 Brüel was a close friend of Niels Bohr and frequently attended dinners at Bohr's house, where they would dine and discuss physics.When Brüel and Bohr both lived in Sweden during WW2, Brüel neglected apparent danger and flew to Denmark from time to time, carrying his friend Bohr's documents of his important work on quantum physics.Brüel was instrumental in the construction of the level recorder, which would become one of the most successful products for his company Brüel & Kjær.