Acceleration and
velocity.
Your unit triggers in g. Your limit is written in mm/s. This is where the two meet — and why the answer moves with frequency.
Why the unit thinks in g
A ViB unit is a three-axis accelerometer. It does not measure how fast the ground is travelling; it measures how hard the ground shakes each axis, in g. The threshold that starts a recording is a percentage of the acceleration range the unit is switched to — 5% of a 10 g range is 0.5 g — so what arms the unit is always an acceleration.
Compliance limits are written in millimetres per second. For a steady vibration at a single frequency the two are one step apart: velocity and acceleration differ by a factor of 2πf.
Frequency sits in the denominator, so one fixed g threshold is not one fixed mm/s. It is a line that slopes down as frequency rises: a trigger that needs 120 mm/s to fire at 5 Hz fires at 6 mm/s once the ground is shaking at 100 Hz. Everything below is that line, drawn.
Set a unit up and read it in mm/s
Pick the range the unit is switched to and the threshold percentage you would set, then drag the frequency scrubber to the ground motion you expect. The orange line is the level that starts a recording; the dashed line is where the sensor saturates. Between them, the event is recorded in full.
Starts recording at about 39 mm/s at 20 Hz.
Clips above 1350 mm/s at 20 Hz.
Trigger 0.50 g per axis · clip 17.3 g peak vector sum.
Pure-tone figure. A real blast carries higher-frequency energy that trips an acceleration trigger sooner than this suggests, so read the mm/s number as the highest PPV at that frequency that could slip past — a planning bound, not a measurement.
Clipping is the peak vector sum across all three axes; the trigger is per axis.
What that means at your distance
The scaled-distance law turns a charge and a standoff into a predicted peak particle velocity. Put it beside the trigger line at your frequency and you can see whether the unit would have started recording at all.
At 20 Hz, this setup starts recording at 39 mm/s.
| Distance | Predicted PPV | Would it record? |
|---|---|---|
| 50 m | 86.8 mm/s | Records |
| 100 m | 28.6 mm/s | Below the trigger |
| 300 m | 4.94 mm/s | Below the trigger |
Predictions use the Australian Standards scaled-distance law with K = 1140 and B = -1.6, the same defaults as the vibration calculator. Real attenuation depends on geology, confinement and site conditions — this is a planning guide, not a blast-design prediction.
Questions this raises
Why is my threshold in % and not mm/s?
Because the unit arms on acceleration, and the threshold is a percentage of the g range it is switched to. The same 5% is 0.5 g on a 10 g range and 2 g on a 40 g range, so changing the range changes what starts a recording even though the percentage has not moved.
What frequency should I use?
The dominant frequency of the ground motion you expect. Far-field blasting through soil is often 5–20 Hz; near-field work and hard rock run higher, 30–100 Hz and beyond. If you do not know it, 20 Hz is the convention the ShotTrack vibration calculator uses. Reading the trigger at the lowest frequency you expect is the conservative choice, because that is where a fixed g threshold needs the most ground velocity to fire.
Why does the trigger line slope?
Because velocity and acceleration differ by 2πf. A constant g threshold therefore corresponds to a velocity that halves every time the frequency doubles, which is a straight line of slope −1 on logarithmic axes. It is not the unit changing its mind; it is the same acceleration meaning less travel.
What is clipping?
Above a certain level the accelerometer saturates and the recorded waveform is no longer faithful. The clip figure shown here is the peak vector sum across all three axes, taken from the unit's firmware, while the trigger is per axis. Between the two lines the event is captured in full — which is the band you are choosing when you pick a range.
Put a number on your own shot
The vibration calculator takes a charge, a distance and a frequency and predicts PPV, acceleration and safe standoff — and suggests the ViB unit that fits.
Australian-made hardware, backed by ShockAI cloud reporting.