Sensitivity needs time to recover after a stimulus stops, so an immediate repeat cue lands with reduced effect

What it is

If a tactile alert ends and, one second to a few tens of seconds later, the exact same alert is repeated (a missed-call buzz firing again, an error tone-and-vibration repeating), users typically report the second one as feeling weaker than the first, even though the device's physical output was identical both times. This discount on an immediate repeat is one of the product experience issues most often mistaken for "the device's vibration got weaker."

Why it happens

Local receptors need some time after a stimulus ends before they can respond at full strength again, and that recovery process alone makes a closely spaced repeat feel somewhat weaker. On top of that, if the user has already registered the first alert as "known, already dealt with," an almost-identical second alert gets an additional layer of higher-level filtering — the brain is more inclined to dampen response to a repeat that has been judged not to carry new information. The size of the discount observed at the product level is typically larger, and lasts longer, than peripheral recovery time alone would predict, precisely because both layers are stacking, not just one.

Studying it

This can be measured by testing subjective-intensity ratings or reaction time for the same alert at a range of repeat intervals: after a first alert, participants receive the identical alert again after several different gaps (seconds, tens of seconds, minutes), tracing how rating or reaction time changes with interval, and comparing that against a condition where the alert's pattern is changed before the repeat, to see whether changing the pattern partly offsets the discount.

Where it stops holding

The discount is largest when the repeat is identical, closely spaced, and the user has already acted on or dismissed the first alert. It eases noticeably once the interval is long enough, or once new relevant information has arrived in between (a different caller, a different specific error) — at that point it no longer reads as "repeating the same already-handled event." This addresses repetition of an alert as one complete event; it does not address how much spacing is needed between pulses within a single ongoing vibration waveform, which is a question about the waveform itself.

Applying it

• For situations that need to re-alert a user within a short time (an unacknowledged important notification, an unresolved ongoing warning), don't expect replaying the identical short cue to preserve the same sense of urgency — let later repeats escalate in intensity, duration, or rhythm rather than simply copying the first one.
• Leave enough spacing between repeats; firing an almost-identical second alert before the user has had a chance to respond to the first is exactly when the discount is largest and most likely to be experienced as "the alert isn't working."
• How to check: compare a plain-repeat design against an escalating-repeat design in a real or simulated multi-alert scenario, measuring response time and miss rate, to confirm the escalating design genuinely mitigates the falloff from closely spaced repetition.

Related

• Same group: A4.08.1 A constant, unchanging tactile stimulus gets perceptually dulled · A4.08.2 A varying pattern stays effective longer than a constant intensity · A4.08.3 Adaptation is site-specific — moving the vibration source to a new patch of skin restores sensitivity · A4.08.4 Peripheral receptor adaptation and central habituation are two different mechanisms with different recovery speeds · A4.08.5 Adaptation to one tactile mode does not fully transfer to another
• Nearby: A4.10 Fast-adapting and slow-adapting receptors · A5.16 Habituation and alert fatigue
• Search terms: repeat alert fatigue · tactile cue discount · escalating notification design