Detection outside the sensitive band requires more energy

What it is

The U-shaped vibrotactile sensitivity curve implies something specific: moving away from the roughly 200–250 Hz sensitive band, whether toward low frequency (a few to a few dozen Hz) or high frequency (several hundred Hz and above), the amplitude needed to achieve the same "detected" outcome rises significantly, and the further off-peak, the more energy must be added to compensate — the curve's two ends typically rise steeply, not gently. This entry covers how large that cost is, a direct consequence of the sensitivity band itself.

Why it happens

The threshold rises at the low-frequency end because that band is handled mainly by slowly adapting afferents and other receptors more tuned to static pressure, which transduce fast vibration inefficiently — reaching a detectable firing rate requires larger mechanical displacement to compensate for that inefficient transduction. The threshold rises at the high-frequency end because the Pacinian corpuscle's lamellar capsule itself transmits higher-frequency vibration less efficiently (the structure behaves like a band-pass filter, attenuating more sharply beyond its passband), and at higher frequencies mechanical energy also dissipates more readily within skin tissue, so less of the same drive energy reaches the receptor. The mechanisms at the two ends differ, but the shared outcome is: the further toward either end, the more drive energy is required.

Studying it

Data at both ends of the curve come from the same threshold-measurement paradigm used for the sensitivity-band study itself (a staircase or method-of-constant-stimuli procedure finding the minimum detectable amplitude at various frequencies); the only difference is focusing on the curve's ends rather than its trough. A common engineering supplement is measuring the drive voltage/current an actuator needs at a given frequency to reach the target threshold, converting the psychophysical threshold into an actual power-cost figure for engineering decisions.

Where it stops holding

The magnitude of threshold elevation varies by individual and by site, so a universal "compensation factor" should not be assumed. There is also a physiological ceiling on the amplitude skin can tolerate (excessive amplitude causes discomfort or even pain, covered under tactile safety limits) — if the target frequency is far from the sensitive band and the required amplitude exceeds a safety or hardware ceiling, "just raise the amplitude" is not a viable path at all, and the only option is switching to a frequency within the sensitive band.

Applying it

• If a product is forced by structural constraints (an ultra-thin body, a fixed-size motor) to operate outside the sensitive band, calculate in advance whether the required amplitude exceeds the actuator's physical limit or the user's comfort ceiling — rather than discovering the vibration is imperceptible only after building a prototype.
• When a product line spans multiple device forms, don't assume every device can reuse the same drive parameters — actuators have different natural frequencies, and a device forced into a low-efficiency band needs its own boosted drive energy, or a different actuator model, to compensate.
• How to check: measure the threshold separately at each target frequency that sits off the sensitive band, confirming the required amplitude falls within device and safety limits, rather than linearly extrapolating from parameters at the sensitivity peak.

Related

• Same group: A4.03.1 Vibrotactile sensitivity peaks in a specific frequency band · A4.03.2 Frequency and amplitude jointly determine perceived intensity
• Nearby: A4.07 Pain and safety limits · A4.01 Types of cutaneous mechanoreceptors
• Search terms: threshold elevation · off-peak vibration · actuator power budget