Chapter 6 Underpinnings III: Hearing things


This chapter is the third in the “Underpinnings” sequence. This time, we look at how we hear sounds: a bit about the mechanics of hearing, and an introduction to the science of psychoacoustics which provides a way to obtain quantitative information about human perceptions and judgements.

Section 6.2 describes, briefly, how our ears work. Your inner ear contains a structure, called the basilar membrane, which acts as a kind of mechanical frequency analyser: the frequency components of incoming sounds are spread out along the membrane. This pattern is then encoded into the pattern of nerve fibres activated by the sound, because these fibres are connected to sensors at different places along the membrane.

Section 6.3 describes what is probably the most familiar psychoacoustical finding: a diagram showing how our perception of loudness of sine waves is influenced by their frequency and amplitude. This example gives an opportunity to introduce the kind of experiments that have to be carried out in order to address this kind of perceptual question.

Section 6.4 examines other basic elements of sound perception, including some detail on the filtering action of the basilar membrane. Aspects of the mechanical behaviour of the basilar membrane have a rather direct influence on how we perceive sound: some examples are given.

In section 6.5, we look at one type of musical question that can be addressed using psychoacoustical techniques. This is based on the idea of a threshold of perception, the smallest change in some attribute of sound such you are capable of “hearing the difference”. Experiments have been carried out to determine the threshold of perception for some things we are directly interested in: for example, shifting the body resonance frequencies in the body of a violin or a guitar. The results are of direct interest to instrument makers, by determining how big a change they need to make to the structure of an instrument before a player will know that something is different.

Finally, in section 6.6 we look at a very challenging type of problem: how do we quantify human judgements of quality and preference between instruments? Two examples are discussed, for which careful experiments have been carried out. The first concerns the choice of constructional material for a guitar body: do guitar makers really need to use traditional tropical hardwoods, or are more sustainable alternatives just as good? The second question addresses perhaps the most famous (or notorious) question in musical acoustics: is there really something special about certain old Italian violins, by makers such as Antonio Stradivari, or are contemporary violin makers capable of producing instruments that are just as good (or indeed better)?