Sound Science Library

This library defines every sound type covered on Sound Well and grades the evidence behind it. Grades: 🟢 STRONG (meta-analyses or multiple consistent RCTs), 🟡 MODERATE (peer-reviewed studies, mixed results), 🟠 WEAK (few or small studies, unverified), 🔴 ANECDOTAL (personal reports or marketing — never stated as science). We never claim a sound "cures", "treats" or "prevents" any condition.

Colored noise

The three "colors" differ only in how sound energy is spread across frequencies — the single cleanest concept in this whole field.

[SN-01] White noise 🟡

What it is: equal energy at every audible frequency — the "hiss" of an untuned radio or a fan. It sounds slightly harsh because human ears are sensitive to high frequencies.

How it works: (1) auditory masking — a constant sound floor stops sudden noises (a door, a bark) from standing out and waking you; (2) a conditioned cue — your brain learns "this sound = time to sleep."

Evidence: masking environmental noise and reducing awakenings 🟡 MODERATE; soothing newborns 🟡 MODERATE; improving task performance in people with ADHD 🟢 STRONG for a small effect (Nigg et al. 2024 meta-analysis, 13 randomized studies, N=335, g≈0.25). Notably, the same meta-analysis found people without ADHD performed slightly worse with noise (g≈−0.21) — so "noise helps everyone focus" is simply not true.

[SN-02] Pink noise 🟡

What it is: energy falls as frequency rises (−3 dB/octave) — softer and "fuller" than white noise. Similar to steady rain, rustling leaves, or distant surf.

How it works: masking, plus a promising research direction where pink noise timed to slow brain waves may deepen slow-wave sleep.

Evidence: improving overall sleep quality 🟡 MODERATE (the best positive-result ratio for sleep among the noises); supporting deep sleep / memory in older adults 🟡 MODERATE (but the strong version needs slow-wave-tracking equipment — you cannot reproduce it by just playing a video); focus in ADHD 🟢 STRONG for a small effect. Pink noise is easy to listen to for hours, which is why it suits long sleep videos.

[SN-03] Brown noise 🟠

What it is: energy drops even faster (−6 dB/octave) — very deep, like a distant waterfall or the rumble inside an aircraft cabin.

Evidence — important: 🟠 WEAK / 🔴 ANECDOTAL. Brown noise went viral on TikTok, but no controlled study has tested brown noise on its own; the 2024 meta-analysis found none that qualified. A 2024 Ghent University study (Rijmen & Wiersema) directly testing the underlying "stochastic resonance" idea found a pure 100 Hz tone worked as well as pink noise — challenging the theory that the randomness of the noise is what matters. Honest position: no evidence yet does not mean useless — it means science has not caught up with the trend.

Brainwave & frequency claims

[BB-01] Binaural beats 🟡

What it is: play a slightly different frequency into each ear (e.g. 200 Hz and 210 Hz) and the brain perceives a "beat" at the difference (10 Hz). Headphones are mandatory — over speakers the effect disappears entirely.

Frequency map: Delta 0.5–4 Hz (deep sleep), Theta 4–8 Hz (deep meditation), Alpha 8–13 Hz (relaxed alertness), Beta 13–30 Hz (focused work), Gamma 30–100 Hz (higher cognition).

Evidence: 🟡 MODERATE — and better than many assume. A meta-analysis of 22 studies (Garcia-Argibay et al.) found a medium effect (g≈0.45) on memory, attention, anxiety and pain perception. But the effect depends heavily on listening before vs. during a task, duration, and band used, and whether brainwaves truly "entrain" is still debated — some of the benefit likely comes from relaxation and expectation. Fair summary: real, medium-sized, not a miracle, and not everyone responds.

[BB-02] "Healing" frequencies — 432 Hz, 528 Hz, Solfeggio 🔴

The claims: 432 Hz "aligns with the universe"; 528 Hz is the "love frequency" that "repairs DNA"; an ancient Solfeggio scale of six tones.

Evidence: 🔴 ANECDOTAL. There is no scientific basis for DNA repair. A few tiny studies comparing 432 vs 440 Hz report minor heart-rate/anxiety differences, but samples are very small and confounded by expectation. The "ancient Solfeggio" story is actually a modern (1970s) construction. You can still make pleasant 432 Hz relaxation audio (real search demand, and calm music does relax) — but never make a medical promise.

[BB-03] 40 Hz gamma stimulation 🟠

What it is: sound/light flickering at exactly 40 Hz, studied for Alzheimer's (MIT's GENUS line of research).

Evidence: 🟠 WEAK in humans (mostly mouse results; human trials early and small). Belongs in "what science is currently exploring" — never in a video promising "40 Hz stops memory loss."

Nature sounds & ASMR

[NT-01] Nature sounds (rain, waves, streams, birds, forest, fire) 🟢

How it works: Attention Restoration Theory (nature holds attention "softly," letting directed attention recover), Stress Reduction Theory (natural stimuli trigger positive affect and lower physiological arousal), plus masking.

Evidence: 🟢 STRONG — the firmest foundation in this whole library. Systematic reviews and meta-analyses confirm reduced stress and anxiety and improved mood/cognition (Buxton et al. 2021, 18 studies). Recovery from stress is faster than with urban noise (measured via skin conductance and heart rate). Nuance: versus silence, the advantage is much smaller than versus noise — nature's biggest value is replacing bad noise. Water and birdsong are the most studied and most reliable.

[NT-02] ASMR 🟡

What it is: a tingling sensation from the scalp downward, triggered by whispering, tapping, and gentle personal-attention sounds.

Evidence: 🟡 MODERATE for responders — studies record lowered heart rate and positive affect in people who experience ASMR. Key caveat: a large share of people do not feel it at all, and for some (misophonia) these sounds are unpleasant. Best treated as its own channel/playlist because the audience differs sharply.

Music

[MU-01] Instrumental / ambient / lo-fi 🟡

Evidence for focus: 🟡 MODERATE, leaning "does no harm" rather than "helps." A 2023 study (Journal of Cognition) found instrumental lo-fi neither reliably improved nor hurt performance — while lyrics clearly hurt. Its real value is masking noise and creating a start-of-work ritual, not "boosting IQ."

[MU-02] Music with lyrics 🟢 (negative)

Evidence: 🟢 STRONG in the negative direction. Lyrics reduce verbal memory, visual memory and reading comprehension (d≈−0.3) via the irrelevant speech effect — language in the song competes with the language system you are using to read/remember. The interference is strongest when lyrics are in your native language and weakens for a second/foreign language — which is why studying to Korean/Japanese pop hurts less than to songs in your mother tongue.

[MU-03] Music therapy 🟡–🟢

What it is: not "listening to music" — a trained therapist, goals, and often singing or playing. Evidence in dementia: consistent improvement in mood, anxiety, depression and verbal fluency; cognitive effects are small (SMD≈0.30) and mixed. A moving, well-supported fact: in Alzheimer's, musical ability is often preserved even after language is lost.

Animal hearing

[AN-01] Dogs 🟡

Hearing range: ~67 Hz – 45,000 Hz (humans: 20–20,000 Hz) — dogs hear ultrasound from electronics and LED drivers we cannot. Nearly 1 in 4 dogs fear loud noises, most commonly fireworks (Storengen & Lingaas 2015). Calming music: 🟡 MODERATE — shelter studies (Kogan 2012; Scottish SPCA 2017) found classical, reggae and soft rock reduced barking, pacing, heart rate and cortisol; heavy metal increased agitation. Dogs habituate, so playlists must be varied.

[AN-02] Cats 🟡

Hearing range: up to ~64,000 Hz. Cats respond weakly to human music, even classical — but music composed for cats (using feline vocal range, tempos based on purring and suckling) relaxes them noticeably (clinic studies). The best line in the whole project: "Your cat doesn't like Mozart — it likes music written in cat."

[AN-03] Making sound for pets — ground rules

Grades and figures summarize secondary sources (systematic reviews, meta-analyses). Verify the original study before stating a specific number publicly. See each topic guide's Sources section for full citations.