This paper reviews the theory and describes the method used at Bell Telephone Laboratories of measuring the transmission loss through partition walls. The partition to be tested is built into an opening between two adjacent but structurally isolated rooms. A loud speaker acts as a source of sound in one room and a portion of the sound energy is transmitted into the second room through the test partition. The transmission loss is taken as TL = L1 − L2 − 0 log10 (a2/A), where L1 and L2 are the intensity levels in the source and test room respectively, expressed in db, a2 is the absorption in the test room and A is the area of the partition. The levels L1 and L2 are measured and plotted with a moving coil microphone and an automatic level recorder, and a beat frequency oscillator is used as a source of tone so that the frequency may be varied continuously. Measurements with a continuous variation in frequency enable resonances in the partition to be much more easily and quickly detected than is possible when measurements are made at discrete frequency intervals. Both pure and frequency modulated tones have been used for the measurements. Results of measurements on a few partitions are given.
A device has been developed which will record the intensity of audiofrequency currents directly on a moving strip of paper by means of a stylus. A wide range of intensities may be covered and the deflection of the stylus is proportional to the logarithm of the input current; the recorder thus has a uniform scale in decibel units. It is capable of following fluctuations in the input current up to 150 per second, and will follow changes in intensity at a rate up to 240 db per second. When used in conjunction with a microphone the recorder lends itself to many types of acoustical measurements. It has been extensively used for reverberation time measurements for which it is particularly well adapted because it will plot the intensity of the reverberant sound on a logarithmic scale during the decay period, a complete decay curve being obtained each time the test tone is interrupted. When used with a beat frequency oscillator curves of intensity vs. frequency may be obtained. This type of curve is useful for loudspeaker and microphone calibrations for determining the distribution of sound in rooms and for intensity measurements for obtaining the sound transmission through partitions. Examples of the above types of curves and others obtained with speech and music are given.
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