The Quick Answer
A decibel (dB) is a unit that measures sound intensity on a logarithmic scale. Every 10 dB increase means the sound is 10× more intense (and roughly twice as loud to human ears).
Key reference points:
- 0 dB — threshold of human hearing
- 30 dB — quiet library
- 60 dB — normal conversation
- 85 dB — noise becomes a hearing risk with prolonged exposure
- 120 dB — threshold of pain
- 140 dB — immediate hearing damage risk
The rest of this article explains why decibels work this way, how to interpret the numbers, and when sound levels become dangerous.
What Is a Decibel?
A decibel is one-tenth of a bel, a unit named after Alexander Graham Bell. It expresses a ratio between two values — in acoustics, between a measured sound pressure and a reference pressure (the quietest sound a healthy human ear can detect).
The formula:
dB SPL = 20 × log₁₀(measured pressure ÷ reference pressure)
The reference pressure is 20 micropascals (20 µPa), which corresponds to 0 dB SPL (Sound Pressure Level).
You don't need the formula to use decibels. The key insight is that decibels are logarithmic, not linear — and that matters for how we interpret the numbers.
Why Logarithmic? Why Not Just Use a Linear Scale?
Human hearing covers an enormous range. The loudest sounds we can tolerate are about 1 trillion times more intense than the quietest sounds we can detect. A linear scale from 1 to 1,000,000,000,000 would be impractical.
The logarithmic decibel scale compresses this range into roughly 0–140 dB, making it manageable. It also happens to match human perception more closely: our ears perceive loudness roughly logarithmically, not linearly.
How the Scale Works in Practice
| Change | What happens to intensity | How it sounds |
|---|---|---|
| +3 dB | Intensity doubles | Barely noticeable |
| +6 dB | Intensity quadruples | Noticeable increase |
| +10 dB | Intensity × 10 | Sounds roughly twice as loud |
| +20 dB | Intensity × 100 | Sounds roughly four times as loud |
This means:
- 70 dB is not "twice as loud" as 35 dB. It's thousands of times more intense.
- Two identical 70 dB sources playing together don't produce 140 dB. They produce about 73 dB (a 3 dB increase, since the intensity doubles).
- Reducing noise by 10 dB cuts perceived loudness roughly in half, even though the physical intensity drops by 90%.
Common Sound Levels: A Reference Chart
| dB Level | Example | Category |
|---|---|---|
| 0 | Threshold of hearing | — |
| 10 | Breathing, rustling leaves | Very quiet |
| 20 | Whisper at 5 feet | Very quiet |
| 30 | Quiet rural area, library | Quiet |
| 40 | Quiet residential street, birdsong | Quiet |
| 50 | Moderate rainfall, quiet office | Moderate |
| 60 | Normal conversation at 3 feet | Moderate |
| 70 | Shower, dishwasher, busy traffic | Moderate–Loud |
| 80 | Alarm clock, garbage disposal, busy restaurant | Loud |
| 85 | Heavy traffic, power tools (prolonged exposure risk begins) | Loud |
| 90 | Lawn mower, motorcycle at 25 feet | Very loud |
| 95 | Subway train, electric drill | Very loud |
| 100 | Handheld drill, factory machinery | Very loud |
| 105 | Personal stereo at max volume | Very loud |
| 110 | Rock concert (average), chainsaw | Extremely loud |
| 120 | Siren at 100 feet, thunder | Threshold of pain |
| 130 | Military jet takeoff at 100 feet | Painful |
| 140 | Gunshot, fireworks at close range | Immediate damage risk |
These are approximate. Actual levels depend on distance, environment, and specific equipment.
When Does Noise Become Dangerous?
Hearing damage depends on both loudness and duration of exposure. The general guideline from occupational safety organizations:
| Sound Level | Maximum Safe Exposure Time |
|---|---|
| 85 dB | 8 hours |
| 88 dB | 4 hours |
| 91 dB | 2 hours |
| 94 dB | 1 hour |
| 97 dB | 30 minutes |
| 100 dB | 15 minutes |
| 103 dB | 7.5 minutes |
| 106 dB | 3.75 minutes |
| 110 dB | ~2 minutes |
| 115 dB | ~30 seconds |
The pattern: every 3 dB increase halves the safe exposure time. This makes sense because +3 dB means double the intensity.
Key Thresholds
- 85 dB is the level where occupational safety regulations typically begin requiring hearing protection for workers exposed over an 8-hour shift.
- 100 dB — at typical headphone-listening volumes at maximum, you may reach this level. Fifteen minutes of exposure per day at this level carries risk over time.
- 120 dB is generally considered the threshold of pain — sound becomes physically uncomfortable.
- 140 dB and above — single impulse events (gunshots, explosions) at this level can cause immediate, permanent hearing damage.
Note: These are general guidelines based on occupational health standards (NIOSH, WHO). Individual susceptibility varies. Consult a hearing health professional for personalized advice.
How Sound Level Meters Work
A sound level meter measures the pressure variations in air caused by sound waves and converts them to a decibel reading.
Basic Components
- Microphone — converts air pressure changes into an electrical signal
- Preamplifier — boosts the weak microphone signal
- Frequency weighting filter — adjusts the signal to match how human ears perceive different frequencies
- RMS detector — calculates the root mean square (average energy) of the signal
- Display — shows the result in dB
Frequency Weighting: A, C, and Z
Not all frequencies sound equally loud at the same intensity. Human ears are less sensitive to very low and very high frequencies. Sound level meters account for this using weighting curves:
- A-weighting (dBA) — mimics human hearing sensitivity. De-emphasizes low frequencies below 500 Hz and high frequencies above 6 kHz. Most commonly used for noise measurements, workplace safety, and environmental regulations.
- C-weighting (dBC) — flatter response, with less low-frequency rolloff. Used for measuring peak levels and for environments with significant bass content.
- Z-weighting (dBZ) — flat, unweighted response across all frequencies. Used for technical/scientific measurements.
When you see a decibel reading without a specified weighting, it's usually A-weighted (dBA).
Calibrated vs. Uncalibrated Meters
Professional sound level meters (Class 1 or Class 2 per IEC 61672) are calibrated against known reference sources. They provide accurate SPL readings within ±1–2 dB.
Browser-based and smartphone meters are uncalibrated. They measure relative levels using whatever microphone is available. The readings are useful for comparison (is this room louder than that one?) but should not be relied upon for absolute SPL values. Microphone sensitivity, frequency response, and automatic gain control all affect the result.
dB vs. dBA vs. dBFS: Different Scales, Different Meanings
You'll encounter several "dB" variants. They all use the same logarithmic math but measure different things:
- dB SPL — sound pressure level in air, referenced to 20 µPa. What a sound level meter measures.
- dBA — A-weighted SPL. The standard for noise regulations and hearing safety.
- dBFS — decibels relative to full scale in digital audio. 0 dBFS is the maximum level a digital system can represent. All values are 0 or negative. A signal at −6 dBFS is half the maximum amplitude.
- dB HL — decibels hearing level. Used in audiometry (hearing tests), referenced to the average threshold of normal hearing at each frequency.
These are not interchangeable. A reading of −20 dBFS on a recording does not mean the room is at −20 dB SPL.
Worked Examples
Example 1: Adding Sound Sources
Two speakers each producing 80 dB are placed in a room. What is the combined level?
Equal sources add 3 dB: 80 + 3 = 83 dB
Not 160 dB. Decibels don't add linearly.
Example 2: Distance and Sound Drop-off
A jackhammer measures 100 dB at 1 meter. What level would you expect at 10 meters (outdoors, no reflections)?
Sound drops by 6 dB each time distance doubles (inverse square law):
- 1 m → 2 m: 100 − 6 = 94 dB
- 2 m → 4 m: 94 − 6 = 88 dB
- 4 m → 8 m: 88 − 6 = 82 dB
- 8 m → 16 m: 82 − 6 = 76 dB
At 10 meters (between 8 and 16), roughly 80 dB. Indoors, reflections keep levels higher.
Example 3: Headphone Volume
Your phone shows volume at 70%. At maximum volume, many earbuds can produce 100–110 dB. Each 10% reduction in volume roughly lowers the level by a few dB (the relationship varies by device). At 70%, you might be at 85–95 dB — approaching or exceeding the safe limit for extended listening.
The practical rule: if someone standing next to you can hear your headphones, you're likely above 85 dB.
Common Misconceptions
"Decibels measure volume"
Decibels measure sound intensity (physical energy). Volume is a subjective perception. Two sounds at the same dB level can sound different in loudness depending on their frequency content.
"0 dB means silence"
Zero dB SPL is the threshold of hearing — the quietest sound a typical healthy young person can detect. It's not "no sound." Negative dB SPL values are possible (sounds quieter than the threshold).
"Doubling the dB means twice as loud"
A sound at 80 dB is not twice as loud as one at 40 dB. Perceived loudness roughly doubles every +10 dB. So 80 dB sounds about 16 times louder than 40 dB.
"My phone's dB reading is accurate"
Smartphone microphones are not calibrated for SPL measurement. Readings can be off by 5–15 dB or more. They're useful for relative comparisons, not for regulatory compliance or precise safety assessments.
Practical Tips for Managing Noise
- Use the 60/60 rule for headphones: Listen at no more than 60% volume for no more than 60 minutes at a time.
- In noisy environments, if you have to raise your voice to be heard at arm's length, the ambient level is likely above 85 dB. Consider hearing protection.
- Sound drops off with distance. Moving twice as far from a noise source reduces the level by about 6 dB outdoors.
- Soft surfaces absorb sound. Carpets, curtains, and upholstered furniture reduce reflected noise indoors.
- Consistent low-level noise (like an office HVAC at 50 dB) is generally not a hearing risk, but it can affect concentration and sleep quality.
FAQ
What does dB mean?
dB stands for decibel, a logarithmic unit that measures the ratio between two values. In acoustics, it measures sound pressure relative to the quietest sound humans can hear (20 micropascals). Every 10 dB increase represents a tenfold increase in sound intensity.
How loud is 50 dB?
50 dB is roughly the level of moderate rainfall or a quiet office. It's comfortable for conversation and not a hearing risk. For reference, normal conversation happens at about 60 dB, and a whisper is around 20 dB.
How loud is 70 dB?
70 dB is about the level of a running shower or a dishwasher. It's noticeable but generally considered safe for extended exposure. Many people find it the upper limit of comfortable background noise.
How loud is 100 dB?
100 dB is about the level of a handheld drill or factory machinery. At this level, hearing damage can occur after just 15 minutes of continuous exposure. It's loud enough that you'd need to shout to be heard by someone a few feet away.
At what decibel level does hearing damage occur?
Prolonged exposure to sound above 85 dB can cause gradual hearing damage. The higher the level, the less time is safe: 85 dB for 8 hours, 100 dB for 15 minutes, 110 dB for about 2 minutes. A single impulse above 140 dB (like a gunshot) can cause immediate damage.
Why is the decibel scale logarithmic?
Because human hearing spans an enormous range — the loudest tolerable sounds are about a trillion times more intense than the quietest detectable ones. A logarithmic scale compresses this range into a practical 0–140 range and better matches how we perceive loudness changes.
What is A-weighting (dBA)?
A-weighting is a frequency filter applied to sound measurements that mimics human hearing sensitivity. It reduces the contribution of very low and very high frequencies, which humans perceive as quieter. Most noise regulations and safety standards use dBA measurements.
Is a phone decibel meter accurate?
Smartphone decibel meters are not calibrated and can be off by 5–15 dB. They're useful for rough comparisons (is this louder than that?) but should not be used for workplace safety assessments or regulatory compliance. For accurate readings, use a calibrated Class 1 or Class 2 sound level meter.
What is the difference between dB and dBA?
dB (unweighted) measures raw sound pressure across all frequencies equally. dBA applies A-weighting, which adjusts the measurement to reflect how human ears perceive different frequencies. Most everyday noise measurements — workplace safety, environmental noise, appliance specs — use dBA.
How loud are 85 decibels in everyday terms?
85 dB is roughly the level of heavy city traffic or a noisy restaurant. It's the threshold where occupational safety guidelines recommend hearing protection for workers exposed for more than 8 hours. You can still have a conversation at this level, but you'd need to raise your voice.
Do two equal sound sources double the decibels?
No. Two identical sources add only 3 dB. If each speaker produces 70 dB, together they produce about 73 dB — not 140 dB. This is because decibels are logarithmic: doubling the intensity adds 3 dB, not double the dB number.
How much does sound decrease with distance?
Outdoors with no reflections, sound decreases by about 6 dB each time the distance doubles (inverse square law). A sound at 90 dB at 1 meter would be about 84 dB at 2 meters and 78 dB at 4 meters. Indoors, reflections keep levels higher than this rule predicts.
What is dBFS in digital audio?
dBFS stands for decibels relative to full scale. It's used in digital audio recording and processing. 0 dBFS is the maximum signal a digital system can represent — going above it causes clipping (distortion). Typical recording levels are −12 to −6 dBFS to leave headroom.
How do noise-canceling headphones work in dB terms?
Active noise-canceling headphones typically reduce ambient noise by 20–35 dB, depending on frequency. They're most effective at low-frequency, continuous sounds (airplane engine hum, HVAC). This means a 75 dB airplane cabin might be reduced to 40–55 dB — a significant perceived reduction.
Measure It Yourself
Use the sound level meter to get a real-time relative dB reading from your microphone — useful for comparing noise levels between rooms, checking if your workspace is noticeably louder than a quiet environment, or simply understanding how the decibel scale looks in practice. For generating test tones at specific frequencies, try the tone generator.