What Is a Decibel? The dB Scale Explained

A decibel is a comparison, not a simple count

Ask how tall a table is, and you expect a simple unit such as meters or inches. Ask how loud a room is, and the answer might be 35 dB(A), 62 dB SPL, or 100 dB(C) Peak. The decibel feels strange because it is not a direct count of sound waves. It is a logarithmic comparison.

Sound pressure in air spans a huge range. The quietest sound a healthy young ear can detect near 1 kHz is tiny compared with a siren, concert, or jet engine. A linear pressure scale would require awkward strings of zeros. The decibel scale compresses that range into manageable numbers.

When you use the online sound meter, the display is showing sound pressure level derived from the microphone signal. The exact value depends on weighting, response time, calibration, and the limits of the phone microphone. To understand the number, you need three ideas: reference level, logarithms, and weighting.

dB SPL starts at 20 micropascals

In air acoustics, sound pressure level is usually expressed as dB SPL. SPL means sound pressure level. The reference pressure is 20 micropascals, written 20 µPa. That reference is close to the threshold of human hearing at sensitive mid frequencies, not a claim that everyone can hear every 0 dB sound.

The formula compares measured root-mean-square sound pressure with that reference. You do not need the formula to use a meter, but the consequence matters: decibels express ratios. A higher dB SPL means the sound pressure is larger relative to 20 µPa.

Zero dB SPL is not silence. It is the reference level. Negative dB SPL can exist in specialized measurements when sound pressure is below that reference. In normal rooms, your phone will not meaningfully measure such levels because the microphone’s own self-noise is higher.

The scale is logarithmic

On a logarithmic scale, equal steps represent equal ratios, not equal additions. That is why 80 dB is not merely a little more than 70 dB. A 10 dB increase means ten times the sound energy. A 20 dB increase means one hundred times the sound energy. A 30 dB increase means one thousand times the sound energy.

Human loudness perception is also not linear. As a rough everyday rule, a 10 dB increase is often heard as about twice as loud, while a 10 dB decrease is often heard as about half as loud. This is approximate, because pitch, duration, background noise, and listener sensitivity all matter.

The most useful rules are simple:

Change	Energy meaning	Common practical meaning
+1 dB	Small energy increase	Barely noticeable in many situations
+3 dB	About double energy	Clear on a meter, modest to the ear
+6 dB	About four times energy	Also the free-field change for halving distance to a point source
+10 dB	Ten times energy	Often perceived around twice as loud
+20 dB	One hundred times energy	A major loudness change

This is why careful measurement matters. A change from 85 to 88 dB may look small, but under the NIOSH 3 dB exchange rule it halves recommended exposure time.

Adding sound sources: two 60 dB sources are not 120 dB

Decibels cannot be added like ordinary numbers. If one machine measures 60 dB and a second identical independent machine is added, the total is about 63 dB, not 120 dB. Doubling equal sound energy raises the level by 3 dB.

If the sources are very different in level, the quieter one contributes little. A 70 dB vacuum plus a 60 dB background is about 70.4 dB, not 130 dB. The louder source dominates the sum.

Source A	Source B	Approximate total
60 dB	60 dB	63 dB
60 dB	57 dB	62 dB
60 dB	50 dB	60.4 dB
70 dB	60 dB	70.4 dB

This explains why reducing the loudest source is usually the best first move. If your room has a loud fan and a quiet computer, replacing the fan changes the total more than obsessing over the computer.

Distance matters: about 6 dB per doubling in free field

For a small point source in open space, sound level falls by about 6 dB every time distance doubles. Move from 1 meter to 2 meters, and a free-field point source drops about 6 dB. Move from 2 meters to 4 meters, and it drops another 6 dB.

Real rooms are messier. Reflections from walls, floors, ceilings, furniture, and windows reduce the neatness of the rule. A low-frequency hum may fill a room more evenly than a high-frequency beep. Outdoors, wind, ground absorption, barriers, and directionality matter.

Still, the distance rule is practical. Put a white noise machine, speaker, appliance, or tool farther away and the level at your ear often drops significantly. If a measurement at 0.5 meter looks alarming, also measure at the actual listening position.

For examples on the scale, compare how loud is 60 dB, how loud is 85 dB, and the decibel chart.

dB, dBA, dBC, and dBZ

A bare “dB” label is incomplete unless the context is obvious. In sound level measurement, you will often see dB(A), dB(C), or dB(Z). These are frequency weightings.

A-weighting reduces very low and very high frequencies to approximate the ear’s sensitivity at moderate levels. It is common in environmental noise, workplace exposure, and community guidance, including WHO 1999/2018 discussions and many regulations. C-weighting is flatter through low frequencies and is often used for loud sounds, bass-heavy sources, and peak assessment. Z-weighting is nominally flat within the instrument bandwidth.

If a bass-heavy neighbor noise reads 38 dB(A) but feels intrusive, the A-weighted value may be hiding low-frequency energy. Looking at C-weighted level or the octave spectrum can give useful context, even if the legal limit is written in dB(A).

For a full comparison, see A, C, and Z weighting explained.

Fast, Slow, Leq, Max, and Peak

Decibel meters also average over time. Fast response has a 125 ms time constant and reacts quickly. Slow response has a 1 second time constant and is easier to read for steady sound. Impulse response exists on some sound level meters for short events, though modern standards and regulations often specify exact metrics rather than relying on a casual “impulse” display.

Leq means equivalent continuous sound level. It is the steady level that would contain the same acoustic energy as the varying sound over the measurement period. If you measure traffic for 10 minutes, Leq summarizes that whole energy history. Max is the highest time-weighted level during the measurement. Peak is the highest instantaneous pressure level and is especially important for impulse noise, though phone microphones can miss true peaks.

The online sound meter shows Leq, Min, Max, Peak, a timeline, and an octave spectrum so you can see both the summary and the pattern.

dB SPL is not dBFS

One common confusion comes from audio production. dBFS means decibels relative to full scale in a digital audio system. In dBFS, 0 dBFS is the maximum digital level before clipping, and normal signals are negative numbers such as -18 dBFS or -6 dBFS. It does not directly tell you how loud the sound is in the room.

dB SPL, by contrast, is a physical sound pressure level in air relative to 20 µPa. A recording could peak at -1 dBFS whether the microphone was next to a whisper or a drum kit; the SPL depends on microphone sensitivity, gain, distance, and calibration.

That is why calibration matters. A phone microphone signal becomes a meaningful SPL estimate only when the system knows how the digital input relates to real pressure at the microphone.

Standards and safety references

IEC 61672 defines performance requirements for sound level meters. Certified meters are designed, tested, and calibrated to meet classes in that standard. A phone web meter is not an IEC 61672 class instrument, even when carefully calibrated.

For health and safety, several well-known references appear often. NIOSH recommends an 85 dBA 8-hour exposure limit with a 3 dB exchange rate. OSHA’s permissible exposure limit is 90 dBA for 8 hours with a 5 dB exchange rate. WHO environmental-noise guidance from 1999 and 2018 discusses community and transportation-noise health effects. These references use decibels precisely, so pay attention to weighting, duration, and context.

If your concern is headphones, concerts, or gyms, read safe decibel levels for listening. If your concern is sleep, read noise level for sleeping.

The takeaway

A decibel is a compact way to describe enormous differences in sound energy. The scale is logarithmic, the reference for dB SPL is 20 µPa, and the label is incomplete unless you know the weighting and time response. Once you understand those pieces, the numbers become less mysterious.

Do not ask only, “How many dB is it?” Ask, “Measured where, for how long, with which weighting, and compared with what?” That is the difference between a number and a useful measurement.