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A sound level meter is 10

10. Method for Measuring Equivalent Noise Levels (Overview of JIS Z 8731:2019)

JIS Z8731 has also been revised based on ISO 1996-1:2016 and ISO 1996-2:2017, following the years 1983 and 1999.
It is structured as follows. For details regarding the content of the standard, please refer to the standard document.

4. Measurement
4.1 General Information
4.2 Measuring Instruments
4.3 Measurement Points
4.4 Weather Conditions
4.5 Methods for Measuring Steady-State Noise
4.6 Methods for Assuming Maximum Noise Levels
4.7 Method for Measuring N-Percent Time-Rate Noise Level
4.8 Methods for Measuring Single-Shot Noise Exposure Levels
4.9 Method for Calculating Equivalent Noise Levels
5 Records
Frequency Weighting Characteristics of Appendix JA (Reference) sound level meter
Attachment JB (Reference) sound level meter Time Weighting Characteristics
Annex JC (Reference) Methods for Measuring Environmental Noise in General Areas
Annex JD (Reference) Methods for Measuring Environmental Noise in Road-Facing Areas
Annex JE (Reference) Methods for Measuring Aircraft Noise
Annex JF (Reference) Methods for Measuring Shinkansen Railway Noise
Annex JG (Reference) Methods for Measuring Noise from Conventional Railways
Annex JH (Reference) Methods for Measuring Wind Turbine Noise
Annex JI (Reference) Methods for Measuring Noise Generated at Specified Factories, etc.
Appendix JJ (Reference) Methods for Measuring Noise Generated in Specific Construction Work
Annex JK (Reference) Comparison Table Between JIS and Corresponding International Standards
Annex JL (Reference) Comparison Table of Technically Important Amendments

In light of the measurement circumstances of environmental noise in our country, standards JC to JJ are referenced as a guide for a quick understanding of the manuals and regulations published by the Ministry of the Environment.

What was previously referred to as 'ambient noise' has now been defined as 'background noise'.

10-1 Environmental conditions affecting measurements

  1. weather conditions, topography, surface shape

    When noise propagates outdoors, it can be greatly influenced by weather conditions such as wind and temperature, as well as terrain and surface shapes. For example, regarding the influence of wind, the intensity of sound generally increases with a tailwind compared to when there is no wind, and decreases with a headwind. Additionally, concerning temperature, sound is more difficult to propagate when the vertical distribution of temperature is such that it is cooler aloft and warmer at the surface, whereas it propagates more easily when it is warmer aloft with an inversion layer present. When sound propagates along the ground, it generally experiences greater attenuation over highly absorptive surfaces like fields and grasslands compared to reflective surfaces like paved areas, making it harder to travel long distances. Therefore, it is essential to clearly record weather conditions such as wind direction, wind speed, temperature, and relative humidity, as well as terrain and surface shapes, as conditions during noise measurement.

  2. the influence of wind noise

    When strong wind hits the microphone of a sound level meter, so-called wind noise occurs at that point, and especially when the sound being measured is relatively small compared to the wind noise, the signal-to-noise ratio becomes insufficient, making measurement impossible. Therefore, when measuring noise outdoors or near machines that generate wind, it is necessary to attach a windscreen designed to reduce wind noise. However, as wind speed increases, there are limits to the wind noise reduction effect of the windscreen, so measurements should be avoided during strong winds.

  3. the influence of other environmental conditions

    In the vicinity of electrical machinery, strong electric and magnetic fields are often formed, and placing a sound level meter in such locations can affect the microphone and the electrical circuit parts of the sound level meter, resulting in inaccurate readings. When using an extended microphone cable, the extension can become more susceptible to these influences. Furthermore, vibrations generated by various machines can be transmitted to the sound level meter, affecting measurements. Additionally, under high temperature and high humidity conditions, instruments such as sound level meters may be hindered.

As mentioned above, due to various influences, it is necessary to confirm the presence and extent of these influences in advance. If the influences are likely to be problematic, it is important to devise appropriate measures such as shielding or vibration isolation for each influencing factor, as well as to pay careful attention to the selection of measurement points.

10-2 Sound Propagation and Distance Attenuation

As mentioned in Chapter 1, sound (sound waves) propagates through the air at a speed of about 340 m/s. At that time, especially without any obstacles such as blocking or absorption, the strength (sound pressure level) decreases with the distance it has propagated. In other words, sound waves have the property of attenuating by spreading out in all directions (diverging). This property is called distance attenuation of sound. Since sound waves are wave phenomena, as one moves away from a sound source, the area over which they propagate expands, causing the strength of the sound (the energy of sound per unit area) to weaken. In particular, when the sound source can be considered a point source, it is often thought to propagate on a spherical surface in free space, and that area is 4πr 2 (where r is the distance from the sound source), so the strength of the sound attenuates in inverse proportion to the square of the distance. This is called the inverse square law.

 

In Figure 10-1, let the sound pressure levels at distances r 1 and r 2 from the point sound source P be L 1 and L 2, respectively;


Formula 10-1

This relates to the fact that, for example, if the distance doubles, it decreases by 6 dB.

  • Figure 10-1 Diffusion from a Point Sound Source and Distance Attenuation