Frequency and amplitude jointly determine perceived intensity
Aliases: equal-sensation contour · subjective vibration intensity · magnitude estimation
What it is
How strong a vibration "feels" is not set by amplitude (raw physical energy) alone — frequency plays a role too. The same amplitude near the sensitive band (roughly 200–250 Hz) feels strong, while outside that band it feels much weaker; conversely, making two different-frequency vibrations "feel equally strong" requires giving them different amplitudes — the further a frequency sits from the sensitivity peak, the larger the amplitude needed to match it. This entry covers how intensity perception is jointly governed by two variables, rather than being a simple linear proxy for amplitude.
Why it happens
Perceived intensity is the result of an amplitude signal passing through the transduction efficiency specific to its frequency, not a direct mapping of the raw physical quantity. As noted above, different frequency bands are handled mainly by different receptor channels, and each channel converts mechanical amplitude into neural firing rate at a different efficiency — a high-efficiency band (the Pacinian channel's peak region) reaches a high firing rate with a smaller amplitude, while a low-efficiency band needs a larger amplitude to reach the same firing rate. Perceived intensity roughly tracks firing rate, so the same physical amplitude produces different firing rates — and thus different subjective intensity — at different frequencies. This is also why amplitude alone is insufficient to describe "how strong" a vibration is; frequency must be considered alongside it.
Studying it
The standard method is magnitude estimation: participants report the subjective intensity of vibrations across different frequency-amplitude combinations, either as numbers or by comparison against a reference stimulus, plotting equal-intensity contours (equal-sensation curves) where any combination on a given curve corresponds to the same subjective intensity level. This family of curves resembles the shape of detection-threshold curves (see the companion entry on the sensitivity band), but reflects supra-threshold intensity perception rather than mere detectability.
Where it stops holding
Equal-sensation curves are measured under fixed contact conditions (a specific probe, a specific contact force); how a real device makes contact (pressed flush against skin, or through a casing) changes the actual vibration-transfer efficiency, so a curve should not be reused for a product with a different contact mode without verification. Sustained continuous vibration also causes adaptation, and perceived intensity drops over time — equal-sensation curves usually reflect an immediate judgement under brief stimulation, and don't directly apply to vibrotactile feedback lasting tens of seconds or more.
Applying it
- When unifying a "vibration intensity level" across devices or frequencies, don't align purely on amplitude values — reference the equal-sensation curve so vibrations nominally labelled the same intensity level at different frequencies actually feel similarly strong.
- To cut power draw while keeping the same felt intensity, tune frequency toward the sensitivity peak and scale down amplitude proportionally — this saves more power than forcing up amplitude at an off-peak frequency.
- How to check: have users perform pairwise intensity comparisons (which feels stronger/weaker) across several frequency-amplitude combinations, and calibrate the product's intensity grading from that behavioural data rather than from drive voltage or nominal amplitude alone.