In the previous issue (October), we covered "equivalent sound level measurement" as an example of sound measurement, so this time we will introduce measurement methods including other measurement items that can be measured with a sound level meter.
The measurement items we will be discussing are as follows:
- A-weighted time-averaged sound level (LAeq: equivalent noise level)
- A-weighted acoustic exposure level (LAE: Single-event noise exposure level or noise exposure level)
- Maximum value of time-weighted sound level (LMX)
- Time-weighted minimum sound level (LMN)
- C-weighted peak sound level (LCpeak)
- Time-weighted sound level, time-weighted noise level (LN or Lx)
Please note that the above includes items called "maximum sound level" and "peak sound level," but these are two separate measurement items that are measured using different methods. We will also explain the differences between these two measurement items in this column.
A-weighted time-averaged sound level (L Aeq: equivalent noise level)
A-weighted time-averaged sound level, also known as equivalent noise level, is a value that expresses the energy average of fluctuating noise over a certain period of time (e.g., 10 minutes, 1 hour).
Environmental standards and workplace noise measurements require the measurement of this equivalent sound level. For details, please refer to the previous measurement column, "Measurement of Equivalent Sound Level."
Sound Measurement Examples - Part 2: "Measurement of Equivalent Noise Level"
A-weighted acoustic exposure level (L AE: single-event noise exposure level, noise exposure level)
A-weighted acoustic exposure level, also known as single-event noise exposure level or noise exposure level, is defined as a quantity that measures short-duration noise that occurs sporadically or intermittently. It calculates the total energy of noise generated over a short duration (from time t1 to t2) and converts it to the noise level of a steady-state sound with the same energy but a duration of 1 second.
It is used to evaluate noises that occur sporadically or intermittently, such as the sound of a train passing or the sound of pile driving.
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Figure 1: Meaning of Single-Event Noise Exposure Levels
Maximum value of time-weighted sound level (sound pressure level) (L MX)
This is the maximum sound level (sound pressure level) during a given measurement period (e.g., 10 minutes, 1 hour). Frequency weighting characteristics such as A-weighting or Z-weighting are used as needed. The maximum A-weighted sound level is also called the maximum noise level.
The "Standards for the Regulation of Noise Generated in Specific Factories" stipulate in Article 2 of the method for determining the magnitude of noise that, "If the reading of the sound level meter fluctuates periodically or intermittently, and the maximum value of the reading remains roughly constant, the average of the maximum readings for each fluctuation shall be used." This means that it is necessary to measure the maximum value of the noise level.
In general fields, "maximum value" and "peak value" often refer to the same thing, but in sound level measurement, there are two measurement items: the maximum sound level and the peak sound level, which are treated as separate measurement items.
Minimum value of time-weighted sound level (sound pressure level) (L MN)
This is the minimum sound level (sound pressure level) during a given measurement period (e.g., 10 minutes, 1 hour). Frequency weighting characteristics such as A-weighting or Z-weighting are used as needed.
C-weighted peak sound level (L Cpeak)
Sound level (sound pressure level) is calculated by determining the effective value from the instantaneous sound pressure (AC output waveform) and converting it to a level (dB value). However, the peak sound level value is obtained by converting the maximum absolute value of the instantaneous sound pressure (AC output waveform) during the measurement period to a level (dB value).
When measuring the peak sound level (L peak) and the maximum sound level (L MX) during a certain measurement period (e.g., 10 minutes, 1 hour), L peak > L MX, resulting in a difference of 3 dB for a steady sinusoidal signal and a difference of 10 dB or more for typical impact noise.
C-weighted peak sound level is required to be measured for European CE marking regulations, and is also specified as a measurement quantity in the measurement method for radiated sound pressure level of mechanical noise (JIS Z 8737 series).
Time-weighted sound level, time-weighted noise level (L N or L x)
Time-weighted noise level has long been used as a measure to evaluate the distribution of noise levels, which fluctuate irregularly and significantly.
When the duration for which the noise level exceeds a certain value corresponds to x percent of the measurement period (from time t1 to t2), that value is called the x percent time-weighted noise level and is represented by LN or Lx.
For example, if the measurement time is 100 seconds and the total time during which the level exceeds 90 dB is 5 seconds (5%), then L05 = 90 dB. Similarly, if the total time during which the level exceeds 85 dB is 10 seconds (10%), then L10 = 85 dB.
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Figure 2. Fluctuating noise and time-weighted noise level.
The "Standards for the Regulation of Noise Generated in Specific Factories" stipulate in item 3, regarding the method of determining the magnitude of noise, that "if the reading of the sound level meter fluctuates irregularly and significantly, the upper limit of the 90 percent range of the measured value shall be used." The 90 percent range of the measured value refers to a range (upper and lower limits) where, for 90% of the measurement time, the noise level falls between the upper and lower limits, for 5% of the measurement time it exceeds the upper limit, and for the remaining 5% it falls below the lower limit. The upper limit of the 90 percent range corresponds to the 5% time-weighted noise level L05, and the lower limit corresponds to the 95% time-weighted noise level L95.
Figure 3 shows an example of time-factored noise level measurement. Each time-factored noise level follows the relationship L05 ≥ L10 ≥ L50 ≥ L90 ≥ L95.
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Figure 3: Example of measuring time-weighted noise level
Measurement method
With the exception of some older models, our sound level meters (noise meters) can simultaneously measure time-weighted average sound level, acoustic exposure level, maximum and minimum sound level, peak sound level, and time-weighted sound level.
This section describes how to measure each parameter using the LA-1440 integral average sound level meter. Each measurement can be viewed after measurement by switching the display. While the switch names and menu configurations may differ when using other sound level meters (noise meters), the measurement procedure is largely the same.
1. Press the [A/C/Z] key to switch the frequency weighting characteristics.
2. Press the [FAST/SLOW] switch to toggle the time weighting characteristics.
3. Switch the level range.
Each time you press the △/▽ (level) switch, the level range cycles through "20 - 90dB" ⇔ "30 - 90dB" ⇔ "40 - 100dB" ⇔ "50 - 110dB" ⇔ "60 - 120dB" ⇔ "70 - 130dB" ⇔ "40 - 120dB". If the sound being measured exceeds the measurable range within the set level range, an overload (ovl) or underrange (udl) warning will be displayed.
Normally, setting it to a wide range ("40 - 120 dB"), which minimizes the risk of warning displays, will cover most measurements.
The set level range values are displayed in the lower left and lower right corners of the bar indicator at the bottom of the screen.
4. Set the measurement time (000:00:00 to 199:59:59) on the MEASURE screen in the MENU.
The diagram below shows an example where the measurement time is set to 10 minutes.
5. Press the [START] switch to begin the measurement.
During measurement, a calculation/measurement mark (▶▶) will be displayed. Once the set measurement time has elapsed, the calculation will automatically stop and the mark (▶▶) will disappear.
6. After the measurement is complete, press the [Lp/Leq] switch. Each press of the switch will cycle through the displayed items in the following order: LA, LAeq, LAE, LAMX, LAMN, LAPK, LA05, LA10, ... (The notation is for frequency weighting with A-weighting).
If a specific measurement item is set to be hidden, pressing the [Lp/Leq] switch will not display the measurement item. Press the [MENU] switch and set the measurement item you want to display to ON in the DISPLAY/LpLeq ON/OFF menu.
7. Pressing the [DISP] switch will switch to the list screen of measurement items.
If LIST 1 screen (L, Leq, LE, LMX, LMN, LPK) or LIST 2 screen (L05, L10, L50, L90, L95, ...) is set to hidden, pressing the [DISP] switch will not display the list screen. Press the [MENU] switch and set the screens you want to display to ON in the DISPLAY/DISP ON/OFF menu.
summary
This time, we introduced the measurement items and methods that are often measured in acoustic measurements. For more information on sound level meters, noise measurement methods, environmental standards, etc., please refer to the following materials.
Ono Sokki Technical Report: "What is a Sound Level Meter?"
(Excerpt from the email newsletter issued on December 20, 2012)
