The Overall Level represents the total RMS energy of a signal integrated across a specified frequency bandwidth. It condenses an entire spectrum into a single scalar value, making it the most widely used indicator for noise and vibration severity assessment.
The broadband overall level is computed by summing the power spectral density across the analysis bandwidth and taking the square root:
In discrete form, this becomes a summation over FFT spectral lines weighted by the frequency resolution Δf. When frequency weighting is applied, each spectral line is multiplied by the corresponding filter transfer function magnitude squared before summation.
Three standard weighting curves are available, defined in IEC 61672-1:
| Weighting | Characteristic | Typical Use |
|---|---|---|
| A-weighting (dBA) | Approximates human ear sensitivity; attenuates low and very high frequencies | Environmental noise, occupational exposure, NVH |
| C-weighting (dBC) | Nearly flat from 31.5 Hz to 8 kHz; mild rolloff at extremes | Peak sound pressure, low-frequency noise assessment |
| Z-weighting (dBZ) | Flat (no weighting) across the full bandwidth | Vibration, structural diagnostics, machine monitoring |
The level in decibels depends on the physical quantity being measured. Different reference values are used for different domains:
| Domain | Reference Value | Unit |
|---|---|---|
| Sound Pressure | 20 μPa (2 × 10−5 Pa) | dB SPL |
| Vibration Acceleration | 1 μm/s² (1 × 10−6 m/s²) | dB re 1 μm/s² |
| Vibration Velocity | 1 nm/s (1 × 10−9 m/s) | dB re 1 nm/s |
| Parameter | Options | Description |
|---|---|---|
| Lower Frequency (f1) | 0 Hz – fs/2 | Lower bound of the integration bandwidth. |
| Upper Frequency (f2) | f1 – fs/2 | Upper bound of the integration bandwidth. |
| Frequency Weighting | A, C, Z | Weighting filter applied before integration. |
| Spectrum Size | 256 – 65536 | FFT block size, determines frequency resolution. |
| Window | Hanning, Hamming, Flat Top, etc. | Time-domain window function. |
| Averaging | Linear, Exponential | Spectral averaging mode before level computation. |
Computes the OA for each successive time block, producing a time history of the broadband level. Useful for monitoring transient events, run-up/coast-down tests, and level stability checks.
When a tachometer signal is available, the OA is plotted against rotational speed. This reveals speed-dependent resonances and critical speeds. The RPM axis is derived from the tacho signal using zero-crossing or pulse-counting methods.
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