["Ionisation"] is built on a most sensitive handling and contrast of different kinds of percussive sounds. There are those indefinite in pitch, like the bass drum, snare drum, wood blocks, and cymbals; those of relatively definite musical pitch, such as the piano and chimes; those of continually moving pitch, like the sirens and 'lion's roar.' It is an example of 'spatial construction,' building up to a great complexity of interlocking 'planes' of rhythm and timbre, and then relaxing the tension with the slowing of rhythm, the entrance of the chimes, and the enlargement of the 'silences' between sounds. There are suggestions of the characteristic sounds of modern city life.