Understanding sound levels
Decibels, log scale, and the threshold of hearing.
Why sound is logarithmic, the dB reference for hearing, and why "twice as loud" isn't 2× the dB number.
Sound pressure level.
Sound pressure level (SPL) is measured in decibels relative to the quietest pressure a healthy young ear can detect — 20 μPa (micropascals). SPL = 20 × log₁₀(p / 20μPa). Whisper: 30 dB. Conversation: 60 dB. Busy street: 70 dB. Loud restaurant: 80 dB. Subway: 90 dB. Live concert: 110-120 dB. Threshold of pain: 130 dB. Jet engine at 30 m: 140 dB.
SPL = 20·log₁₀(p / 20μPa)
The 10× / 2× / 3 dB rules.
The log scale produces these useful tricks. +10 dB = 10× the sound power, perceived as roughly twice as loud. +6 dB = 4× the power. +3 dB = 2× the power. Two identical sources at the same loudness combine to +3 dB (twice the power, only marginally louder). Same noise at half the distance = +6 dB. Memorising these makes ballpark sound math tractable.
A-weighting.
Human hearing isn't flat — we're most sensitive to 2-4 kHz (speech range) and much less so to bass and very high frequencies. A-weighted dB (dBA) applies a frequency-weighting curve that matches human perception. Almost every regulatory limit (workplace noise exposure, building code, traffic noise) is in dBA. Raw SPL (dB SPL, unweighted) is mostly used in acoustic engineering and audio production.
Hearing damage thresholds.
OSHA in the US permits 90 dBA for 8 hours of workplace exposure — higher levels need protection, exposure halves for every 5 dB above. NIOSH recommends the stricter 85 dBA / 3 dB exchange rate. Damage accumulates: a 100 dB concert for 2 hours is roughly equivalent to a 90 dB workshop for 8 hours. Recreational noise (concerts, headphones, power tools) is the most common preventable cause of adult hearing loss; earplugs at -20 dB are cheap and work.
Loudness, the perceptual unit.
dB SPL is physical; loudness in phons or sones is perceptual. A 1000 Hz tone at 40 dB SPL is defined as 40 phons / 1 sone. The same SPL at 100 Hz is much quieter to the ear — fewer phons. The complexity is why audio engineering uses LUFS (covered in the audio-normalizer chapter) for integrated loudness rather than raw dB. For everyday work, dB SPL or dBA is enough; loudness models matter at the mastering stage.
Phone microphones lie.
Free phone apps that claim to measure decibels are calibrated by guess on whichever phone they're installed on. Real sound-level meters cost £50-500 and are calibrated to standards (Class 1 or 2 per IEC 61672). For "is this concert loud enough to damage my hearing" the phone is fine to within ±5 dB; for legal noise complaints, workplace safety audits, or anything requiring evidence, use a calibrated meter.