So what is vibration frequency? It’s how many times something shakes back and forth each second, measured in hertz (Hz). A washer drum spinning at 1,200 RPM shakes the machine about 20 times a second, so its vibration frequency is 20 Hz. Frequency is the fastest way to find out what is shaking, because each moving part shakes at its own rate. The vibration frequency analyzer measures it with your phone and lists the strongest peaks in Hz and RPM.
What is vibration frequency, in plain terms?
Picture a spot on a washing machine lid. It moves up, comes back down past where it started, and returns. That round trip is one cycle. Frequency counts the cycles in one second.
- 1 Hz is once a second. You can count along, like a slow wobble.
- 10 Hz is ten times a second. You feel a buzz but can’t count it.
- 30 Hz is about as fast as a phone in a browser can measure.
The time for one cycle is the period, and it’s 1 divided by the frequency. At 12.5 Hz, each cycle takes 0.08 seconds.
Frequency is not strength. They are two separate questions: how fast (Hz) and how hard (mm/s or g). A guitar string is a good picture. The note you hear is the frequency. How loud it is, is the strength. A tiny buzz and a violent shake can have the same frequency.
Hz and RPM are the same thing
Hz counts per second. RPM (revolutions per minute) counts per minute. So:
- RPM = Hz × 60
- Hz = RPM ÷ 60
| Hz | RPM |
|---|---|
| 1 | 60 |
| 5 | 300 |
| 10 | 600 |
| 12.5 | 750 |
| 20 | 1,200 |
| 25 | 1,500 |
| 30 | 1,800 |
| 50 | 3,000 |
| 60 | 3,600 |
The Hz to RPM converter and RPM to Hz converter do the math for any number.
Why frequency points to the cause
Anything that turns and is slightly out of balance shakes once per turn. So the vibration frequency matches its speed: a fan blade turning at 90 RPM shakes at 1.5 Hz. Mechanics call this “1×”. Other faults leave other marks:
- Imbalance usually shows up at 1× the running speed.
- Misalignment or loose mounts often show up at 2×.
- Engines shake at their rotation speed, and also at their firing frequency. For a four-stroke engine that’s RPM ÷ 60 × cylinders ÷ 2.
- Car wheels turn at speed ÷ (π × tire diameter).
A worked example: a 225/65R17 tire is about 724 mm tall. At 60 mph (26.8 m/s) it turns about 11.8 times a second, or 707 RPM. If a passenger measures a steady shake near 11.8 Hz at 60 mph, the wheels are the prime suspect. The tire vibration frequency calculator works this out for your tire and speed.
Common sources and their frequencies
| Source | Speed | Frequency | Can a phone read it? |
|---|---|---|---|
| Dryer drum | 45 to 60 RPM | under 1 Hz | Too slow to read well |
| Ceiling fan on low | roughly 60 to 100 RPM | 1 to 1.7 Hz | Yes |
| Bouncy floor when someone walks | not a rotation | often 4 to 8 Hz | Yes |
| Top-load washer spin | often 600 to 900 RPM | 10 to 15 Hz | Yes |
| Car wheel at 60 mph (225/65R17) | about 707 RPM | about 11.8 Hz | Yes |
| Engine at 750 RPM idle | 750 RPM | 12.5 Hz | Yes |
| Front-load washer spin | 1,000 to 1,400 RPM | 17 to 23 Hz | Yes |
| 4-cylinder firing at 750 RPM | fires twice per turn | 25 Hz | Yes, near the limit |
| 6-cylinder firing at 750 RPM | fires three times per turn | 37.5 Hz | No |
| Fixed-speed fridge compressor | 3,000 to 3,600 RPM | 50 to 60 Hz | No |
| Transformer hum (US) | not a rotation | 120 Hz | No |
Harmonics: why you see more than one peak
Real shaking is rarely one clean frequency. A machine usually shows its main speed plus weaker peaks at 2×, 3× and so on. These are harmonics. Two machines running at once add peaks that aren’t multiples of each other.
The analyzer lists the top three peaks. When you see evenly spaced peaks, the lowest one is usually the running speed. When the peaks don’t line up, look for a second source, like a pump, a fan or traffic outside.
Natural frequency and resonance
Everything has frequencies at which it likes to vibrate. Flick a wine glass and it rings at one note. That’s its natural frequency. Floors, shelves, car steering and a washer drum on its springs all have one too.
When a machine’s speed matches a natural frequency, the shaking builds up far more than the push alone would cause. This is resonance. You’ve probably felt it:
- A washer shakes hardest for a moment as the spin speeds up, then calms at full speed.
- A car shakes in one speed band, like 55 to 65 mph, and smooths out above it.
- A vibratory pile driver shakes the ground hardest as it starts and stops, which Caltrans (2020) links to resonance of the soil and pile.
Floors are a special case. AISC Design Guide 11 treats floors with a natural frequency below about 9 Hz as low-frequency floors whose response to walking has to be checked. Very bouncy floors are often in the 4 to 8 Hz range. The floor vibration test shows your floor’s bounce frequency.
To beat resonance, change the speed, stiffen the part, or add rubber isolation feet so less shake passes through.
Frequency, velocity and acceleration
There are three ways to describe how hard something shakes:
- Acceleration in g or m/s². This is what the phone’s sensor measures.
- Velocity in mm/s or in/s. This is what most machine and building guidelines use.
- Displacement in micrometers (µm) or mils. How far it actually moves.
Frequency links them. Velocity = acceleration ÷ (2π × frequency), and displacement = velocity ÷ (2π × frequency). Here is the same velocity at three frequencies:
| Velocity | Frequency | Acceleration | Displacement |
|---|---|---|---|
| 1 mm/s | 2 Hz | 0.013 m/s² | 80 µm |
| 1 mm/s | 10 Hz | 0.063 m/s² | 16 µm |
| 1 mm/s | 30 Hz | 0.19 m/s² | 5 µm |
Slow shaking moves much farther for the same velocity. That’s why DIN 4150-3 sets a lower guideline for homes at slow frequencies: 5 mm/s at 1 to 10 Hz, rising to 15 to 20 mm/s at 50 to 100 Hz. The vibration unit converter switches between all these units at any frequency.
How your phone measures vibration frequency
Your phone’s accelerometer takes about 60 readings per second in a browser. The tool times each reading, spaces them evenly, and splits the signal into its frequencies with a Fourier transform (FFT). The result is a spectrum: a bar for each frequency, with the tallest bar marking the main vibration.
The highest frequency a phone can measure is half its sample rate, about 30 Hz or 1,800 RPM. Your phone’s exact limit is shown in the details.
Faster vibration doesn’t simply disappear. It folds back and shows up as a false, lower frequency. This is called aliasing. At 60 samples a second, a 50 Hz compressor hum can look like 10 Hz, and 45 Hz can look like 15 Hz. When the tool suspects this, it says “The main vibration may be faster than your phone can measure.” If you know a machine runs above 1,800 RPM, don’t trust a low peak from it.
Longer, steadier measurements give cleaner peaks. That’s why the analyzer runs for 60 seconds and averages overlapping slices of the recording. It also ignores anything slower than 1 Hz.
How to measure it
- Place your phone flat, screen up, on a hard part of the machine, away from moving parts. Tape it down if the surface is smooth or sloped.
- If you can, switch the machine off. Tap Start (on iPhone, tap Allow) and keep still for the 2-second “Hold still…” step.
- Switch the machine on and let it run at a steady speed for the full 60 seconds.
- Read the top three peaks. Compare the main one with the running speed ÷ 60, and with twice that.
- Check the details for your phone’s highest measurable frequency, and watch for the aliasing warning.
Phone readings are a good guide, not a certified measurement. Put it into practice with the vibration frequency analyzer, or check a motor or pump with the machine vibration test.