The equivalent sound level is a quantity that expresses fluctuating noise as an energy average and evaluates how much noise a person is exposed to and for how long. It is a value expressed as a level by averaging the total energy of the noise over a certain period of time.
Environmental standards for noise are stipulated in "Environmental Standards for Noise (Ministry of the Environment Notification No. 64, September 30, 1998, revised: Ministry of the Environment Notification No. 45, May 26, 2005)," and prefectures or mayors of municipalities further establish environmental standards within the scope of these standards. Under these standards, noise is, in principle, evaluated by the equivalent sound level over the entire period of time for each time division.
This time, we will introduce the method for measuring equivalent sound levels using our company's sound level meter as an example.
Regarding the provisions in JIS C 1509-1
JIS C 1509-1:2005 "Sound level meters (noise meters) - Part 1: Specifications" specifies a measurement quantity called A-weighted time-averaged sound level, which corresponds to the equivalent sound level.
JIS C 1509-1 specifies that a sound level meter only needs to be able to measure one of the following three types of quantities:
- Time-weighted sound levels
- Time-averaged sound level
- Acoustic exposure level
Furthermore, the sound level meter must have a frequency weighting characteristic A, and the measured quantities weighted by frequency weighting characteristic A are referred to as follows:
- A-weighted time-weighted sound level (also called noise level)
- A-weighted time-averaged sound level (also called equivalent noise level)
- A-weighted acoustic exposure level (also called noise exposure level or single-event noise exposure level)
The instruction manuals for sound level meters conforming to JIS C 1509-1 may refer to the A-weighted time-averaged sound level, but this measured quantity corresponds to the equivalent noise level.
Evaluation methods for noise in environmental standards
The "Environmental Standards Related to Noise (Environmental Agency Notification No. 64, September 30, 1998; revised: Ministry of the Environment Notification No. 45, May 26, 2005)" stipulates that the standard values for environmental standards shall be the values obtained when evaluated by the following method.
- The evaluation shall be based on the noise level affecting individual residences, and shall be evaluated based on the noise level on the noise-sensitive side of the building used for residential purposes. In this case, the standard for noise transmitted into the building shall be evaluated by subtracting the sound insulation performance value of the building from the noise level on the noise-sensitive side of the building.
- The noise evaluation method shall be based on the equivalent sound level, and the evaluation shall, in principle, be based on the equivalent sound level over the entire duration for each time period.
- The evaluation period shall be selected to reflect the average noise levels throughout the year.
- When measurements are taken for evaluation purposes, the noise level measurement method specified in JIS Z 8731 shall be used as a general rule. If the effect of reflection from the building in question cannot be ignored, measurements shall be taken at a location where this effect can be avoided, and if this is difficult, appropriate measures shall be taken, such as correcting the measured value. Furthermore, if the necessary measurement time cannot be secured, the noise level may be estimated from conditions such as road traffic volume instead of taking measurements.
- Furthermore, factories and businesses that generate significant noise, construction sites, airports, railway premises, and similar locations will be excluded from the measurement locations.
- These environmental standards do not apply to aircraft noise, railway noise, and construction noise.
Section 4 of this standard references JIS Z 8731:1999, "Methods for Displaying and Measuring Environmental Noise." This standard stipulates that, unless otherwise specified, the setting of measurement points shall be as follows:
Outdoor measurements
When it is necessary to minimize the effects of reflection to a negligible degree, measurements should be taken at a distance of at least 3.5m from any reflective objects other than the ground, whenever possible. Unless otherwise specified, the height of the measurement point should be 1.2m to 1.5m above the ground. The height of other measurement points should be determined according to the purpose.
Measurements around the building
When investigating the degree of noise impact on a building, unless otherwise specified, measurements should be taken at a distance of 1 to 2 meters from the exterior wall surface of the building affected by noise, and at a height of 1.2 to 1.5 meters above the building's floor level.
Measurement inside a building
Unless otherwise specified, measurements should be taken at a height of 1.2m to 1.5m above the floor, at a distance of at least 1m from walls and other reflective surfaces, and approximately 1.5m from openings such as windows that are affected by the noise.
For specifications other than measurement points, please refer to JIS Z 8731:1999.
Procedure for measuring equivalent noise level using the LA-1440 integral average sound level meter.
Press the [A/C/Z] key to switch the frequency weighting characteristic to A-weighting.
Press the [FAST/SLOW] switch to change the time weighting characteristic to FAST.
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.
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.
Press the [START] switch to begin the measurement.
During measurement, a measurement progress indicator ( ) will be displayed. Once the set measurement time has elapsed, the calculation will automatically stop and the indicator ( ) will disappear.
After the measurement is complete, press the [Lp/Leq] switch to change the display item to A-weighted time-averaged sound level (equivalent noise level, LA eq).
To measure again, press the [START] switch to clear the calculation results and start the measurement again. At this time, you can press the [Lp/Leq] switch to switch the display item to A-weighted time-weighted sound level (noise level, LA), or you can leave it as LA eq.
If you are using a different sound level meter (noise meter), the switch names and menu configuration may differ, but the procedure for measuring the equivalent noise level is almost the same.
Measurement procedure using LA-1440: Repeated L Aeq measurements every 10 minutes for 24 hours and automatic saving.
For traffic noise and similar conditions, long-term measurements are conducted, such as 24-hour measurements from 6 AM to 6 AM the following day. For instructions on automatic measurement and saving every 10 minutes, please refer to the simplified operating manual below (clicking the URL will open the linked page in your browser).
summary
This time, we focused on the measurement method for equivalent sound level, which is often measured in acoustic measurements. Next time, we plan to introduce the measurement procedures for other measurement quantities.
Please also refer to the following documents for information on sound level meters, noise measurement methods, environmental standards, etc.
Ono Sokki Technical Report: "What is a Sound Level Meter?"
(Excerpt from the email newsletter issued on October 18, 2012)
