The Sound Machine (Symbol)
The inaudible sound–detection device from which the story gets its title is a symbol for Klausner's attempt to turn to his subjective experience of reality into an objective, observable phenomenon. As a scientific recording instrument, the sound machine is Klausner's way of bridging the gap between what he theorizes to be true (i.e. that the world is full of sounds too high-pitched for the ear to hear) and what he can prove to be true. Thus, when the tree branch smashes the sound machine, Klausner loses not only his invention but his connection to objective reality.
The Axe (Symbol)
While the sound machine symbolizes Klausner's connection to objective reality as a genius inventor, the axe symbolizes Klausner's descent into madness. While the axe enters the story as a scientific instrument that Klausner uses to conduct an experiment, after the machine is smashed, the axe transforms into a symbol of Klausner's violent break with reality. With the axe in his hands, Klausner orders the Doctor to paint iodine on the tree's "wounds." The Doctor objects to the idea until he sees Klausner's hands menacingly tighten on the axe. In depicting this subtle gesture, Dahl cements the axe as a symbol of Klausner's madness.
Iodine (Symbol)
Although the story ends with Klausner having seemingly lost his mind by threatening the Doctor, the iodine that he demands the doctor paint on the tree's wounds symbolizes Klausner's sensitivity. Believing he has caused harm to the tree, Klausner wishes to make things right by helping heal the tree's axe wounds. While the iodine, intended for humans, is unlikely to do anything to the tree, Klausner is convinced that he can right his wrong and help the tree heal from its injuries.
