Measuring Vibration Frequency
Frequency is one of the most important parameters in vibration monitoring. Understanding how it is measured helps you interpret compliance reports, set correct thresholds, and protect structures more effectively.
Every vibration has two defining characteristics: how fast the motion occurs (frequency) and how intense it is (amplitude). Standards like DIN 4150-3 and SBR-A do not simply set a single velocity limit. They set limits that change depending on frequency. Without accurate frequency measurement, compliance assessment is impossible.
What Is Vibration Frequency?
Frequency describes how many complete oscillation cycles occur per second. It is expressed in Hertz (Hz). A vibration at 10 Hz completes 10 full back-and-forth cycles every second. A vibration at 50 Hz completes fifty.
In practice, the vibrations caused by pile driving, demolition, or heavy machinery are not a single clean frequency. They contain a mix of frequencies at the same time. The dominant frequency is the one carrying the most energy. This is the frequency that monitoring systems extract for comparison against standard limit curves.
Key concept: Dominant frequency is the frequency at which a vibration event carries its highest energy. Building standards use this value to select the correct limit from their frequency-dependent curves. Measuring PPV (Peak Particle Velocity) alone, without frequency, gives an incomplete and potentially misleading picture.
Why Frequency Matters for Compliance
Standards like DIN 4150-3, BS 7385-2, and SBR-A define limits as a curve rather than a single number. The allowable Peak Particle Velocity (PPV) at 5 Hz is very different from the allowable PPV at 50 Hz. Lower frequencies are generally more damaging to structures, so limits are stricter in that range.
This means a project on a soft-soil site producing low-frequency ground movement faces tighter constraints than a project with high-frequency vibration at the same PPV level. Getting the frequency wrong leads to either unnecessary work stoppages or actual risk to surrounding buildings.
| Standard | Frequency Range | What Changes with Frequency |
|---|---|---|
| DIN 4150-3 | 1 to 80 Hz | PPV limit rises with frequency for industrial and commercial buildings. |
| SBR-A | 1 to 100 Hz | Three vibration classes with frequency-dependent A1 reference values. |
| BS 7385-2 | 1 to 250 Hz | Limit curves split into low, medium, and high sensitivity structures. |
| ISO 4866 | 0.1 to 80 Hz | Methodological reference for frequency weighting and filtering. |
How Frequency is Measured in Practice
Modern vibration monitors measure frequency through signal processing. The sensor captures a continuous time-domain signal: a waveform that shows how particle velocity or acceleration changes over time. That raw signal is then analyzed to extract frequency content.
Sensor captures the waveform
A geophone or MEMS accelerometer records particle velocity or acceleration as an analog signal in three axes: vertical, transverse, and longitudinal.
Analog-to-digital conversion
The continuous waveform is sampled at a high rate (typically several hundred to several thousand samples per second) and converted to a digital time series.
Fast Fourier Transform (FFT)
The logger applies an FFT to transform the time-domain signal into a frequency spectrum. This shows the amplitude at each frequency component present in the event.
Dominant frequency identification
The system identifies the frequency with the highest spectral amplitude. This is stored as the dominant frequency alongside the PPV for each recorded event.
Comparison with the limit curve
The monitoring system plots the measured PPV and dominant frequency against the applicable standard curve to determine compliance automatically.
Geophones vs. MEMS Accelerometers
The choice of sensor directly affects how accurately frequency is captured, particularly at the low end of the spectrum.
Geophone Sensors Electromechanical sensors with a natural frequency typically around 4.5 Hz. Very accurate in the 4 to 300 Hz range but begin to roll off below their natural frequency. A common source of missed low-frequency events in older systems.
MEMS Accelerometers Micro-electromechanical sensors with a flat response from near 0 Hz upward. Modern systems like the Vibra 5+ use MEMS combined with advanced signal processing to achieve accurate measurements from 0.5 Hz, covering frequencies that geophones often miss.
Tri-axial Measurement measurement Vibration frequency can differ per axis. A tri-axial sensor captures all three spatial directions simultaneously and reports the dominant frequency and PPV per axis as well as the resultant vector.
Low-Frequency Vibration: A Critical Blind Spot
Low-frequency vibrations, typically in the 1 to 10 Hz range, are particularly relevant near soft soil conditions, during deep foundation work, and for sensitive structures like heritage buildings.
Low-frequency vibrations, typically in the 1 to 10 Hz range, are particularly relevant near soft soil conditions, during deep foundation work, and for sensitive structures like heritage buildings. Standards like SBR-A set their strictest limits precisely in this range.
A monitoring system that underperforms below 4 Hz can give the impression that measured values are safely within limits while the actual ground motion is significantly higher. This is one of the most common causes of disputed measurements on construction projects.
When selecting monitoring equipment, always check the specified lower frequency limit in the technical datasheet. For Dutch projects under SBR-A and for most DIN 4150-3 applications, a system capable of accurate measurement from 1 Hz or lower is the correct requirement.
Tri-Axial Measurement and Frequency per Axis
Ground and structural vibrations do not travel equally in all directions. A vibration event from pile driving will often show different dominant frequencies on the vertical axis compared to the horizontal axes. Compliance standards typically require assessment in all three directions separately and as a vector sum.
Relying on a single-axis sensor and hoping it captures the worst case introduces risk. Tri-axial systems record all three channels simultaneously, making it possible to identify which direction is driving the compliance result and to store complete raw traces for later review if a dispute arises.
Storing Raw Traces for Dispute Resolution
A logging system that stores only peak values and dominant frequencies records enough information for routine compliance checks. But in the event of a damage claim or regulatory challenge, the raw time-domain trace is essential. It allows an independent expert to reprocess the data with any standard or filter and verify the original measurement independently.
Projects near sensitive buildings or in litigious environments should always configure their monitoring system to store full raw waveforms for all exceedance events and, ideally, for all significant events above a defined pre-trigger threshold.
Common Mistakes in Frequency Measurement
Using a sensor with inadequate low-frequency response. A geophone with a natural frequency of 4.5 Hz will not accurately capture events dominated by 1 to 3 Hz content. This is especially problematic near soft soil or during vibrocompaction work.
Setting thresholds based on PPV alone. Without entering the applicable standard and the correct structure category, a monitor cannot automatically evaluate compliance. The threshold at 5 Hz and at 50 Hz are not the same number. Poor sensor coupling. A sensor that is not firmly attached to the structure or ground will introduce its own resonance and give a distorted frequency reading. Always follow the mounting instructions and verify coupling before measurement begins.
Not storing raw traces. If only processed peak results are saved, an independent technical review of a disputed event becomes impossible. Configure the system to store raw data from the start of the project.
Frequently Asked Questions
What frequency range is most important for building protection?
For building protection under DIN 4150-3 and SBR-A, the range from 1 to 50 Hz is most critical. Low-frequency content between 1 and 10 Hz is subject to the strictest limits. Systems that cannot measure reliably below 4 Hz risk missing the most damaging part of the vibration event.
Is dominant frequency the same as peak frequency?
These terms are often used interchangeably in construction monitoring. Both refer to the frequency at which the most energy is concentrated in a given event. In formal signal processing, the peak in the FFT spectrum corresponds to the dominant frequency used in compliance assessment.
Can frequency change during a construction activity?
Yes. The dominant frequency of ground vibration from pile driving, for example, can shift depending on the pile type, driving energy, and soil layering. This is why continuous logging rather than spot checks is the correct approach for compliance monitoring.
Do I need a separate instrument to measure frequency and PPV?
No. Modern vibration monitors record both simultaneously from the same sensor. A single tri-axial logger will capture PPV and dominant frequency in all three axes and compare them to the selected standard automatically.
What standard should I apply on a Dutch construction project?
SBR-A applies to nuisance assessment near residential buildings and SBR-B applies to structural damage risk. For projects with an international or German client or specification, DIN 4150-3 may also be required. Always confirm the applicable standard with the permit authority before starting measurement.
Accurate from 0.5 Hz upward
The Vibra 5+ uses MEMS technology combined with advanced processing to deliver reliable frequency and PPV measurement across the full range required by DIN 4150-3, SBR-A/B, and BS 7385-2. No low-frequency blind spots, no manual data retrieval, no gaps in your compliance record.
Want to ensure your vibration measurements are accurate?
We help you select the right equipment and interpret frequency data according to DIN 4150-3, SBR, and BS 7385-2.