Vibration is measured by attaching a sensor to the object of interest, capturing motion data over time, and converting that data into meaningful values like displacement, velocity, or acceleration. The right approach depends on what you’re measuring, why you’re measuring it, and the frequency range you care about. Here’s how each piece fits together.
Three Ways to Quantify Vibration
Vibration can be described using three physical quantities, and each one tells a different part of the story. Displacement measures how far something moves from its resting position, typically in thousandths of an inch (mils) or micrometers. Velocity measures how fast it moves, in millimeters or inches per second. Acceleration measures how quickly the speed changes, in g-forces or meters per second squared.
These aren’t interchangeable. Low-frequency events like imbalance or misalignment in rotating machinery show up most clearly in velocity data, which is effective roughly from 10 Hz to 1,500 Hz. High-frequency events like bearing defects or gear mesh problems are best captured with acceleration measurements. Displacement is the go-to for very low frequencies, particularly on equipment with sleeve bearings (like some motors and compressors) where accelerometers don’t give reliable readings. In those cases, a proximity probe that measures shaft displacement in mils is the standard tool.
If you’re just getting started with vibration measurement on general rotating equipment, velocity is the most broadly useful parameter. It covers the middle of the frequency spectrum where most common mechanical faults appear.
Choosing the Right Sensor
The most common vibration sensor is the accelerometer, and the two dominant technologies are piezoelectric and MEMS (micro-electromechanical systems). Both convert motion into an electrical signal, but they differ in ways that matter for your application.
Piezoelectric accelerometers have a lower noise floor, roughly 10 µg/√Hz compared to about 200 µg/√Hz for MEMS sensors. That means piezoelectric sensors can detect much smaller vibrations. They also offer wider bandwidth, up to about 15 kHz versus 8 kHz for MEMS, so they capture higher-frequency content. And they handle heat better: piezoelectric sensors are commonly rated to 120°C (sometimes 150°C), while MEMS sensors rarely go beyond 105°C.
MEMS accelerometers cost less, draw less power, and are easier to integrate into wireless monitoring systems. For permanent installations on equipment that doesn’t get extremely hot or vibrate at very high frequencies, MEMS sensors work well. For demanding industrial environments, high-temperature machinery, or situations where you need to pick up faint high-frequency signals, piezoelectric is the stronger choice.
Non-Contact Measurement
Sometimes you can’t attach a sensor to the surface at all. The object might be too light (the sensor’s mass would change how it vibrates), too hot, or a liquid. In these cases, a laser Doppler vibrometer measures vibration by bouncing a laser beam off the surface and detecting the frequency shift of the reflected light. It’s the standard tool for lightweight structures, high-temperature components, and applications in medical imaging or materials research where physical contact would alter the response you’re trying to measure.
How You Mount the Sensor Changes Everything
An accelerometer is only as good as its connection to the surface. A poorly mounted sensor will distort or miss high-frequency content entirely. The mounting method sets a hard ceiling on your usable frequency range.
Stud mounting, where the sensor is screwed directly into a tapped hole on the surface, provides the best coupling and the highest frequency response. It’s the reference standard in calibration testing. When a threaded hole isn’t an option, adhesive and mechanical alternatives trade some frequency range for convenience:
- Thin foam tape: accurate up to about 2,000 Hz
- Thick foam tape: accurate up to about 2,500 Hz
- 3D-printed quick mounts: accurate up to about 1,000 Hz
- Hose clamps and zip ties: accurate up to about 800 Hz
- Foam dots and poster strips: unreliable, failing at frequencies as low as 200 Hz
These limits are based on a ±3 dB accuracy threshold, meaning the measured signal stays within about 30% of the true value. Beyond those frequencies, the mounting acts like a filter and the data becomes unreliable. If you need to capture bearing defect frequencies above 2,000 Hz, a zip tie mount won’t cut it. Stud mounting or a strong adhesive bond is essential.
Magnetic mounts fall somewhere in between and are popular for portable route-based data collection, though their upper frequency limit depends heavily on the magnet’s pull strength and the surface flatness. ISO 5348 provides detailed guidance on all of these methods.
Single-Axis vs. Three-Axis Sensors
A uniaxial accelerometer measures vibration in one direction. A triaxial accelerometer measures in three directions simultaneously (vertical, horizontal, and axial). In theory, triaxial gives you a more complete picture of what’s happening.
In practice, the difference is often smaller than expected. Comparative studies have found that uniaxial and triaxial measurements differ by less than 2.1%, with agreement improving at higher vibration intensities. For routine condition monitoring on rotating machinery, taking single-axis readings in the three standard directions (one at a time) often provides the same diagnostic information as a triaxial sensor at lower cost. Triaxial sensors save time when you need simultaneous data from all three axes, such as in modal analysis or when monitoring transient events that you can’t reproduce.
Turning Raw Data Into Useful Information
A vibration sensor outputs a time-domain signal: a wavy line showing amplitude over time. This is useful for spotting impacts or transient events, but it doesn’t tell you which frequencies are present. That’s where spectrum analysis comes in.
The fast Fourier transform (FFT) is the core tool. It takes the raw time signal and breaks it into its individual frequency components, producing a spectrum that shows which frequencies are being excited and at what amplitude. Each peak in the spectrum corresponds to a specific source of vibration. A peak at the machine’s running speed usually points to imbalance. A peak at twice running speed often indicates misalignment. Peaks at bearing defect frequencies reveal wear in specific bearing components.
The FFT also highlights harmonics, which are peaks at whole-number multiples of a fundamental frequency. These patterns are diagnostic signatures. A series of harmonics at multiples of the gear mesh frequency, for example, can indicate gear wear or damage. Tracking how these peaks change over time is the basis of predictive maintenance: a bearing defect frequency that grows in amplitude over weeks signals a bearing approaching failure.
Calibration and Accuracy
Vibration sensors drift over time, and their sensitivity can shift after drops, temperature extremes, or cable damage. Industry practice is to recalibrate accelerometers at least every 12 months. Some organizations calibrate more frequently when the stakes are higher, since any data collected with an out-of-calibration sensor could be questioned or invalidated.
Calibration involves comparing your sensor’s output against a known reference on a precision shaker. Many labs offer this service, and some portable calibrators allow you to verify sensor sensitivity in the field between formal calibration cycles. If you notice a sudden shift in your baseline vibration readings and nothing has changed on the machine, suspect the sensor or its cable before assuming a mechanical fault.
Measuring Vibration for Human Safety
Vibration measurement isn’t only about machines. Workers who use handheld power tools like rock drills, grinders, and chainsaws face hand-arm vibration exposure that can cause nerve damage, blood vessel disorders, and joint problems over time. ISO 5349 defines the standard method for quantifying this risk.
Exposure is measured as acceleration in meters per second squared (m/s²), expressed as a root-mean-square value normalized to an eight-hour workday (written as A8). In most countries, the daily exposure limit is 5 m/s² A8. How quickly you reach that limit depends on the tool’s vibration intensity. A tool producing 19 m/s² hits the daily limit in just 33 minutes of use. At 28.5 m/s², you reach the limit in 15 minutes.
To measure this, accelerometers are attached either to the tool handle or directly to the worker’s hand. The sensor captures acceleration during actual use, and the data is processed to calculate the daily dose. If you manage workers who use vibrating tools, these measurements determine how long each person can safely operate specific equipment before rotating to a different task.