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Ohmforge Vibration Spectrum (accelerometer CSV)

Accelerometer CSV in, vibration spectrum out: peak frequencies, amplitudes, rpm and RMS.

One row per sample: time, then 1–3 acceleration columns; header optional. Up to 200 000 rows.

Used when no file is uploaded. Comma, semicolon, tab or space separated.

More options

Larger = finer frequency resolution but fewer averages (noisier).

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More about this tool

How it works

Finds the frequencies a machine shakes at from accelerometer samples — a washing machine during spin, an unbalanced fan, a pump. Paste or upload a CSV with a time column and one to three acceleration columns (for example a recording from Ohmforge's Pocket instruments or a phone sensor app). You get the spectrum, the strongest peaks with amplitude and the matching rotation speed in rpm, and RMS acceleration per axis.

  • The samples are resampled to a uniform grid at the median sample interval (linear interpolation, because phone sensor events arrive with jitter) and each axis' mean is removed — that takes gravity and sensor offsets out.
  • Each axis gets a Welch spectrum (Hann window, 50 % overlap) with the same FFT as Ohmforge's capture analyzer and the Soundscape audio analysis; the combined spectrum is √(x² + y² + z²) per frequency bin. Amplitudes are the peak amplitude of a sine (window gain corrected), so a 0.5 m/s² sine reads 0.5 m/s².
  • Peaks are local maxima that stand out from the median of their neighbourhood, refined by a parabola through three bins. rpm = frequency × 60 assumes one disturbance per turn (imbalance); blade-pass and gear frequencies are multiples of the rotation speed.
  • Only frequencies below half the sample rate can be seen (Nyquist); phones sample at about 50–500 Hz. Linear interpolation lowers amplitudes close to the Nyquist frequency.

Phone accelerometers are not vibration sensors: their sample rate, resolution and mounting limit what they can show. Use the results to compare and find frequencies, not to judge machine condition against ISO 10816/20816 limits.

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How to use the Ohmforge Vibration Spectrum tool

Turn accelerometer samples into a vibration spectrum and find the frequencies, amplitudes and rotation speeds a machine shakes at.

What it is for

A washing machine walking across the floor, a humming fan, a rattling pump: which part shakes, and how fast? Every rotating part has a frequency. An unbalanced drum shakes once per turn, a three-blade fan pushes air three times per turn. If you know the frequencies in the vibration, you know where to look.

This tool takes accelerometer samples — a time column and one to three acceleration columns — and computes the vibration spectrum. It lists the strongest peaks with their amplitude, the axis that shakes most and the matching rotation speed in rpm. It also gives the RMS acceleration per axis and overall. The recording can come from Ohmforge's Pocket instruments (Vibration tab, Download CSV) or from any sensor logging app that writes CSV.

A phone samples motion only 50 to 500 times a second. That is fine for washing machines, fans and pumps. It is far too slow for bearing faults, which live in the kilohertz range.

Worked example

The second example is a desk fan at 1500 rpm recorded at 200 Hz, with the phone lying on the fan's base. With the FFT size on Automatic the tool picks 256 points and averages 11 frames:

  • Strongest peak 75.00 Hz (4500 rpm), 0.0727 m/s², mostly on the ay axis.
  • Second peak 25.00 Hz (1500 rpm), 0.0602 m/s², mostly on ax.
  • RMS of all axes 0.0709 m/s²; resolution 0.781 Hz.

The 25 Hz peak is the rotor itself: 25 × 60 = 1500 rpm, a small imbalance. The 75 Hz peak is three times that, the blade-pass frequency of a three-blade fan. It is stronger than the imbalance here, so the noise comes from the blades moving air past the guard, not from a bent shaft.

Setting the FFT size to 1024 instead gives a finer resolution of 0.195 Hz but only 2 averaged frames. The peaks stay at 75.01 and 25.00 Hz, while the noise floor becomes more ragged.

How to read the result

  • Strongest peak and as rotation speed: the dominant frequency and its rpm, assuming one disturbance per turn. Multiples (2×, 3×) point to misalignment, blades or gear teeth.
  • Peak amplitude is the amplitude of a sine at that frequency in m/s², combined over all axes.
  • Strongest axis tells you the direction. With the phone flat, x and y are horizontal and z is vertical.
  • RMS summarises the overall vibration. Compare it before and after a fix, such as levelling the machine's feet.
  • Resolution is the bin spacing. Two peaks closer than about three bins merge into one.

Tips and common mistakes

  • Put the phone flat and still on the machine. Do not hold it: your hand adds a few hertz of its own shake.
  • Record at least 10 seconds: more averaged frames, cleaner spectrum.
  • Check the Nyquist note: frequencies above half the sample rate fold back and show up as false low peaks.
  • Gravity is removed by subtracting each axis' mean, so you can upload accelerometer data with or without gravity.
  • Time in milliseconds or microseconds is detected from the header or the step size. Set it by hand if your file has neither.
  • Linear interpolation lowers amplitudes close to the Nyquist frequency. Compare those peaks only with each other.

FAQ

Why do I need a time column?

Phones deliver motion events with jitter. The tool resamples them to an even grid at the median interval, which needs the real time of each sample.

Can it tell me whether my machine is damaged?

No. It shows frequencies and amplitudes. Judging machine condition against ISO 10816 or 20816 limits needs a proper vibration sensor, mounted firmly.

What does the rpm column assume?

One shake per turn, like an imbalance. If the peak is a blade-pass or gear frequency, divide the rpm by the number of blades or teeth.

Which units can the file use?

Accelerations in m/s² or in g (converted with 9.80665 m/s²); time in seconds, milliseconds or microseconds; comma, semicolon, tab or space separated.

Guide last updated:

For scripts and AI agents

  • JSON API: POST https://ohmforgevibrationspectrum.usefulweb.app/api/v1/calculate (same calculation as this page)
  • Interface description: OpenAPI 3.1 · AI guide: llms.txt
  • MCP server (Streamable HTTP): https://ohmforgevibrationspectrum.usefulweb.app/mcp · tool name ohmforge_vibration_spectrum
Example request
curl -s -X POST https://ohmforgevibrationspectrum.usefulweb.app/api/v1/calculate \
  -H 'content-type: application/json' \
  -d '{…see OpenAPI examples…}'
Feedback · ideas & bug reports welcome