Chapter 10 Underpinnings V: Getting your hands dirty

This chapter is about measurements and experiments, in all their guises relevant to the study of musical instruments. The first section gives a discussion of some important background material: What are the different types of measurement and experiment? What are the different motivations for wanting to do measurements? What are the pitfalls you need to avoid in the design of any experiment?

The next section looks at various experimental techniques for “seeing the invisible”: microscopy of various kinds, CT scanning (similar to the use of a medical scanner) to see inside things, and a method for using acoustical measurement to reconstruct the internal bore profile of a wind instrument.

After that comes a section devoted to the particular question of describing and measuring the properties of wood. Many musical instruments are, of course, made of wood, and the selection of suitable wood is a big concern for many instrument makers. There are two separate threads to this concern. First, for many instruments the vibrational properties of wood are directly important: density, various kinds of stiffness, and the level of internal damping. These factors can all be quantified by measurement, and this may help makers when selecting wood. The second thread is to do with sustainability. Many of the traditional timbers used by instrument makers are tropical hardwoods, and in many cases these are now threatened, and trade in them is legally restricted. Makers are very interested in alternative materials, whether that means other timber species that are more sustainable, or new man-made materials.

Section 10.4 addresses something we have encountered many times already. For many purposes, it is valuable to measure a frequency response function of some kind. This section gives an account of some aspects of such measurements. There are many options for actuators and sensors, each with their own advantages and disadvantages. The choice depends in part on having a clear sense of what exactly you want to get out of your measurement, and how accurately you need to know it. Finally, there is a subsection devoted specifically to measurements involving microphones and radiated sound. Any such measurements raise important issues to do with the acoustics of the room in which the measurement is done, as some examples illustrate.

Section 10.5 described one particular use to which frequency response functions may be put: experimental modal analysis. We have already seen some examples of measured mode shapes; this section gives a more detailed description of how the measurement method works.

The final section addresses a different type of measurement technique: it illustrates some ways to visualise fluid flows and acoustic sound fields. Fluid flows will become important in the next chapter, when we look in some detail at wind instruments.

In writing this rather wide-ranging chapter I have benefited enormously from conversations with several friends and colleagues: especially Evan Davis and Claire Barlow. Many of their good ideas are incorporated into what follows. Other colleagues have also freely contributed material for inclusion here, in areas that lie outside my own comfort zone: I thank Murray Campbell, David Sharp, Mico Hirschberg, Earl Williams, Lily Wang, George Stoppani and Rudolf Hopfner from Violinforensic of Vienna.