Linear resonant actuators start and stop fast, giving a crisp tap

What it is

A linear resonant actuator (LRA) drives a mass back and forth in a straight line near its own resonant frequency. Because it does not have to overcome rotational inertia, it reaches target amplitude and returns to zero within a few milliseconds, which makes a sharply bounded "tap."

Why it happens

LRA output approximates a controlled envelope with steep ends, so a single tap is perceived as a discrete event rather than a smeared buzz. Working at resonance also allows very short inter-pulse gaps while keeping pulses separate. The cost is that efficiency drops sharply away from the resonant point, so the useful frequency range is narrow; resonant frequency differs by vendor, and the same drive parameters behave differently across motors.

Studying it

Measure minimum perceptible duration and minimum discriminable gap, comparing against an ERM on the same device to confirm that "short and crisp" holds. Behaviorally, tap-identification and rhythm-reproduction tasks show whether users can separate a single tap from a double and reproduce a given rhythm. Report whether the drive frequency matches the motor's resonance, or the result may reflect poor driving rather than motor capability.

Where it stops holding

The advantage is largest where fine temporal structure matters; for "something happened" cues, the gap is not a reason to choose. Working near resonance also means large timbre changes are difficult and need more elaborate drive waveforms. Devices and cases change how much energy reaches the hand, so perceived strength must be measured per device.

Applying it

• Prefer LRA devices for taps, double taps, and rhythm discrimination, and record that premise as a design constraint.
• Calibrate drive parameters to the target motor's resonant frequency rather than reusing another model's values.
• Do not treat an LRA as a freely tunable timbre source; validate complex waveforms separately.
• Verification: on the target model, measure identification accuracy for single taps, double taps, and short rhythms, and confirm two pulses at the minimum gap do not merge.

Related

• Within the group: D3.01.1 Eccentric rotating mass motors start and stop slowly, so short pulses are hard · D3.01.3 Motor type bounds the achievable haptic vocabulary
• Adjacent: D3.02.3 Rhythm carries meaning better than intensity · D3.14.1 Motor types and algorithms make haptics inconsistent across devices
• Search terms: linear resonant actuator · LRA · haptic click