What is sound level meter?
Sound level meters measure sound and display it as sound pressure level (Lp) and A-weighted sound pressure level (LA). In addition to these parameters, integrating sound level meters compute and display equivalent continuous A-weighted sound pressure level (LAeq), sound exposure level (LAE) and percentile sound pressure level (Lx). The following pages describe how each of these parameters is determined.
In the acoustic field, sound pressure level is a physical measure of the intensity (sound pressure) of sound waves. The unit of measurement is dB (deci Bel).
Sound with a high sound pressure level has high intensity; sound with a low sound pressure level has low intensity. The primary range of sound pressure level is 0 to 130 dB. Frequency weighting Z (FLAT) is used.
Sound pressure level is defined as follows.
(Equation 9-1) |
|
The reference sound pressure p0 is the minimum audible sound pressure for humans, i.e. 20 µPa = 2 × 10-5 Pa. Therefore, the minimum audible value is:
| (Equation 9-2) | |
Time-weighted sound level is the sound pressure level, which is a measure of the physical loudness of sound, given frequency weighting and time weighting, and is displayed as LAF or LCS on a sound level meter. (The first subscript represents the frequency weighting, and the second subscript represents the time weighting.) A-weighting is also called the noise level. Noise level is used as a measure of the loudness of noise.
A-weighted sound pressure level, normally abbreviated to LA, is used as a measure of noise, the unit of which is dB (deci Bel).
The old Measurement Act used phon as the unit of measurement; phon was later superseded by dB, which is equal to dB in sound pressure level, for alignment with international standards. Sound pressure level of 40 dB at 1 kHz is rated as a reference level. The curves below show the relationship between sound pressure levels that are perceived as being the same in loudness and frequency. These curves are called equal-loudness-level contours. (Please refer to Section 6.4, "Loudness level.")
Figure 9-1: Frequency weighting and permissible ranges |
NOTE: Sound level
IEC 61672 (JIS C1509 (Sound level meters)) specifies frequency-weighted sound pressure level as sound level. For example, A-weighted sound pressure level is termed A-weighted sound level. The standard specifies the following three key sound level types.
| Corresponding displayed parameter | |
|---|---|
| 1 Time-weighted sound level | Sound pressure level, noise level |
| 2 Time-average sound level | Equivalent continuous A-weighted sound pressure level |
| 3 Peak sound level | (This is not the maximum noise level.) |
As shown in Figure 8-1 (Block diagram for a sound level meter) and Figure 8-12 (Conversion of acoustic signals into digital forms), time-weighted sound level (1) is time-weighted squared instantaneous sound pressure level. According to IEC 61672 (JIS C1509), A-weighted and time-weighted sound level can be obtained using the following equation:
| (Equation 9-3) | |
|
|
Equation 9-3 shows how to calculate the root mean square value of frequency-weighted instantaneous sound pressure that corresponds to A-weighted and time-weighted sound level, or the conventional noise level. In addition, the equation is a function of measurement time t, indicating that the sound level is a time-dependent variable. With steady noise, the sound level stays nearly the same. With noise that fluctuates with time, time-average sound level (2) must be obtained.
This parameter presents fluctuating noise as average energy and is used to evaluate how long humans are exposed to what levels of noise. As shown in Figure 9-2, the parameter shows the average level of total energy of noise over a period of time. With advances in noise measurement technologies and international trends, the revised environmental standards for noise, enforced in April 1999, adopted A-weighted time-averaged sound level, (LAeq, T) as a parameter for evaluating environmental noise, and the parameter has since been an important indicator of environmental noise. Equivalent continuous A-weighted sound pressure level effectively represents the physiological and psychological reactions of humans to fluctuating noise and presents the physical level of fluctuating noise as the level of steady noise with energy equal to the fluctuating noise over the measurement time T = t2 − t1.
Figure 9-2: Fluctuating noise and equivalent continuous A-weighted sound pressure level |
(Equation 9-4) |
|
NOTE:
1. Equivalent continuous A-weighted sound pressure level is abbreviated to LAeq, T.
2. Equivalent continuous A-weighted sound pressure level corresponds to A-weighted time-average sound level of IEC 61672-1:2002 (JIS C1509-1:2005).
This parameter is defined as a measure of a single or intermittent noise of short duration. As shown in Figure 9-3, this value is converted to the time-averaged sound level of a steady sound that has energy equal to the total energy of a single-shot noise and a duration of 1 second. A-weighting sound exposure level (LAE) is also called noise exposure level.
Intermittent noise such as trains passing by and piles being hammered is measured for LAE, which is then converted into A-weighting time averaged sound level (equivalent continuous A-weighted sound pressure level)
Figure 9-3: Sound exposure level |
Sound exposure level is defined as follows.
| (Equation 9-5) | |
The relationship between sound exposure level and equivalent continuous A-weighted sound pressure level is as follows.
| (Equation 9-6) | |
| NOTE:
In IEC 61672-1:2002 (JIS C1509-1:2005), sound exposure level is termed A-weighted sound exposure level and corresponds to the level of energy of time-integrated sound. |
