In between the chapters of this account are "interludes" that look in more detail at the history of specific instruments: brass instruments, the piano, the violin, acoustics in architecture, woodwind instruments, percussion, electronic instruments, and the voice. These are quite substantial, taking up half the book.
Writing for music students or lay readers, Johnston doesn't assume any prior knowledge of physics. He does explain a fair bit, but does so clearly and without introducing unnecessary details, and some of the technical material is left to appendices or marked so it can be skipped. The concepts of Fourier analysis and impedance are probably the most difficult things covered. Overall Johnston finds a nice balance between sweeping generalisations and detail; and he sensibly restricts his scope by not covering non-Western music or theories of music at all and only touching on psychoacoustics, in a description of the workings of the ear. The result is a work that should have something for anyone at all interested in music and not scared by a bit of simple mathematics and physics — or, as I can vouch myself, for someone with a physics degree but no musical education.
Disclaimer: Ian Johnston taught me physics over a decade ago.