技術レポート騒音計（サウンドレベルメータ）とは 11

Table 11-1 Classification of Frequency Analysis

	analysis range	main uses
Fast Fourier Transform method	Fixed Width Analysis	Investigation of physical causes and countermeasures
Octave Analysis	Fixed Ratio Analysis	Evaluation of noise and vibration.

Octave analysis is also known as Constant Percentage Bandwidth (CPB) analysis. Here, we will explain octave analysis.

To implement noise control, noise analysis = frequency analysis is necessary. A method that has been used for a long time as noise analysis is octave analysis. An octave means double the frequency (see "What is an octave" below).

Octave analysis is often used because the frequency characteristics perceived by the ear are geometric. For the noise being measured, the sound pressure level for each band is determined by passing through bandpass filters specified by the 1/1 octave or 1/3 octave standards within the audible frequency range. The characteristics of the filters are specified in JIS C 1514:2002, so please refer to that for more details.

What is an octave:

An octave refers to the interval where the frequency ratio between two notes, such as from one 'do' note to the next higher 'do' note, is 2:1. An octave band is a range of frequencies centered around a specific frequency, where the upper and lower frequency limits have a ratio of 1 octave, and the center frequency of this band is called the octave band center frequency. When an octave band is divided into thirds, it is referred to as a 1/3 octave band. As shown in the right diagram, the equal temperament intervals in music divide an octave into 12 equal parts, resulting in a 1/12 octave band.

The translation of IEC standards in JIS (IDT) specifies filter characteristics, covering not only octaves and 1/3 octaves but also more generally up to 1/N octave filters. It also specifies tolerances for the required accuracy up to 2 classes (1, 2). This includes not only filter shapes but also requirements as equipment (such as the effects of environmental changes and EMC requirements). This standard requires octave frequency ratios based on powers of 10. The structure of the standard is similar to that of the JIS C 1509 series sound level meters.

Table 11-2 lists the center frequencies of octave and 1/3 octave filters. Note that these center frequency values are the nominal center frequencies specified in ISO 266, and the exact center frequencies are designed using values obtained from the formulas specified in the standard.

Center frequency (Hz)	1/1 Octave	1/3 octave	Center frequency (Hz)	1/1 Octave	1/3 octave	Center frequency (Hz)	1/1 Octave	1/3 octave
0.8		Available	25		Available	800		Available
1	Available	Available	31.5	Available	Available	1000	Available	Available
1.25		Available	40		Available	1250		Available
1.6		Available	50		Available	1600		Available
2	Available	Available	63	Available	Available	2000	Available	Available
2.5		Available	80		Available	2500		Available
3.15		Available	100		Available	3150		Available
4	Available	Available	125	Available	Available	4000	Available	Available
5		Available	160		Available	5000		Available
6.3		Available	200		Available	6300		Available
8	Available	Available	250	Available	Available	8000	Available	Available
10		Available	315		Available	10000		Available
12.5		Available	400		Available	12500		Available
16	Available	Available	500	Available	Available	16000	Available	Available
20		Available	630		Available	20000		Available

Table 11-3 Exact Center Frequencies of 1/3 Octave Bands in the Audible Range

exponent x	Nominal center frequency (1/3 octave) [Hz]	Exact center frequency by powers of 10 10 ^x/10 X 1000 [Hz]	Exact center frequency by power of 2【Reference】 (Filter design based on powers of 2 is not recommended in JIS C 1513-1.) 2 ^x/3 X 1000 [Hz]
-16	25	25.119	24.803
-15	31.5	31.623	31.250
-14	40	39.811	39.373
-13	50	50.119	49.606
-12	63	63.096	62.500
-11	80	79.433	78.745
-10	100	100.00	99.213
-9	125	125.89	125.00
-8	160	158.49	157.49
-7	200	199.53	198.43
-6	250	251.19	250.00
-5	315	316.23	314.98
-4	400	398.11	396.85
-3	500	501.19	500.00
-2	630	630.96	629.96
-1	800	794.33	793.70
0	1000	1,000.0	1,000
1	1,250	1,258.9	1,260
2	1,600	1,584.9	1,587
3	2,000	1,995.3	2,000
4	2,500	2,511.9	2,520
5	3,150	3,162.3	3,175
6	4,000	3,981.1	4,000
7	5,000	5,011.9	5,040
8	6,300	6,309.6	6,350
9	8,000	7,943.3	8,000
10	10,000	10,000	10,079
11	12,500	12,589	12,699
12	16,000	15,849	16,000
13	20,000	19,953	20,159

To convert the known dB values of 1/3 octave data to the corresponding dB values of 1/1 octave band data, calculate the sum of the dB values of the 1/3 octave band data that correspond to the desired 1/1 octave band. For example, when seeking the band data value for the 1/1 octave centered at 1000 Hz, if the corresponding 1/3 octave band data has the following dB values;