Perceptible strength is far below contact-based feedback

What it is

Non-contact haptics deliver far less perceptible strength than contact feedback. Producing force in air means focusing acoustic energy onto a point, and the usable energy density is bounded by safety and efficiency, so users feel a faint pressure rather than a definite push or resistance.

Why it happens

Contact feedback transmits force through an object to the skin with a large contact area and a clear direction; mid-air haptics rely on acoustic radiation pressure, which produces far less force per area and decays quickly with distance. Low strength forces a compromise in one of two directions: require the user to hold still to maximize sensation, or abandon force-type information and keep only presence cues. It also limits the available intensity steps, since the whole range sits near threshold.

Studying it

Measure detection thresholds and discriminable intensity steps: at varying distances and modulation settings, test whether users detect the cue and whether they can separate two intensity levels, yielding the working range and step count. Variables include distance, focus size, modulation frequency, and hand posture. Outcomes include detection rate, level-discrimination accuracy, and rated strength. Report safety limits, since raising intensity often hits the exposure ceiling.

Where it stops holding

Where definite force feedback is required—feeling resistance, confirming a press—contact solutions remain irreplaceable. Mid-air haptics fit as a supplementary cue: adding "something is here" to touchless interaction. With gloves or covering skin, perceptible strength drops further and applicability narrows.

Applying it

• Treat mid-air haptics as cues and contours, not as force, resistance, or clear magnitude.
• Publish the usable distance range and discriminable step count so designs do not exceed physics.
• Switch to contact devices or a visual substitute where force sensation is needed.
• Verification: at the target distance, measure detection and two-level discrimination; if levels are not reliably separable, collapse to a single-level cue design.

Related

• Within the group: D3.06.1 Ultrasound can produce a felt sensation in mid-air · D3.06.3 Coverage is tightly coupled to hand posture
• Adjacent: D3.02.1 Too short a duration falls below the perceptibility threshold · D3.07.1 Force feedback can express resistance, boundaries, and texture
• Search terms: mid-air haptics · ultrasound intensity · weak actuation