Coverage is tightly coupled to hand posture

What it is

Mid-air haptic coverage depends on where the hand is and how it is oriented. The focus is a small region in space, and the sensation is clear only when skin sits inside it and roughly faces the beam. Move or rotate the hand and the sensation changes or disappears.

Why it happens

Acoustic radiation pressure on skin depends on incidence angle and focusing accuracy. A palm facing the transducer array receives the largest normal component, while side or back surfaces receive far less. Hand motion also means the focus must track in real time, and array focusing and tracking both take time, so fast movement separates focus from hand. The effective region is therefore not a fixed volume but one that varies with posture and motion—which is why mid-air haptics demo well statically and lose feedback during real use.

Studying it

Measure detection across postures and speeds: hold the focus fixed and vary hand orientation, distance, and movement speed, recording detection and rated strength. Variables include posture angle, focus-tracking algorithm, speed, and array layout. Outcomes include effective volume, tracking latency, and the proportion of temporary dropouts caused by motion.

Where it stops holding

If the interaction itself requires a still hand—a hover-to-confirm target—posture dependence matters less because users can settle into the best position. With large mid-air gestures, tracking lag and posture change make feedback intermittent, and haptics should be downgraded to occasional cues rather than continuous feedback. Air currents and occluding objects also alter the sound field.

Applying it

• Indicate the recommended holding region and orientation in the interface itself.
• For continuous feedback, measure tracking lag first and verify that feedback does not drop out frequently during movement.
• Provide a substitute when feedback fails, such as an on-screen cue, rather than leaving users to think their action was missed.
• Verification: measure detection across postures and speeds and chart the effective volume; if common postures fall outside it, adjust the array layout or the interaction.

Related

• Within the group: D3.06.1 Ultrasound can produce a felt sensation in mid-air · D3.06.2 Perceptible strength is far below contact-based feedback
• Adjacent: D3.02.2 Perceived intensity depends on how the device is held · C4.06.2 Uncertain boundaries in mid-air gestures
• Search terms: working volume · hand posture · mid-air haptics