Technical Report: What is a sound level meter (sound level meter) 7

The sound level meter measures and displays the sound pressure level (L _p) and the noise level (L _A). Additionally, in the case of integrating sound level meters, it can calculate and display the equivalent noise level (L _Aeq), single event exposure noise level (L _AE), and time-averaged noise level (L _x). Here, we will explain how each of these displayed values is determined.

In the field of noise, it is used as a physical measure of the strength of sound waves (sound pressure). The unit is expressed in dB (decibels).

Sounds with high sound pressure levels can be considered strong sounds, while those with low levels can be considered weak sounds. The range of sound pressure levels primarily targeted is from 0 to 130 dB. Additionally, the frequency weighting characteristic used is Z (FLAT).

The definition formula for sound pressure level is expressed as follows:

Equation 9-1

By the way, the minimum audible sound pressure for humans is 20 μPa = 2 × 10^-5Pa, which serves as the reference sound pressurep₀. Therefore, the minimum audible value is:

Formula 9-2

The sound pressure level, which is a measure of the physical magnitude of sound, is given frequency weighting characteristics and time weighting characteristics, and in the display of the sound level meter, it appears as LAF or LCS (the first subscript represents the frequency weighting characteristic, and the second subscript represents the time weighting characteristic).
Those with A characteristics are also called noise levels.
The noise level is used as a measure of the magnitude of noise. The symbol typically used is L _A, and the unit is dB (decibels). In the old measurement law, the unit "phon" was used, but it is the same quantity as dB (it was revised to dB to align with international standards). As an A characteristic correction value, 40 dB is used as the reference sound pressure level at 1 kHz (0 dB), and equal-loudness contours that are perceived to be of equal magnitude are used (see Chapter 6, Section 4 "Loudness Level").

In JIS C 1509 sound level meters, the sound pressure level determined by a certain frequency weighting characteristic is referred to as 'sound level'; for example, when the frequency weighting is of A characteristic, it is called A characteristic sound level. Furthermore, this standard specifies three main types of sound levels.

The fluctuating noise is expressed as an energy average, which is a measure to evaluate how much noise a person has been exposed to and for how long. As shown in Figure 9-2, it is the time average value of the total energy of noise over a certain period. In light of recent advancements in noise measurement technology and international trends, the environmental standards revised and implemented in April 1999 adopted the A-weighted time average level, known as the equivalent noise level (L _{Aeq, T}), as an important indicator for environmental noise assessment. The equivalent noise level corresponds well to human physiological and psychological responses to fluctuating noise and is physically represented by the noise level of a steady noise that has the same energy as the fluctuating noise over the measurement time T = t ₂- t ₁.

Note

It is defined as the quantity that measures short-duration noise occurring sporadically or intermittently. This value is converted to the time-averaged sound level of a steady sound with a duration of 1 second that has the same energy as the total energy of the noise occurring sporadically, as shown in Figure 9-3.
The A-weighted sound exposure level (L _AE) is also referred to as the noise exposure level.

Intermittent noise such as train passing sounds and pile driving sounds is measured using this L _AE and converted to the A-weighted time-averaged sound level (equivalent noise level), serving as basic data for noise evaluation.

(Note)
In JIS C 1509-1, it refers to the A-weighted sound exposure level, which corresponds to the level of sound energy integrated over a certain period of time.