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) |
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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) | |
9-2 A-weighted sound pressure level (LA)
This parameter is what is commonly called "noise level" in Japan. It is sound level, a physical measure of sound, compensated by A-weighting. 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The old Measurement Act used phon as the unit of measurement; phon was later superseded by dB, which is equal to dB in sound pressure level, for alignment with international standards.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) | |
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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.
9 -3 Equivalent continuous A-weighted sound pressure level (LAeq,T)
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 equivalent continuous A-weighted sound pressure 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) |
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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).
9-4 Sound exposure level (LAE)
This parameter is defined as a measure of a single or intermittent noise of short duration. As shown in Figure 9-3, the total energy of a single sound is converted into a level of steady sound lasting one second with energy equal to the total energy of the single sound.
Intermittent noise such as trains passing by and piles being hammered is measured for LAE, which is then converted into equivalent continuous A-weighted sound pressure level for use as basic data for noise evaluation.
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. |
9-5 Percentile Sound Pressure Level (Lx)
This parameter has been used for many years as a measure of fluctuating sound. It is also listed as a measure of noise in the Noise Regulation Act. The parameter is effective for clarifying the distribution of the levels of irregularly and significantly fluctuating noise.
With reference to Figure 9-4, which shows fluctuating sound over a measurement time period, when the total of the time when the sound exceeds a certain level is x% of the measurement time T = t2 − t1, the sound level is called x % of the percentile sound pressure level and is abbreviated to Lx . Figure 9-5 shows the relationship between percentile sound pressure level (horizontal axis) of typical fluctuating noise and the percentage of level-exceeding time (vertical axis).
NOTE: In ISO 1996-1 (JIS Z 8731:1999), the parameter is abbreviated to LAN, T. |
Figure 9-4: Fluctuating noise and percentile sound pressure level |
Figure 9-5: Relationship between percentile sound pressure level and the percentage of level-exceeding time |
In practice, percentile sound pressure level is obtained by, as shown in Figure 9-6, taking samples of noise level for every time segment Δt and processing the data statistically. In a typical processing, the cumulative frequency distribution is obtained from the samples and the noise level with a percentage accumulation of (100 − x) % is determined as x % of the percentile sound pressure.
In Japan, 50 % of percentile sound pressure level L50 is called median sound pressure level, 5 % of percentile sound pressure level L5 and 95 % of percentile sound pressure level L95 are called the upper and lower limit of the 90 % (= 95 % – 5 %) range, and 10 % of percentile sound pressure level L10 and 90 % of percentile sound pressure level L90 are called the upper and lower limit of the 80 % range. These, collectively called the five parameters, have been widely used for evaluating fluctuating noise. When measuring percentile sound pressure level, normally 50 measurements are taken, with each lasting five seconds (Δt).
Figure 9-6: Sound level samples and percentile sound pressure level |
Figure 9-7: Cumulative frequency distribution of noise level and percentile sound pressure level |
