Common dB Reference Levels

dB SPL	Sound
0	Threshold of hearing
30	Quiet library
60	Normal conversation
85	Heavy traffic (safe limit ~8h)
110	Rock concert
120	Threshold of pain
140	Jet engine at 30 m

How dB is calculated

Decibels are a logarithmic scale, not linear. For sound pressure level (SPL), the formula is:

dB SPL = 20 × log₁₀(P₁ / P₀)

P₁: Measured sound pressure (pascals)
P₀: Reference pressure = 20 µPa (micropascals), the threshold of human hearing

Why logarithmic? Human hearing perceives sound intensity logarithmically. A 10 dB increase sounds roughly "twice as loud" to our ears, but it's actually 10× the sound pressure. The log scale compresses the enormous range of human hearing (~1 trillion:1 in intensity) into a manageable 0–140 dB scale.

Relative vs. absolute dB scales

"dB" alone is always relative-a ratio with no absolute value. The suffix specifies the reference:

dB SPL: Sound pressure level, reference = 20 µPa. Used for acoustics and hearing.
dBm: Power relative to 1 milliwatt. Used in RF (radio frequency) and telecommunications.
dBu: Voltage relative to 0.775 V RMS. Used in professional audio.
dBFS: Decibels relative to digital full scale. 0 dBFS = maximum digital level; all other values are negative. Used in digital audio.

Hearing damage thresholds (NIOSH guidelines)

85 dB SPL is the threshold for occupational hearing protection. NIOSH recommends:

85 dB: Safe for 8 hours per day.
Every 3 dB increase halves the safe exposure time. Examples:
- 88 dB: 4 hours safe
- 91 dB: 2 hours safe
- 94 dB: 1 hour safe
- 100 dB: 15 minutes safe (rock concert level)
- 115 dB: ~30 seconds safe (chainsaw, ambulance siren at close range)
120 dB: Threshold of pain. Immediate risk of hearing damage.

Permanent hearing loss is cumulative and irreversible. Use hearing protection (earplugs, earmuffs) in loud environments.