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Technical Report: What is a sound level meter?

9. Display value of the sound level meter

9-7 Time-Weighted Sound Level (percentile exceedance sound level) (L N)

The evaluation of fluctuating noise has been used for a long time as a measure of noise in noise regulation laws in our country, and it is effective in assessing the distribution of noise levels that fluctuate irregularly and significantly.

When focusing on fluctuating noise during a certain measurement period, as shown in Figure 9-7, if the total time that the noise level exceeds a certain level corresponds to x% of the measurement time T = t 2- t 1, this noise level is referred to as the x% time rate sound level, represented as L x.
Figure 9-8 shows the relationship for general fluctuating noise, with the time rate sound level on the horizontal axis and the percent time rate on the vertical axis.

(Note)
The time rate sound level is not specified in JIS C 1509-1, and the N percent time rate noise level is specified in JIS Z 8731.

  • Figure 9-7 Fluctuating Noise and Time Rate Sound Level
  • Figure 9-8 Relationship between Time Rate Sound Level and Percent Time Rate

When calculating the time rate sound level, noise levels are sampled at regular time intervals Δt, as shown in Figure 9-9, and the results are statistically processed to determine the specified time rate sound level. A common statistical processing method is to derive the cumulative frequency distribution from the sampled values, and the sound level at which the cumulative percentage of sound levels reaches (100 - x)% is defined as the x% time rate sound level.

In our country, the 50% time rate sound level L 50 is referred to as the median sound level, the 5% time rate sound level L 5 and the 95% time rate sound level L 95 are referred to as the upper and lower limits of the 90% range (95% - 5% = 90% range width), and the 10% time rate sound level L 10 and the 90% time rate sound level L 90 are referred to as the upper and lower limits of the 80% range, and these have been widely used as measures of fluctuating noise. Collectively, these five values are conventionally referred to as the "5 values".
As for the measurement method of the time rate sound level, the 5-second (equivalent to Δt) 50 times method is generally used.

  • Figure 9-9 Sound Level Sampling and Time Rate Sound Level
  • Figure 9-10 Cumulative Intensity Distribution of Sound Level and Time Rate Sound Level

[Reference] Maximum value over N seconds (Tact max)

Instantaneous values such as L p and L A display the instantaneous values every second from the sampled data. In contrast, Tactmax displays the maximum value from high-speed sampling data over one second. In Japan, instantaneous values are used as noise levels, while in Europe, Tactmax for 1 second or 3 seconds is commonly used.


(Note)
The old IEC standard for sound level meters (IEC 60651) stipulated that "the numerical display of the instantaneous sound pressure level shall be the maximum sound pressure level for every 1 second (i.e., Tactmax 1s)."

  • Figure 9-11: Sound Level Sampling and Tactmax Value (1 s)