Glossary — Terms used on this site

Amplitude modulation (AM): The cycling-up-and-down of a sound’s overall volume over time. Distinct from the audio frequency of the sound itself. Two sounds at the same average dB(A) can have very different physiological effects depending on whether and how their amplitude is modulated. See Sound.
Catecholamines: The class of fight-or-flight stress hormones (epinephrine, norepinephrine, dopamine). Chronically elevated catecholamines in children exposed to high-noise environments are part of the documented mechanism by which noise impairs cognition.
Cortisol: A steroid stress hormone produced by the adrenal glands. Elevated by noise events; antagonistic to melatonin. Chronically elevated cortisol is a documented driver of cardiovascular disease, immune impairment, and metabolic disruption.
dB(A): A-weighted decibels — a measure of sound pressure level adjusted to approximate human hearing sensitivity across frequencies. The standard unit for occupational and environmental noise. The decibel scale is logarithmic: a 10 dB increase represents a tenfold increase in sound intensity.
Frequency-following response (FFR): The brain’s tendency to synchronize its electrical activity to rhythmic external stimuli. The mechanism by which amplitude-modulated sound can drive neural oscillations at the modulation frequency — including frequencies that disrupt sleep architecture.
Infrasound: Sound at frequencies below the threshold of conscious human hearing — roughly below 20 Hz. Has documented physiological effects despite being inaudible. Sources include HVAC systems, large wind turbines, and internal combustion engines.
Modulation frequency: The rate at which a sound’s amplitude envelope rises and falls. Distinct from the audio frequency of the sound. The brain’s entrainment response tracks modulation frequency.
NIOSH: National Institute for Occupational Safety and Health. The US federal research agency that produces evidence-based recommended exposure limits (RELs). Distinct from OSHA, which enforces. NIOSH’s noise REL is 85 dB(A) TWA with a 3 dB exchange rate — more protective than OSHA’s PEL.
NRR (Noise Reduction Rating): A standardized rating, in decibels, of the noise attenuation provided by a hearing-protection product. Higher is better. The pharmacy “comfort plug” segment typically rates at 22–25 dB; serious sleep / occupational use calls for 30+ dB.
OSHA: Occupational Safety and Health Administration. The US federal regulator of workplace safety. Sets and enforces the permissible exposure limit (PEL) for occupational noise — 90 dB(A) TWA with a 5 dB exchange rate.
PEL (Permissible Exposure Limit): OSHA’s legally enforceable workplace exposure limit. For noise, 90 dB(A) as an 8-hour time-weighted average. Above this, hearing-protection programs are mandatory.
REL (Recommended Exposure Limit): NIOSH’s evidence-based recommendation, more protective than the OSHA PEL. For noise, 85 dB(A) TWA. Not legally enforced, but the more medically-defensible threshold.
SORE (Small Off-Road Engines): The regulatory category in California that includes leaf blowers, lawn mowers, chainsaws, and similar gas-powered equipment under 25 horsepower. The category targeted by California Assembly Bill 1346 (2021).
Steady-state white noise: White noise whose amplitude envelope is constant over time — no rising or falling, no rhythmic pattern. Generally benign and often beneficial for sleep masking. To be distinguished from pulsed white noise, in which the amplitude varies on a rhythm that can drive neural entrainment.
TWA (Time-Weighted Average): The average exposure over a defined period, typically an 8-hour shift for occupational noise. Lets short bursts of louder sound be averaged with quieter periods to produce a single comparable number.
Two-stroke engine: An internal combustion engine in which the power cycle completes in two strokes (rather than four) of the piston. Common in handheld and backpack lawn equipment because of their high power-to-weight ratio. Burn an oil-fuel mixture, lack modern emissions controls, and produce disproportionate air pollution and noise per unit of work.

Glossary.