Modulation of audible low frequency ventilation fan noise reproduced in a laboratory has been shown to trigger the startle reflex in people sensitised to that noise. Using a range of low frequency and infrasound noise signals to sensitised and unsensitized subjects can show a causal relationship between an acoustic trigger and a physiological stress response, which engineers call “annoyance” or “noise annoyance” symptoms, and which biologists recognize as the “startle reflex.” Utilizing low level amplitude and frequency modulation as the source triggers a “startle reflex” response for comparison with the typical “startle reflex” to high level noise impulses.