Page 65 - Spring 2015

for bright and highly qualified students like Per, he chose Per to become his assistant. Per started working on sound transmission through walls. Later on, Pedersen suggested that Per should write a doctoral dissertation (the Dr techn) on acoustical impedance measured using a standing wave tube. The work on the doctoral dissertation took place over the subsequent years.
In September 1939, the Germans invaded Poland and the Danish army was called in. This included Per Brüel, who, as an electronics engineer, was transferred to a laboratory tasked with making radio communication equipment for armored cars. At this laboratory, Per, in his free time, built the world’s first battery-operated acoustic analyzer with a constant relative bandwidth. The analyzer color was strongly influenced by the military, with a light-green front plate and a dark-green box. These colors still characterize instruments produced by Brüel & Kjær A/S.
After the German occupation of Denmark on April 9, 1940, Per returned to work with Professor Pedersen and continued work on his doctoral dissertation. Per strongly admired Ped- ersen as a key person in all acoustics in Denmark, but unfor- tunately Pedersen died in 1941. Per now became involved in acoustics of buildings as an assistant to Professor Chr. Nøk- kentved, who was responsible for building the new Danish Broadcasting House. This job gave Per a deeper knowledge of room acoustics and sound absorbers and permitted him to work on his doctoral dissertation. In 1942, it was decided not to finish the Broadcasting House as long as the Germans were in Denmark. This finalized the work for Per in Den- mark for a while.
He had, however, maintained a cooperation with his friend from his studies at the TUD, Viggo Kjær, and they founded the company, Brüel & Kjær, which in 1946 became a lim- ited (A/S). But Per Brüel and Viggo Kjær both felt that they needed more industrial experience to be gained from work in other companies and they did not want to do work for the Germans.
While Viggo Kjær stayed in Denmark to gain experience in other companies, Per went to Sweden because he felt that their future main market would be Sweden. In Stockholm, Per became head of the acoustics department of Höganäs- Billesholm, working with sound absorption panels. At the beginning of 1944, Per was invited by Chalmers University in Gothenburg, Sweden, to establish an acoustical laborato- ry. Thus, he worked in building acoustics from 1944 to 1947, with objects like sound insulation, sound transmission, and
Figure 2. Dr. Brüel as the pilot.
acoustics in concert halls. One of Per’s assistants at Chalmers University was Dr. Uno Ingard, who later became a famous professor of acoustics at MIT and another was a Danish col- league, Freimuth Larris, who developed the level recorder for acoustic measurements that became the main product of Brüel & Kjær for many years to come.
Per frequently visited Denmark during his time in Sweden. During the two last years of WWII, Per operated as a courier and brought messages to and from Denmark. These messag- es were frequently wrapped in his dissertation manuscript and were written on the same type of paper as the manu- script to look similar to it. Professor Niels Bohr, who had fled to Sweden, used Per as a messenger when he needed to have communication with Denmark. Toward the end of the war, the Germans became extremely nervous and had soldiers onboard the ferry between Sweden and Denmark. On his way back to the defense of this doctoral dissertation at the TUD, Per was stopped onboard the ferry by one of the Germans. After ruffling all the papers around in the cabin, the German discovered a crack in the heel of one of Per’s shoes. He ripped off the heel and to be sure he also ripped off the heel of the other shoe. This brutal examination made Per very nervous, but the German did not find any “illegal” matters. For the defense of his dissertation the next day, Per turned up in his father’s tuxedo and in shoes with glued-on heels. The defense went well and he got his Dr techn. The results of his dissertation with the title, Application of the Tube-Method in Room Acoustics, became a product that for many years was manufactured and sold by the company Brüel & Kjær A/S.
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