Seismograph and Vibration Meter

Separating vibration from gravity

A phone accelerometer measures proper acceleration, and most of what it reads while at rest is simply gravity: a constant vector of about 9.81 m/s² pointing at the zenith. To see vibration, that constant component has to come out of the equation. Some devices deliver this ready-made, in the linear acceleration field computed by the operating system sensor fusion. When that field comes back empty, the tool does the work itself: it keeps a slow running average of the full vector, which converges on the direction of gravity, and subtracts it. What remains is the fast motion, that is, the vibration. It is an improvised high-pass filter, and it is why the reading takes a second or two to settle after you start it. The on-screen trace is drawn on a canvas rather than in SVG, for a practical reason: at sixty samples per second, a vector chart would rebuild hundreds of DOM nodes every frame and stutter on a modest device. The scale is automatic, with a floor, so the trace does not look like an earthquake when all that exists is the sensor noise floor. Dominant frequency is estimated by counting zero crossings rather than by a Fourier transform. That sounds like a step backwards, but it is the honest choice in this range: the browser delivers samples at a low and irregular rate, and building an FFT on top of that would produce numbers that look precise without being backed by anything. Counting crossings is coarse and sufficient to tell a motor vibration from a footstep.

Rest the device on whatever you want to measure

1. Tap Start sensor and grant motion permission. Wait a second or two for the filter that separates gravity to settle.
2. Rest the device firmly on the surface or equipment you want to assess. Holding it in your hand measures the tremor of your hand, not that of the object.
3. Watch the trace and read the peak and RMS values. Use Reset highest peak to clear the record before a new comparison.

Sources and references for this tool

These references help contextualize formulas, standards, APIs and limitations used on this page. They do not replace professional validation when a result has legal, financial, medical or operational impact.

• DeviceMotionEvent.accelerationMDN Web Docs, Gravity-free acceleration, when the device provides it, used as the vibration signal.
• Relationships between Peak Ground Acceleration, Peak Ground Velocity, and Modified Mercalli Intensity in CaliforniaWald, Quitoriano, Heaton e Kanamori (1999), Earthquake Spectra, Regression used to convert peak acceleration into Mercalli intensity, valid between V and VIII.

Frequently asked questions

Where to go next