60S Bell System technical journal 



2. The rule of combination is specified so that the power indicated 



for a complex wave shall be the sum of the powers which would 

 be indicated for each of the single frequency components of the 

 complex wave acting alone. 



3. The dynamic characteristic of the indicating instrument is to be 



such that the deflection of the indicating instrument for a 

 constant 1000-cycle sinusoidal input shall be equalled by the 

 maximum deflection of the indicating instrument for a pulse 

 of 1000-cycle power which has the same magnitude as the 

 constant input and a time of duration lying between 0.2 and 

 0.25 second. 



In addition, the method of reading the sound level meter is important. 

 Where the noise is steady, it is fairly obvious how the meter should 

 be read. When, however, the noise fluctuates, a certain amount of 

 judgment is involved in obtaining an average. A satisfactory pro- 

 cedure in this event is to take a series of instantaneous readings of the 

 noise peaks at approximately 5-second intervals for a period of time 

 sufficient to include all noise sources. One or more of these series of 

 measurements may be made depending on the regularity of occurrence 

 of the noises of interest. The average and standard deviation of the 

 fluctuating noise may then be determined from these measurements. 



Using simplifying approximations based on these specified charac- 

 teristics a peak may be defined as follows: 



A peak is an impulse integrated by the measuring device. Its frequency 

 components are weighted in accordance with the loudness weighting incor- 

 porated in the meter and combined by direct power addition. 



It will be seen from the foregoing that the duration of the source 

 peaks depends on the period of the indicating meter. It has been 

 found that 0.2 second gives satisfactory correlation between computed 

 values and actual sound level meter readings, and is in reasonable 

 agreement with the above specified characteristics. Due to the meter 

 characteristics, full magnitude is not indicated for impulses shorter 

 than 0.2 second. Several impulses in the same integration period 

 appear as a single peak on the meter. Impulses lasting longer may 

 be regarded as producing a number of consecutive peaks. A steady 

 noise, for instance, would be considered as consisting of a series of 

 consecutive peaks of equal magnitude. 



On the assumption of discrete integration intervals the average 

 reading on a single source is the arithmetic mean of the intensities of 

 the source peaks. Hence for a source, j, producing on the average 

 mj peaks per minute of intensities, I\., h., /„,., the average reading on 

 the meter is given by 



