Octave band analysis and FFT analysis
The topic of this article is Octave Band Analysis and FFT Analysis.
The analysis in octave bands and analysis by FFT (Fast Fourier Transform) are complementary techniques. FFT provides the maximum possible spectral resolution, while the analysis in octaves aggregates energy into bands of normalized width, approaching the way many physical phenomena and human hearing perceive sound and vibration.
1 Advantages of octave analysis compared to FFT
The main advantages of octave analysis over FFT are the following:
| Analysis in octaves | Advantage over FFT |
| Less amount of data | Reduces thousands of FFT spectral lines to a few dozen bands, facilitating interpretation. |
| Greater robustness to noise | Energy is integrated within each band, making the measurement less sensitive to small spectral variations. |
| Better comparison between measurements | Small deviations in frequency (for example due to variation in rotation speed) have little influence on the band level. |
| Standards compatibility | Most acoustic and vibration standards use octave or octave bands. 1/3 octave. |
| Representation of energy | Highlights the global energy content of a spectral region rather than individual peaks. |
| Less dependence on spectral resolution | It is not necessary to choose a high number of FFT lines to obtain consistent results. |
| Greater visual stability | Spectra show fewer fluctuations between successive measurements. |
| Better correlation with human perception | The octave and 1/3 of octave approximate the frequency resolution of the human ear. |
2. Simplification of interpretation
An FFT spectrum can contain between 400 and more than 3.000.000 spectral lines. Octave analysis reduces this information to about:
- 10 octave bands;
- 30 bands of 1/3 octave;
- 90 bands of 1/12 octave.
This makes it much simpler to identify where energy is concentrated.
3. Less sensitivity to speed variations
On rotating machines, the frequency of a component may vary by a few percent due to load or speed.
By FFT, a peak can shift from one spectral line to another, making comparison difficult.
In octave analysis, continues normally within the same band, maintaining practically the same energy level.
This feature is especially useful when monitoring machines with variable speed.
4. Better representation of energy
FFT shows amplitude at very specific frequencies.
Octave analysis calculates the total energy in an entire band.
For example:
- FFT:
- 238 Hz → 0,3 mm/s
- 244 Hz → 0,5 mm/s
- 251 Hz → 0,4 mm/s
The band of 250 Hz brings together all this energy in a single representative value.
When the objective is to evaluate exposure, sound power or vibratory energy, This representation is more significant.
5. Standards compatibility
Most international standards use octave or octave bands. 1/3 octave, for example:
- ISO 2631
- ISO 5349
- ISO 1996
- IEC 61260
For this reason, Practically all professional sound level meters present results in octave and octave bands. 1/3 octave.
5. Greater statistical stability
How each band integrates energy from many FFT lines, the result presents:
- lower variance;
- less influence of random noise;
- better repeatability.
This is particularly advantageous in environmental or long-term measurements.
6. Reduced influence of FFT resolution
By FFT, Insufficient resolution may result in:
- spectral escape (spectral leakage);
- amplitude error;
- division of energy across several lines.
In octave analysis, These effects have less impact because the energy is added over the entire band.
7. Better trend visualization
In continuous monitoring It is often more useful to follow:
- evolution of the band 125 Hz;
- evolution of the band 1 kHz;
- evolution of the band 8 kHz;
than tracking thousands of FFT lines.
This allows you to easily identify global changes in vibratory behavior or acoustic.
8 Limitations of octave analysis
Despite the advantages, analysis in octaves loses detailed information.
For example, two machines can present exactly the same level in the bandwidth. 1 kHz, but:
- one has a dominant peak 980 Hz;
- another a dominant peak at 1120 Hz.
The FFT clearly distinguishes these cases; analysis in octaves does not.
Therefore, octave analysis is not suitable for:
- accurate identification of fault frequencies;
- bearing diagnosis;
- gear analysis;
- identification of harmonics;
- calculation of rotation orders.
9 When to use each method
| Application | octave bands | FFT |
| Environmental noise assessment | ★★★★★ | ★★☆☆☆ |
| Compliance with standards | ★★★★★ | ★★☆☆☆ |
| Human exposure to noise and vibration | ★★★★★ | ★★☆☆☆ |
| Energy monitoring | ★★★★★ | ★★★☆☆ |
| Machine diagnostics | ★★☆☆☆ | ★★★★★ |
| Bearing identification | ★☆☆☆☆ | ★★★★★ |
| Gear Analysis | ★☆☆☆☆ | ★★★★★ |
| Identification of natural frequencies | ★★☆☆☆ | ★★★★★ |
| Balancing | ★☆☆☆☆ | ★★★★★ |
10 Octave band analysis and FFT analysis Conclusion
The main advantage of analysis in octave bands is to transform a high-resolution spectrum, often complex, in a representation more compact, robust and directly interpretable, suitable for energy assessment, comparison between measurements and compliance with standards. In contrast, FFT preserves all spectral information and is indispensable when the objective is the detailed diagnosis of machines or the precise identification of the frequencies of the phenomena under study.
