Understanding Notch Filters

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A notch filter — also called a band-stop filter, band-reject filter, or frequency trap — is a frequency-selective signal-processing element that strongly attenuates vibration components inside a narrow frequency band while letting everything outside that band pass essentially unchanged. It is the exact opposite of a band-pass filter: rather than passing one band and blocking the rest, it blocks one band and passes the rest. Within the toolkit of signal filtering, the notch is the surgical instrument.

In vibration analysis, notch filters remove dominant interference such as 50/60 Hz electrical noise, suppress an overwhelming vibration component (a very large 1× unbalance that masks everything else), or knock down a resonance that obscures diagnostic detail. In effect they let the analyst “see around” a dominant frequency to reveal weaker but diagnostically important components hiding beneath it in the spectrum.

1. Filter Characteristics

Center (Notch) Frequency

• The frequency of maximum attenuation — the one being “notched out”.
• Tuned to a specific interference or unwanted frequency.
• Attenuation is typically 40–60 dB at the center.

Notch Bandwidth

• Narrow notch: rejects a very selective frequency range (high Q).
• Wide notch: rejects a broader frequency band (low Q).
• Q factor: center frequency divided by bandwidth.
• Typical: Q = 10–50 for vibration applications.

Attenuation Depth

• How far the notch frequency is reduced.
• Typically 40–60 dB, equivalent to a 100–1000× reduction.
• Higher-order filters provide deeper notches.
• Adjacent frequencies are minimally affected when the design is good.

2. Common Applications

Electrical Interference Removal

Eliminating power-line noise is the classic use:

• 60 Hz notch: removes 60 Hz electrical pickup in North America.
• 50 Hz notch: removes 50 Hz interference in Europe and Asia.
• Harmonics: additional notches at 120/180/240 Hz or 100/150/200 Hz.
• Benefit: a cleaner spectrum that reveals the underlying mechanical vibration.
• Caution: do not use it if 2× line frequency (120 or 100 Hz) carries diagnostic value — that band is a key indicator of electrical faults in motors.

Dominant Component Suppression

• Severe unbalance: notch out an overwhelming 1× to expose other components.
• High gear mesh: remove a dominant gear mesh frequency to reveal bearing frequencies.
• Strong resonance: suppress a structural resonance to see the excitation behind it.
• Purpose: reveal masked diagnostic information.

Sensor Resonance Elimination

• Remove artifacts from sensor mounting resonance.
• Place the notch at the mounting-resonance frequency, which varies with the mounting method.
• Ensures the measurement represents the machine, not the sensor.

Avoiding Aliasing Artifacts

• Notch out a specific high frequency before downsampling.
• Prevents aliasing of a known strong component.
• Complements the anti-aliasing low-pass filter rather than replacing it.

3. Design Considerations

Narrow Notch (High Q)

• Advantage: surgical removal of a single frequency with minimal effect on neighbours.
• Disadvantage: the target frequency must be precisely known and stable.
• Example: a 60.0 Hz ± 0.5 Hz notch for electrical interference.

Wide Notch (Low Q)

• Advantage: captures frequency variations, so the tuning is less critical.
• Disadvantage: may affect frequencies you wanted to keep.
• Example: a 1× ± 5 Hz notch to remove unbalance that drifts with running-speed fluctuations.

Depth vs. Width Trade-off

• Deeper notches (greater than 60 dB) often require a wider bandwidth.
• Very narrow notches may not achieve deep attenuation.
• Optimise the balance to suit the application.

4. Advantages and Limitations

Advantages

• Removes dominant interfering frequencies.
• Reveals masked diagnostic components.
• Improves use of the available dynamic range.
• Lets the analyst focus on weaker but important signals.

Limitations and Cautions

• Removes information: the notched-out content is lost permanently from the filtered record.
• Can hide problems: if the notched frequency had diagnostic value, the fault is missed.
• Phase distortion: notch filters significantly affect phase near the notch frequency.
• Ringing: sharp notches can create time-domain artifacts in the time waveform.
• Use cautiously: a notch should complement, never replace, unfiltered analysis.

5. Best Practices

When to Use a Notch Filter

• Known interference such as electrical noise is obscuring the measurement.
• A dominant component (severe unbalance) is preventing full use of dynamic range.
• Unfiltered analysis has already confirmed the notched frequency is not diagnostic.
• You want to reveal a weak signal for detailed examination.

When Not to Use One

• Routine screening measurements — use unfiltered data for general diagnosis.
• When the notched frequency has diagnostic value.
• Before you understand the full unfiltered spectrum.
• As a substitute for fixing the actual interference source.

Documentation

• Always document when a notch filter was used.
• Record the notch frequency and bandwidth.
• Keep the unfiltered data for reference.
• Note the reason for notch filtering in the diagnostic report.

6. Implementation

Hardware Notch Filters

• Fixed frequency, typically 50 or 60 Hz.
• Switched in and out as needed.
• An analog circuit inside the instrument.
• Operates in real time.

Software Notch Filters

• Applied to digitised data after acquisition.
• Adjustable center frequency and bandwidth.
• Different notch parameters can be tried and compared.
• Non-destructive — the original data is preserved.

7. The Notch Filter in Field Practice

In day-to-day field work the notch filter earns its keep when a single frequency drowns out everything you need to see. A common case is a rotor with a large unbalance: a portable analyser such as the Balanset-1A measures the 1× amplitude and phase directly for balancing, and once that dominant 1× component has been corrected, the spectrum opens up so the smaller bearing and gear signatures become readable without any notch at all. That illustrates the golden rule — wherever possible, cure the source rather than mask the symptom. The notch remains the right tool when the offending frequency is genuinely external, such as 50/60 Hz pickup on the cabling, and is confirmed to carry no mechanical meaning.

Notch filters are specialised signal-processing tools that selectively remove narrow frequency bands from a vibration signal. They are powerful for eliminating interference and revealing masked components, but they must be used judiciously and with full awareness of what is being discarded — always confirming that the notched frequencies hold no important diagnostic content.

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