The Decibel Scale: Understanding Sound Levels, Loudness and Hearing Safety

Two 60 dB machines together measure 63 dB, not 120. How the logarithmic decibel scale works, what dB(A) means, and the levels that damage hearing.

Introduction

Put two machines in a room, each measuring 60 decibels on its own, and the pair together will read about 63 dB. Not 120. Not even 70. That single fact tells you almost everything about why the decibel confuses people: it is not a quantity you can add up like grams or litres. It is a ratio on a logarithmic scale, and the ordinary arithmetic of measurement simply does not apply to it.

Once the logic clicks, the scale becomes genuinely intuitive, and the numbers on a noise meter, a pair of headphones and a workplace safety notice all start to mean something concrete. This guide covers how the scale is built, how to combine levels correctly, what the A in dB(A) is doing, and which numbers matter for protecting your hearing.

Why the Scale Is Logarithmic

Human hearing has an extraordinary range. The quietest sound a healthy ear can detect involves a pressure variation of about 20 micropascals. The point at which sound becomes painful is around 20 pascals - a million times greater in pressure, and a million million times greater in intensity.

A linear scale covering that span would be unusable. You would be comparing a reading of 0.00002 against one of 20 and trying to keep the decimal places straight. The decibel compresses those twelve orders of magnitude into a range that runs from 0 to about 120, which is why it survives despite being awkward to reason about.

The Reference Point: What 0 dB Actually Means

A decibel on its own means nothing. It is always a ratio against a reference, and for airborne sound that reference is a pressure of 20 micropascals - roughly the threshold of human hearing.

dB SPL = 20 × log₁₀(pressure ÷ 20 μPa)

That definition produces some useful anchors:

Because 0 dB is a reference and not a floor, negative decibel readings are perfectly legitimate. An anechoic chamber can measure well below 0 dB. Sound does not stop existing there; it is simply quieter than the average person can hear.

The Three Numbers Worth Memorising

Nearly every decibel question resolves to one of three steps on the scale:

That last one is the gap between physics and perception, and it is the source of most misjudgement. A sound at 80 dB carries ten times the energy of one at 70 dB, but the ear reports it as merely "twice as loud". Conversely, a 3 dB increase doubles the energy arriving at your ear while being barely noticeable. Small numbers on this scale are not small effects.

How to Actually Add Decibels

To combine independent sources, convert each level back to intensity, add those, and convert the total back:

Total = 10 × log₁₀(10^(L₁/10) + 10^(L₂/10))

Two 60 dB sources give 10 × log₁₀(2,000,000) = 63 dB. Ten identical 60 dB sources give 70 dB - ten times the machines for one doubling of apparent loudness.

The more useful consequence is what happens with unequal sources. Add a 60 dB source to a 70 dB one and the total is 70.4 dB. The quieter machine contributes almost nothing. This is why silencing the loudest item in a noisy room is worth more than quietening everything else combined, and why a difference of 10 dB or more lets you ignore the smaller source entirely.

Everyday Sound Levels

Approximate levels for calibrating your intuition:

Distance: The Six Decibel Rule

For a point source outdoors, every doubling of distance drops the level by 6 dB. A machine reading 100 dB at one metre reads 94 dB at two metres, 88 dB at four, and 82 dB at eight.

This makes distance the cheapest hearing protection available. It also explains why indoor measurements disappoint: walls reflect sound back, so the level falls off far more slowly than the rule predicts, and a room can stay loud well beyond where open air would have thinned it out.

What the A in dB(A) Means

The ear is not equally sensitive across frequencies. It is far less responsive to low bass than to the midrange where speech sits, so a sound meter that treated every frequency identically would overstate the impact of rumble.

A-weighting corrects for this by discounting frequencies the ear handles poorly. At 1 kHz the correction is zero by definition; at 125 Hz it subtracts about 16 dB. This is why almost every noise regulation, hearing conservation limit and product specification is quoted in dB(A) - the number is meant to track hazard and annoyance, not raw physical energy.

C-weighting is far flatter and is used for peak measurements and low-frequency work, where the bass energy A-weighting discards is exactly what you need to capture. A reading with a large gap between its dB(C) and dB(A) values is telling you the sound is bass-heavy.

Phons and Sones: Measuring Loudness Instead of Pressure

Decibels describe the physical signal. Two other units describe the sensation.

A phon is a loudness level: a tone measured at 40 phon sounds as loud as a 1 kHz tone at 40 dB SPL, whatever its actual frequency. At 1 kHz the two scales coincide by definition.

A sone goes further and scales with perception directly. One sone is defined as 40 phon, and loudness doubles with every additional 10 phon - so 2 sones is 50 phon, and 4 sones is 60 phon. Doubling the sone value genuinely means "twice as loud", which is precisely what the decibel cannot offer. Appliance manufacturers often rate fans and extractors in sones for this reason.

Hearing Safety: The Numbers That Matter

Damage depends on level and duration together. The widely used NIOSH recommendation is a limit of 85 dB(A) over eight hours, with a 3 dB exchange rate: every 3 dB increase doubles the energy, so it halves the safe exposure time.

Worth knowing: the US OSHA legal standard is more permissive, using 90 dB(A) with a 5 dB exchange rate. The two frameworks disagree, and the gap is not academic - at 100 dB(A), NIOSH allows 15 minutes where OSHA permits two hours. Where you have the choice, the 3 dB rule is the one grounded in the physics of energy.

Audiograms use yet another reference, dB HL, or hearing level. It is measured against the threshold of normal hearing at each frequency rather than against 20 μPa, so 0 dB HL is not a fixed pressure - it moves with frequency. That is why an audiogram is plotted as a curve against normal hearing rather than as raw sound levels.

Common Mistakes Worth Avoiding

Quoting decibels without a reference

dB SPL refers to 20 μPa, dBm to one milliwatt, dBFS to digital full scale. A bare "dB" figure in a specification is incomplete, and the three are not interchangeable.

Adding levels arithmetically

Two 90 dB machines make 93 dB, not 180. Halving a noise source removes 3 dB, not half the number.

Assuming twice the number is twice as loud

A 100 dB sound is not twice as loud as 50 dB. It carries roughly one hundred thousand times the intensity and sounds perhaps thirty times louder.

Comparing weighted and unweighted figures

A dB(A) value and an unweighted dB SPL value for the same sound will differ, sometimes considerably. Check which one a specification is quoting before comparing two products.

Conclusion

The decibel is a ratio, not an amount. Remember that +3 dB doubles the energy, +10 dB sounds twice as loud, and doubling your distance from a source removes 6 dB, and the scale stops fighting you. For anything involving hearing, work in dB(A) and treat 85 dB(A) over eight hours as the line worth respecting.

Our decibel to pascals converter handles the logarithmic maths for you across the whole sound category, including A- and C-weighted levels at a frequency you choose, and the sone to phon converter covers the loudness side. Since weighting depends entirely on frequency, our guide to hertz and kilohertz is a useful companion for understanding the scale those corrections are applied across.

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