騒音計（サウンドレベルメータ）とは 12

When performing frequency analysis, the levels for each frequency band are displayed. The composite level, which is the sum of the noise levels (sound pressure levels) for each band, is referred to as the overall level, and this value is also displayed simultaneously on the analysis equipment.

In frequency analysis, A-weighting correction may or may not be applied (FLAT characteristic), depending on the purpose. To calculate the overall level with A-weighting correction from the frequency analysis values with FLAT characteristic, you can derive it from the measured values for each band by incorporating the correction values from Table 11-3 (for example, the corrected value at 200 Hz of 55.2 dB is calculated as 55.2 - 10.9 = 44.3 dB). Now, let the band levels for each frequency band be L ₁, L ₂, ..., L _n (dB), then the overall level L (dB) can be calculated using the following formula (this calculation corresponds to the dB addition in Section 1 of Chapter 12).

["source":"img-noise12-01"] Equation 11-8

This method is commonly used in soundproofing measures, applied to predictions based on the characteristics table of soundproofing effects after implementation.

Table 11-3 Frequency Weighting A Characteristics Table

Center frequency (Hz)	Correction value (dB)	Center frequency (Hz)	Correction value (dB)	Center frequency (Hz)	Correction value (dB)
12.5	-63.4	160	-13.4	2000	+1.2
16	-56.7	200	-10.9	2500	+1.3
20	-50.5	250	-8.6	3150	+1.2
25	-44.7	315	-6.6	4000	+1.0
31.5	-39.4	400	-4.8	5000	+0.5
40	-34.6	500	-3.2	6300	-0.1
50	-30.2	630	-1.9	8000	-1.1
63	-26.2	800	-0.8	10000	-2.5
80	-22.5	1000	0	12500	-4.3
100	-19.1	1250	+0.6	16000	-6.6
125	-16.1	1600	+1.0	20000	-9.3

(1) All Pass
This is the value obtained by directly averaging the total power (RMS) of the AC output (instantaneous sound pressure) signal without frequency analysis and displaying it as a level.

(2) Overall
This is the value obtained by performing 1/1 and 1/3 octave analysis on the AC output (instantaneous sound pressure) signal, calculating the level values from the 1/1 and 1/3 octave band data and Table 11-3 using Equation 11-8, and displaying it.

Figure 11-7 shows the result of a 1/3 octave analysis of normal noise with Z (FLAT) characteristics, where it can be seen that the All Pass value and Overall value are almost equal (upper data). The lower data is an example of calculating the A-weighted Overall value by recalculating the upper data using Table 11-3 and Equation 11-8.

There may be differences in the values of the all-pass and overall, but both are valid values, so please be careful. In the analysis of normal steady noise data that does not include transient noise, the values of both are almost equal (even if there is a difference, it is about 0.1 to 0.2 dB). Considering the frequency band, the frequency band of the all-pass is the entire range of the input signal (for example, the frequency characteristics of the microphone from 20 Hz to 20 kHz), while the overall is the band of the frequency band obtained, so caution is required. In special cases, such as when the noise level is high around or above 20 kHz, the all-pass value may be larger. Furthermore, recent sound level meters perform all digital processing after the effective value circuit (the dotted part of the figure) in the block diagram of the sound level meter (Figure 8-1). Although not shown in this figure, when converting the analog signal that has passed through the frequency correction circuit into a digital value by A/D conversion at a unique sampling frequency f _S (e.g., 64 kHz or 48 kHz) depending on the type of sound level meter, the maximum frequency band of the input signal (within 1/2 of the sampling frequency) is limited. The all-pass value of the sound level meter includes the power value up to that maximum frequency band, so in special cases where noise data above 20 kHz exists, there may also be differences in the comparison of all-pass values depending on the model.
Our sound level meter, capable of octave analysis, can obtain two simultaneous all-pass values with different frequency and time weighting. Figure 11-8 shows an example of the two all-pass values (ALLPASS1, ALLPASS2) measured and displayed.

	frequency weighting	the weight of time
ALLPASS1	A	Fast
ALLPASS2	C	Slow

In the frequency analysis of low-noise machinery's noise levels and background noise levels, even if the overall value (or overall value) is a positive dB value, the band levels can be negative dB. Being in negative values means that the power is smaller than the reference value (20 μPa), but the noise power itself is not a negative value. By the way, using Equation 11-8 to sum the power of each band will equal the overall value (or overall value). Figure 11-9 shows that the OA value of our microphone's self-noise characteristics is about 11 dB, while each band value is around 0 dB or below negative dB values.

The evaluation metric proposed by Beranek, L.L. in 1957 in the United States targets steady-state noise with a wideband spectrum, such as air conditioning noise, and is based on a large-scale survey of office noise and a questionnaire survey of the staff working there.

For the targeted noise, the sound pressure levels for each octave band are plotted on the NC curve shown in Figure 11-10, and when all bands fall below the reference curve, the value of that curve is defined as the evaluation metric (NC number).