The HIC (Head Injury Criterion) calculation sounds simple — integrate the acceleration signal over time. In practice, getting a valid HIC number from a head impact test requires careful attention to signal conditioning, filtering, and sampling rate. Here's the technical detail.

Sensor Setup

The standard headform accelerometer setup per ECE 22.06, FMVSS 208, and related standards:

• Triaxial accelerometer at headform centre of gravity (CG)
• Accelerometer range: ±2000g (motorcycle) or ±500g (industrial)
• Sensitivity: 1–2 mV/g typical
• Natural frequency: >10 kHz (to avoid mechanical resonance in measurement band)

Mount the accelerometer with minimal mass adapter — any added mass shifts the CG and invalidates the test.

Signal Conditioning Chain

The acquisition chain runs the raw charge signal through four stages before it reaches the DAQ:

👁 Headform Signal Conditioning Chain

The charge amplifier holds under 0.1% linearity across a DC–10 kHz bandwidth. The anti-alias filter is a 4th-order Butterworth set at fc = 1650 Hz, matching SAE J211 channel class 1000. The ADC samples at 16-bit resolution minimum, at ≥10 kHz, and the DAQ timestamps each sample to within ±10 μs.

SAE J211 Filtering

All automotive and helmet impact standards require SAE J211 Channel Class filtering applied to raw acceleration data before HIC calculation:

• Channel Class 1000: fc = 1650 Hz (head acceleration in crash testing)
• Filter type: 4-pole Butterworth (implemented as zero-phase forward-backward filter)
• Apply to raw data BEFORE calculating any derived quantities

👁 SAE J211 Channel Class Filtering

The cutoff frequency follows fc = 0.6 × channel class, so Channel Class 1000 resolves to fc = 600 Hz. This filter must run on the raw signal first — every downstream metric (peak g, HIC) is derived from the filtered trace, never the raw one.

HIC Calculation

HIC is found by a sliding-window search: for every pair of time points within a 0.036 s window (per ECE 22.06), compute the mean acceleration over that interval and evaluate the HIC expression, keeping the maximum value found across all candidate windows.

👁 HIC Calculation — Sliding-Window Search

The same resultant trace also gives the peak resultant acceleration, computed as the maximum of the vector magnitude across the triaxial channels — a separate pass/fail criterion alongside HIC.

Pass/Fail Check

Each standard sets its own limits on peak resultant acceleration and, where applicable, HIC:

👁 Pass/Fail Limits by Standard

An impact only passes overall if it clears the peak-g limit and, for standards that define one, the HIC limit as well.

The Neometrix Head Impact Test Rig implements this signal chain with integrated SAE J211 filtering and automated HIC reporting.
→ https://neometrixgroup.com/products/head-impact-test-rig