Technical Report: What is a Sound Level Meter? (Part 9)

This quantity has long been used as an evaluation quantifier for fluctuating noise and is used in Japan as an evaluation quantifier for noise under the Noise Regulation Law. It is effective for evaluating the distribution of noise levels that fluctuate irregularly and significantly.

As shown in Figure 9-7 below, when focusing on fluctuating noise within a certain measurement period, if the total time during which the noise level exceeds a certain level corresponds to x% of the measurement period T = t2 - t1, then that noise level is called the x% time-weighted sound level and is represented by Lx.
Figure 9-8 below shows the relationship described above for typical fluctuating noise, with the time-rate sound level on the horizontal axis and the percentage time-rate on the vertical axis.

(Note)
Time-weighted sound level is not specified in JIS C 1509-1; however, N percent time-weighted noise level is specified in JIS Z 8731.

In practice, when determining the time-weighted sound level, the noise level is sampled at regular time intervals Δt, as shown in Figure 9-9 below, and the results are statistically processed to obtain a predetermined time-weighted sound level. A common statistical processing method involves calculating the cumulative frequency distribution from the sampled values and defining the x-% time-weighted sound level as the sound level at which the cumulative percentage of the sound level equals (100 - x)%.

In Japan, the 50% time-weighted sound level L50 is called the median sound level, the 5% time-weighted sound level L5 and the 95% time-weighted sound level L95 are called the upper and lower limits of the 90% range (95% - 5% = 90% range width), respectively, and the 10% time-weighted sound level L10 and the 90% time-weighted sound level L90 are called the upper and lower limits of the 80% range, respectively. These five values have been widely used as evaluation quantities for fluctuating noise. These five values together are conventionally called the "five values."
Generally, the method used to measure time-weighted sound levels is the 50-times-5-second (equivalent to Δt) method.

Instantaneous values such as Lp and LA display the instantaneous value every second within the sampled data. In contrast, Tactmax displays the maximum value within the high-speed sampled data over one second. In Japan, instantaneous values are used as noise levels, but in Europe, Tactmax of 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 per second (i.e., Tactmax 1s)."