Ryden L.

After a couple of devastating fires, where it was found that fire had propagated through unknown cracks and ducts, the Swedish National Property Board initiated a research project with the object of developing a method for finding risks that are not found by visual inspection. The Property Board administers the national building heritage like palaces, museum, theatres etc., often old buildings that have been rebuilt many times. During rebuilding old ventilation ducts and chimney flues might have been boarded up without any indication of where they are. In the case of a fire there is a very real risk that these might conduct hot fire gases, spreading the fire to other parts of the building. After discussions and preliminary tests it was agreed that the potentially best method was an acoustic one. The method was first field-tested in the Royal Palace in Stockholm and it was found to work well. In further work three methods were investigated: sound insulation measurements, listening tests and scanning with a semi-enclosed microphone. In all cases a loudspeaker was used to produce a high level wide band noise in the sending room. In the course of the project a semi-enclosed microphone was developed where the microphone is enclosed in an acoustically damped cup with a sealing lip so that sound coming from the outside is reduced when scanning the surface. The cup is mounted by an articulated fitting to a telescopic handle. The microphone is connected to a sound level meter which indicates the sound pressure level within the cup. Scanning surfaces with the semi-enclosed microphone indicates where and how large the leakages are and is a fast method that has proven its usefulness also in solving purely acoustical problems.