This measurement column addresses frequently asked questions received by our customer support center and provides answers to those questions.
Let's consider a scenario where measurements are taken repeatedly with frequency weighting: A, time weighting: FAST, and calculation time: 0.1 seconds. The equivalent noise level is calculated using the following formula: (JIS C 1509 Standard for Electroacoustic Performance of Sound Level Meters)
L Aeq: Equivalent noise level [Time t] 1 From time t 2 [The noise level of a stationary sound that has the same (equivalent) energy as all the fluctuating noise over a time period T (seconds) up to that point.]
T: Measurement time (t1 to t2)
p A (t) A-weighted sound pressure
p 0 Reference sound pressure (20 μPa)
What I want you to note here is that the above formula does not include time weighting. The equivalent sound level value is not affected by the time weighting setting of the sound level meter.
In contrast, because the L max and L min values are time-weighted, the noise level values over short intervals cannot keep up with fluctuations in sound pressure levels.
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Figure 1
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Figure 2
Figure 1 on the previous page plots the equivalent noise level over a 0.1-second period and the maximum and minimum noise levels over a 4-second period. It can be seen that when a sudden sound enters the room, the equivalent noise level becomes higher than the maximum value or lower than the minimum value.
Figure 2 plots the equivalent noise level and the maximum and minimum noise levels measured over an 80-second period, with a calculation time of 2 seconds. The equivalent noise level is plotted between the maximum and minimum values, giving an idea of the average.
As the equivalent noise level value does not include time weighting as described above, a discrepancy may occur between it and the noise level value (frequency weighted A-weighted, time weighted FAST (125 ms)) if the calculation is performed for a very short time. A setting of at least several seconds is appropriate.
Reference
In some cases, the peak level value 𝐿𝐿peak is used as an evaluation value for sudden sounds. The peak value is the maximum instantaneous sound pressure (AC signal) converted into a level. Since it takes the absolute value of the instantaneous sound pressure and converts that value into a level, it depends only on frequency weighting and does not involve time weighting.
In JIS C 1509-1, this is called peak sound level.
Figure 3 shows Figure 1 with the peak sound pressure level added. It can be seen that it has a steep rise and fall characteristic compared to the maximum value.
However, since peak values are not time-weighted, they behave differently from noise levels. While they are a useful indicator of whether the sound being measured is impactful, please note that they are separate from noise level values.
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Figure 3
(Excerpt from the email newsletter issued on April 15, 2020)
