What is a sound level meter?

4. Measurement
4.1 General
4.2 Measuring equipment
4.3 Measurement points
4.4 Weather conditions
4.5 Measurement Method for Steady-State Noise
4.6 Method for estimating the maximum noise level
4.7 Method for measuring N percent time-weighted noise level
4.8 Method for measuring single-event noise exposure levels
4.9 Method for Calculating Equivalent Sound Level
5. Record Items
Annex JA (Reference) Frequency Weighting Characteristics of Sound Level Meters
Annex JB (Reference) Time-weighted characteristics of sound level meters
Annex JC (Reference) Measurement Methods for Environmental Noise in General Areas
Annex JD (Reference) Method for measuring environmental noise in areas facing roads
Annex JE (Reference) Methods for Measuring Aircraft Noise
Annex JF (Reference) Measurement Methods for Shinkansen Railway Noise
Annex JG (Reference) Measurement Methods for Conventional Railway Noise
Annex JH (Reference) Method for Measuring Wind Turbine Noise
Annex JI (Reference) Method for measuring noise generated in specific factories, etc.
Annex JJ (Reference) Method for measuring noise generated in connection with specific construction work
Annex JK (Reference) Comparison Table of JIS and Corresponding International Standards
Annex JL (Informative) Comparison Table of Technically Important Amendments

In light of the environmental noise measurement situation in Japan, JC to JJ are listed as references to facilitate a quick understanding of the manuals and regulations published by the Ministry of the Environment, which refer to this standard.

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

Weather conditions, topography, and ground surface shape.

When noise propagates outdoors, it can be greatly affected by weather conditions such as wind and temperature, as well as topography and ground surface shape. For example, regarding the effect of wind, generally, the magnitude of propagated sound increases with a tailwind and decreases with a headwind compared to when there is no wind. Regarding the effect of temperature, generally, sound propagates more difficult when the vertical temperature distribution is lower in the upper atmosphere and higher at the ground surface, and more easily when the temperature is higher in the upper atmosphere and there is an inversion layer. When sound propagates along the ground, generally, sound attenuation is greater on highly sound-absorbing ground surfaces such as fields and grasslands compared to reflective ground surfaces such as paved surfaces, making it difficult for sound to propagate over long distances. Therefore, when measuring noise, it is necessary to record as clearly as possible weather conditions, wind direction, wind speed, temperature, relative humidity, and topography and ground surface shape near the measurement point.
Wind noise

When a strong wind hits the microphone of a sound level meter, so-called wind noise is generated at that point. In particular, when the sound being measured is relatively quiet compared to the wind noise, the signal-to-noise ratio becomes insufficient, making measurement impossible. Therefore, when measuring noise outdoors or near machinery that generates wind, it is necessary to attach a windbreak screen designed to reduce wind noise. However, as wind speed increases, there are limits to the wind noise reduction effect of the windbreak screen, so measurements should be avoided in strong winds.
Other environmental conditions

Strong electric and magnetic fields are often formed near electrical machinery. Placing a sound level meter in such a location can affect the microphone and the electrical circuits of the sound level meter itself, resulting in inaccurate readings. When using an extended microphone cable, the extension cable is more susceptible to these effects. Furthermore, vibrations generated by various machines can be transmitted to the sound level meter itself, affecting measurements. In addition, high temperature and high humidity can cause malfunctions in sound level meters and other instruments.

As mentioned in Chapter 1, sound (sound waves) propagates through the air at a speed of approximately 340 m/s. At that time, even without any obstructions such as blockages or absorbers, the intensity (sound pressure level) of the sound decreases with distance traveled. In other words, sound waves have the property of attenuating by spreading (diverging) in all directions. This property is called distance attenuation of sound. Since sound waves are wave phenomena, the area over which the sound propagates increases as the distance from a single sound source increases, and therefore the intensity of the sound (sound energy per unit area) decreases. In particular, if the sound source can be considered as a point source, it can be thought of as propagating on a spherical surface in free space, and its area is 4πr² (where r is the distance from the sound source), so the intensity of the sound decreases inversely proportional to the square of the distance. This is called the inverse square law.