FFT Peak Hold maintains a running maximum envelope across successive FFT spectra. At each frequency bin, the peak hold spectrum retains the highest magnitude observed since the measurement started or was last reset. It captures the worst-case spectral content regardless of when individual peaks occurred.
Given a sequence of N averaged spectra Sk(f), the peak hold spectrum is defined as:
The maximum operation is applied independently at each frequency bin, meaning the peak hold spectrum may represent a composite of peaks that occurred at different times. This distinguishes it from a single snapshot spectrum which captures all frequencies at the same instant.
| Application | Description |
|---|---|
| Worst-case noise assessment | Captures the maximum noise level at every frequency across an entire production run or test cycle, ensuring no intermittent exceedances are missed. |
| Transient capture | Preserves spectral signatures of brief events (impacts, clicks, rattles) that would be averaged out by linear or exponential averaging. |
| Pass/fail testing | Compares the peak hold spectrum against a limit curve to verify that no frequency exceeds the specification at any point during the test. |
| Run-up/coast-down envelope | During speed sweeps, the peak hold accumulates the resonance peaks from all speeds, producing an envelope that reveals all excited modes. |
| Intermittent fault detection | Captures spectral signatures of sporadic bearing defects, gear tooth faults, or electrical arcing that appear only briefly. |
| Parameter | Options | Description |
|---|---|---|
| Spectrum Size | 256 – 65536 | FFT block size. Determines frequency resolution. |
| Window | Hanning, Hamming, Flat Top, etc. | Window function. Flat Top recommended for accurate peak amplitudes. |
| Overlap | 0% – 95% | Block overlap. Higher overlap increases temporal resolution and reduces the chance of missing brief transients. |
| Update Rate | Continuous | Each new FFT block is compared against the current peak hold. |
| Reset | Manual / Trigger | Clears the peak hold buffer and restarts accumulation. |
The Peak Hold vs Time variant displays the evolution of the peak hold spectrum as it accumulates over time. Early in the measurement, the peak hold changes rapidly as new maxima are discovered. As the measurement progresses, changes become increasingly rare, occurring only when an exceptionally loud event produces a new record at one or more frequency bins.
This temporal evolution can be visualized as a waterfall plot, where each slice shows the peak hold state at that moment. Comparing successive slices reveals when each peak was first captured.
| Mode | Behavior | Best For |
|---|---|---|
| Linear Average | Arithmetic mean of all spectra | Stationary signals, variance reduction |
| Exponential Average | Weighted toward recent spectra | Slowly varying signals, real-time monitoring |
| Peak Hold | Running maximum at each bin | Worst-case assessment, transient capture |
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