From the 1989 lecture programme:
The origin of musical instruments is lost in the mists of time. It has been suggested that the strings developed from the twang of a bow string and the wind section seems very likely to have developed from the pan-pipes made with lengths of hollow reeds or from the sounds that can be produced by blowing into an animal horn. We shall be more concerned, however, with the essential features that have to be present in any instrument if a usable musical sound is to be produced.
The characteristic of a simple musical note is regularity of the pressure changes and the necessity for their frequency to be within the range to which human ears are sensitive. So we must obviously start with a device that will produce such regularity. It could be a vibration (or "wobble") or it could be rotation (like the wheel of a siren).
Most instruments depend on vibration of air in pipes, of tightly stretched strings, of more-or-less flat plates, or of hollow shapes like bells. So we shall need to start by thinking about how such things vibrate. And we shall have to consider how the vibrations are started and what effect this has on the notes. Pulling a cork out of a bottle makes a musical sound, but it is very short lived; how can we keep feeding in energy to make a continuous note?
Clearly plucking a guitar string, or striking a piano key make quite different sounds from those made by bowing a violin, even though the primary source is in each case a stretched string; so we shall ask how bowing can feed in energy to keep the sound going. Frequently we find that, even if we can keep it going, the sound is too quiet to be heard (a violin string without a body is almost inaudible).
So we need to amplify the sound and this can be done by adding a soundboard (e.g. in the piano), or a hollow body (e.g. in the acoustic guitar), or, in more recent times, by electronic means (e.g. in the electric guitar). But, as soon as we add an amplifier, complications arise. The amplifier does not instantly start to vibrate and the delay produces a sound which is characteristic of the instrument and provides one of the most important signals that help us to identify it. An amplifier will also change the quality of the sound as well as delaying the start and making it louder.
We shall explore all these effects and their consequences for the design of instruments, for the player, and for the listener. We shall also begin to look at their consequences for music produced by synthesizers or computers, a topic to which we shall return in Lecture V.