Technical Report: What is a Sound Level Meter? (Part 7)

Sound level meters measure and display sound pressure level (_Lp) and noise level (_LA). Integrating sound level meters can also calculate and display equivalent noise level (_LAeq), single-event exposure noise level (_LAe), and time-weighted noise level (_Lx) in addition to these two values. This section explains how each of these displayed values is determined.

In the field of noise, it is used as a physical measure of the intensity (sound pressure) of sound waves. Its unit is decibels (dB).

Sounds with high sound pressure levels are considered strong sounds, and sounds with low sound pressure levels are considered weak sounds. The sound pressure level range of 0 to 130 dB is the primary focus. The frequency weighting characteristic used is Z (FLAT).

The definition of sound pressure level is expressed as follows:

Equation 9-1

Incidentally, the minimum audible sound pressure for humans is 20 μPa = 2 × 10⁻⁵^Pa, and this is the reference sound pressurep₀_.Therefore, the minimum audible value is:

Equation 9-2

Sound pressure level, a measure of the physical magnitude of sound, is given frequency weighting and time weighting characteristics. In sound level meters, this is displayed as LAF or LCS (the first subscript represents the frequency weighting characteristic, and the second subscript represents the time weighting characteristic).
A-weighted characteristics are also called noise levels.
Noise level is used as a measure of the loudness of noise. The symbol is usually L _A, and the unit is dB (decibels). The old Measurement Law used the unit "phon," but it is the same quantity as dB (it was revised to dB to conform to international standards). For A-weighting correction, a sound pressure level of 40 dB, 1 kHz is used as the reference (0 dB), and an equal-impedance curve that is perceived as being of the same loudness is used (see Chapter 6, Section 4, "Loudness Level").

In JIS C 1509 Sound Level Meters, the sound pressure level obtained using a certain frequency weighting characteristic is called the "sound level." For example, if the frequency weighting is A-weighting, it becomes the A-weighted sound level. This standard also defines three main types of sound levels.

The equivalent sound level (LAeq, T) 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. As shown in Figure 9-2 below, it is a value that expresses the time average value of the total energy of noise within a certain period of time as a level. In light of recent improvements in noise measurement technology and international trends, the environmental standards revised and implemented in April 1999 adopted the equivalent sound level (_{LAeq, T}), which is an A-weighted time average level, as an environmental noise evaluation quantity, and it has become an important indicator for noise evaluation. The equivalent sound level corresponds well to the physiological and psychological responses of humans to fluctuating noise, and physically, it represents the noise level of fluctuating noise as the noise level of a steady-state noise with the same energy within the measurement time T = _t2-_t1.

(Note)

It is defined as a quantity for measuring short-duration noise that occurs sporadically or intermittently. As shown in Figure 9-3, this is the value converted to the time-averaged sound level of a 1-second steady sound with the same energy as the total energy of the sporadically occurring noise.
A-weighted acoustic exposure level (_LAE) is also known as noise exposure level.

For intermittently occurring noises such as the sound of passing trains or pile driving, this LAE (Last-A-Effect) is measured and converted to A-weighted time-averaged sound level (equivalent sound level), and is used as basic data for noise evaluation.

(Note)
According to JIS C 1509-1, this is called the A-weighted sound exposure level, and it corresponds to the level of sound energy integrated over a certain period of time.