability properties. Subsequently the mutual interplay of the jet and the sound field in the resonator are discussed. In section 5.4 the focus switches to the overblowing, another important transient process related with aerophones, particularly the Turkish ney. With the presented work, the author would like to take the reader into the exciting world of aerophones and to a fundamental understanding of sound production in perhaps the most inspiring musical instruments we have - the aerophones.