What is a sound level meter? 12

Frequency analysis displays the level for each frequency band. The sum of these noise levels (sound pressure levels) for each band is called the overall level, and analytical instruments also display this value.

In frequency analysis, A-weighting correction is sometimes applied and sometimes not (FLAT characteristics), depending on the purpose. To calculate the overall level with A-weighting correction from the frequency analysis values of the FLAT characteristics, the correction value from Table 11-3 is added to the measured value for each band (for example, the corrected value after 55.2 dB at 200 Hz is 55.2 - 10.9 = 44.3 dB). Now, if the band levels for each frequency band are L1, L2, ..., Ln (dB), the overall level L (dB) can be calculated using the following formula (this calculation corresponds to dB addition in Chapter 12, Section 1).

img-noise12-01 Equation 11-8

This method is frequently used in soundproofing measures, including predicting the effectiveness of soundproofing measures based on their characteristics.

Table 11-3 Characteristics of Frequency Weight A

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 value is obtained by directly calculating the average total power (RMS value) of the AC output (instantaneous sound pressure) signal without frequency analysis, and then displaying it as a level.

(2) Overalls
The AC output (instantaneous sound pressure) signal was analyzed in 1/1 and 1/3 octave bands. The level values were then calculated using Equation 11-8 from the 1/1 and 1/3 octave band data and Table 11-3, and the displayed values are shown.

Figure 11-7 below shows the results of a 1/3 octave analysis of normal noise using the Z (FLAT) characteristic, and it can be seen that the all-pass value and the 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 a difference between the all-pass and overall values, but both are valid values, so please be aware of this. In the analysis of normal steady-state noise data that does not include transient noise, the two values will be approximately equal (if there is a difference, it will be around 0.1 to 0.2 dB). When considering the frequency band, the frequency band of the all-pass value is the entire range of the input signal (e.g., the frequency response of a microphone, 20 Hz to 20 kHz), while the overall value is the band of the calculated frequency band, so caution is needed. In special cases, such as when the noise level is high around or above 20 kHz, the all-pass value may be larger. Furthermore, in recent sound level meters, all calculations from the RMS circuit (the dotted line in the diagram) onward in the sound level meter block diagram (Figure 8-1) are performed digitally. Although not shown in this diagram, when an analog signal that has passed through a frequency correction circuit is converted to a digital value using an A/D conversion based on a sampling frequency fS specific to the type of sound level meter (e.g., 64 kHz or 48 kHz), the maximum frequency bandwidth of the input signal (within half of the sampling frequency) is limited. The all-pass value of a sound level meter includes the power value up to that maximum frequency bandwidth, so in special cases, such as when noise data above 20 kHz exists, there may be differences between models even when comparing all-pass values.
Our sound level meter, which is capable of octave analysis, can simultaneously determine two all-pass values with different frequency and time weightings. Figure 11-8 shows an example of measuring and displaying two all-pass values (ALLPASS1, ALLPASS2).

	Frequency Weighting	Time weighting
ALLPASS1	A	Fast
ALLPASS2	C	Slow

In frequency analysis of noise levels and background noise levels of low-noise machinery, the all-pass value (or overall value) may be a positive dB value, while the band levels may be negative dB. A negative value means that the power is less than the standard value (20 μPa), not that the noise power itself is negative. Incidentally, if you sum the power of each band using equation 11-8, it will be equal to the all-pass value (or overall value). Figure 11-9 shows the self-noise characteristics of our microphone, where the OA value is approximately 11 dB, but each band value is around 0 dB or below, resulting in negative dB values.

This evaluation quantification was proposed in 1957 by Beranek, LL. of the United States. It targets steady-state noise with a broadband spectrum, such as air conditioning noise, and was compiled based on a large-scale survey of office noise levels and questionnaires given to the employees working there.

For the noise in question, the sound pressure level for each octave band of the noise is plotted on the NC curve shown in Figure 11-10. When all bands fall below a reference curve, the numerical value on that curve is taken as the evaluation quantity (NC count).