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.
Brainwave bands: The standard EEG-defined frequency bands of human brain activity. Delta: 0.5–4 Hz (deep sleep). Theta: 4–8 Hz (deep relaxation, drowsiness, deep meditation). Alpha: 8–13 Hz (relaxed awareness, eyes closed, pre-sleep). Beta: 13–30 Hz (active thinking, conscious attention). Lower gamma: 30+ Hz (focused cognition, binding). See Schumann.
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.
Cortisol awakening response (CAR): The natural endogenous spike in cortisol that brings the body out of sleep, peaking approximately 30 minutes after waking. The acoustic context in which this spike occurs shapes how the nervous system interprets the wake transition. Pairing the CAR with birdsong (the natural dawn-chorus context) produces a different downstream autonomic state than pairing it with an alarm or traffic noise. See Birdsong.
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. A 2026 Frontiers in Behavioral Neuroscience paper found infrasound exposure elevated cortisol and produced more negative affect in subjects even when they could not consciously detect the sound. Sources include HVAC systems, large wind turbines, internal combustion engines, and notably aircraft. See Sound and Aircraft.
Lenard effect: The physical process by which water droplets breaking apart (rushing rivers, waterfalls, breaking waves) release electrons into the air, charging the local atmosphere with negatively charged molecules. The mechanism that makes riverbanks chemically different from surrounding air. Documented physiological effects of the resulting negative ions include increased oxygen availability to the brain, regulation of serotonin levels, and reduced fatigue. See Riverbank.
Looming sound: A sound growing louder and closer along the horizontal plane. Documented to trigger the strongest threat response in the brain — the midbrain defense circuits fire hardest for sounds that behave like an approaching ground-level threat. The acoustic signature of the predator channel. See Elevation.
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.
Negative ions: Air molecules carrying a net negative electrical charge. Generated abundantly by moving water via the Lenard effect. Documented to increase oxygen availability to the brain, regulate serotonin levels, and reduce fatigue. The biochemical basis for the riverbank’s distinctive restorative effect beyond its acoustic profile. See Riverbank.
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.
Pink noise: A broadband noise spectrum with more energy at lower frequencies, tapering off at higher ones. Distinct from white noise (which has equal energy across all audible frequencies). Wind through trees, distant traffic murmur, and rainfall fall in the pink-noise range. Associated with relaxed brainwave states and improved sleep quality. See Wind chimes.
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.
Schumann resonance: The fixed electromagnetic standing-wave frequencies trapped between Earth’s surface and the ionosphere, continuously excited by global lightning activity. Fundamental at 7.83 Hz, with overtones at approximately 14, 20, 26, and 33 Hz. Predicted mathematically by Winfried Otto Schumann in 1952 and confirmed by measurement. Human brainwaves intermittently synchronize with the fundamental during deep relaxation; the harmonic series spans the entire conscious-waking brainwave spectrum. See Schumann.
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.
Stochastic resonance: An acoustic profile that is irregular but not chaotic — variable enough that the brain does not habituate to it, structured enough that it does not register as threat. The signature sound profile of wind chimes (driven by air motion no one controls) and of certain healthy brain rhythms. The JSTOR brain-rhythms literature documents that rigidly periodic brain rhythms are associated with pathological states, suggesting natural statistical variability is biologically appropriate. See Wind chimes.
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.