Tom Service considers the texture of music. We often talk about the pitches and the rhythms in a piece of music, but how does it strike the ear? Is it rough or smooth, dense or transparent? And how are such textures achieved? He talks to composer Anna Meredith about how she creates excitement through combining different layers of orchestral sound; and to arranger Iain Farrington about how to preserve the textures of a Mahler symphony when it's arranged for only a dozen musicians.