Reliable measurement of hearing by audiometry requires quiet surroundings for which suggested criteria exist. When field studies of hearing are carried out it is frequently most practicable to use a vehicle in which these desired sound levels are obtained. It was decided to incorporate in the design of this vehicle a consulting room and a waiting room so that the patient could adjust to the surroundings for what is essentially a psychological test. Ventilation is effected by input and extractor fans.
As a basis for the acoustic design the sound levels in typical situations where the van was to be used were determined. From these it is possible to estimate the attenuation required. The walls and roof of the vehicle are of solid construction with a surface density of 5 lb./sq. ft., and the floor has a surface density of 6 lb./sq. ft. A commercially made booth is suspended on resilient mountings inside the consulting room. The total weight of all the equipment and the sound proofing is in the region of 2 to 3 tons.
After construction the sound attenuation of the vehicle was measured in nearly uniform sound fields, provided by a power station and jet engines. Measurements were made simultaneously outside the vehicle, in the consulting room, and inside the booth. The readings were taken in ⅓ octave bands with a calibrated condenser microphone and audio-frequency spectrometer. The results show that the attenuation of the vehicle shell closely follows the predicted value for 5 lb./sq. ft. Subtraction of the overall attenuation from the octave band sound levels in typical factory yards gave a measure of the sound levels to be expected within the booth under field conditions. These showed that the vehicle should be suitable for audiometric measurements of frequencies above 100 cycles per second. To get an accurate value of the attenuation of the booth, which is not symmetrically placed in the consulting room, a source giving a uniform field in the horizontal plane was placed inside the booth and sound level measurements made there and in the consulting room.
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