Octave
Two frequencies are an octave apart if the ratio of the higher frequency to the lower frequency is two.
| Band Number | Nominal Centre Frequency Hz | Exact Centre Frequency Hz | Passband Hz |
| 3 | 2 | 2.00 | 1.41 - 2.82 |
| 6 | 4 | 3.98 | 2.82 - 5.62 |
| 9 | 8 | 7.94 | 5.62 - 11.2 |
| 12 | 16 | 15.85 | 11.2 - 22.4 |
| 15 | 31.5 | 31.62 | 22.4 - 44.7 |
| 18 | 63 | 63.10 | 44.7 - 89.1 |
| 21 | 125 | 125.89 | 89.1 - 178 |
| 24 | 250 | 251.19 | 178 - 355 |
| 27 | 500 | 501.19 | 355 - 708 |
| 30 | 1000 | 1000.0 | 708 - 1410 |
| 33 | 2000 | 1995.3 | 1410 - 2820 |
| 36 | 4000 | 3981.1 | 2820 - 5620 |
| 39 | 8000 | 7943.3 | 5620 - 11200 |
| 42 | 16000 | 15848.9 | 11200 - 22400 |
- 1/3 Octave Bands
- Preferred centre frequencies and passbands are defined by ISO R 266 and ANSI S1.6-1984. The nominal centre frequencies are used to identify the bands and are normally what is reported. The true centre frequencies are calculated using
- where
- fc = true centre frequency [Hz]
- n = band number
- 1/3 octave filters are sometimes referred to as 1/10 decade filters.
For convenience, 1/3-octave bands are sometimes numbered from band No. 1 (1.25 Hz third-octave centre frequency, which cannot be heard by humans) to band No. 43 (20000 Hz third-octave centre frequency).
| Band Number | Nominal Centre Frequency Hz | Exact Centre Frequency Hz | Passband Hz |
| 1 | 1.25 | 1.26 | 1.12 - 1.41 |
| 2 | 1.6 | 1.58 | 1.41 - 1.78 |
| 3 | 2 | 2.00 | 1.78 - 2.24 |
| 4 | 2.5 | 2.51 | 2.24 - 2.82 |
| 5 | 3.15 | 3.16 | 2.82 - 3.55 |
| 6 | 4 | 3.98 | 3.55 - 4.47 |
| 7 | 5 | 5.01 | 4.47 - 5.62 |
| 8 | 6.3 | 6.31 | 5.62 - 7.08 |
| 9 | 8 | 7.94 | 7.08 - 8.91 |
| 10 | 10 | 10.0 | 8.91 - 11.2 |
| 11 | 12.5 | 12.59 | 11.2 - 14.1 |
| 12 | 16 | 15.85 | 14.1 - 17.8 |
| 13 | 20 | 19.95 | 17.8 - 22.4 |
| 14 | 25 | 25.12 | 22.4 - 28.2 |
| 15 | 31.5 | 31.62 | 28.2 - 35.5 |
| 16 | 40 | 39.81 | 35.5 - 44.7 |
| 17 | 50 | 50.12 | 44.7 - 56.2 |
| 18 | 63 | 63.10 | 56.2 - 70.8 |
| 19 | 80 | 79.43 | 70.8 - 89.1 |
| 20 | 100 | 100.00 | 89.1 - 112 |
| 21 | 125 | 125.89 | 112 - 141 |
| 22 | 160 | 158.49 | 141 - 178 |
| 23 | 200 | 199.53 | 178 - 224 |
| 24 | 250 | 251.19 | 224 - 282 |
| 25 | 315 | 316.23 | 282 - 355 |
| 26 | 400 | 398.11 | 355 - 447 |
| 27 | 500 | 501.19 | 447 - 562 |
| 28 | 630 | 630.96 | 562 - 708 |
| 29 | 800 | 794.33 | 708 - 891 |
| 30 | 1000 | 1000.0 | 891 - 1120 |
| 31 | 1250 | 1258.9 | 1120 - 1410 |
| 32 | 1600 | 1584.9 | 1410 - 1780 |
| 33 | 2000 | 1995.3 | 1780 - 2240 |
| 34 | 2500 | 2511.9 | 2240 - 2820 |
| 35 | 3150 | 3162.3 | 2820 - 3550 |
| 36 | 4000 | 3981.1 | 3550 - 4470 |
| 37 | 5000 | 5011.9 | 4470 - 5620 |
| 38 | 6300 | 6309.6 | 5620 - 7080 |
| 39 | 8000 | 7943.3 | 7080 - 8910 |
| 40 | 10000 | 10000.0 | 8910 - 11200 |
| 41 | 12500 | 12589.3 | 11200 - 14100 |
| 42 | 16000 | 15848.9 | 14100 - 17800 |
| 43 | 20000 | 19952.6 | 17800 - 22400 |
For 1/3 octave filters the time required for the amplitude to approach the final value is ~1/B where B=filter bandwidth. For 100Hz centre, 23.1Hz bandwidth 1/B=1/23=0.04s.
|
See also: Bandwidth Time Product, Constant Percentage Bandwidth Filter, Frequency.
Subjects: Noise & Vibration