Why doesn't 70 dB + 70 dB equal 140 dB?

Because decibels are logarithmic, not linear. Doubling acoustic energy raises the reading by only 3 dB, so 70 + 70 = 73 dB.

How many dB must noise drop for it to sound half as loud?

A 10 dB reduction is needed for the brain to perceive noise as half as loud. This means 90% of the sound energy has been absorbed.

What Are Decibels (dB) and Why Don't They Work Like Normal Numbers: The Logarithmic Scale of Sound

If you buy 1 kilogram of potatoes and add another kilogram, you have 2 kilograms. If you walk 10 metres and then another 10, you've covered 20 metres. We're all accustomed to thinking in this linear way.

But when we enter the world of acoustics and try to insulate a house, this logic collapses. The unit for measuring sound intensity is the decibel (dB). And decibels hide an enormous mathematical trap: they are logarithmic, not linear.

1. The Vacuum Cleaner Paradox: Why 70 + 70 = 73

Let's run a thought experiment. You switch on your vacuum cleaner. The sound meter reads 70 dB. Now you bring a second, identical vacuum cleaner and switch it on right next to the first. How much noise do you have in total?

Paradox - two 70 dB vacuum cleaners produce only 73 dB, not 140

🤔 Logic Says...

70 dB + 70 dB = 140 dB. If that happened, your eardrums would burst, because 140 dB is the noise of a fighter jet taking off right next to you!

🎯 Reality

70 dB + 70 dB = 73 dB. Just 3 decibels more! This happens because decibels aren't "normal" numbers - they follow a logarithmic scale. Every time the acoustic energy doubles, the reading goes up by only 3 dB.

2. The Physics Behind the Magic: Why We Need Logarithms

Human hearing has a colossal range. We can hear from the infinitesimal rustle of a leaf to the deafening roar of thunder (which carries billions of times more acoustic energy). To fit all these numbers onto a readable scale, scientists use logarithms.

Logarithmic scale - formula for adding sound pressure in dB

📐 The Formula

The formula for adding sound pressure levels (where L is the noise level in decibels) is: L_total = 10 · log₁₀(Σ 10^(Lᵢ/10)). Thanks to this formula, every time the acoustic energy doubles, the meter reading rises by only 3 dB.

💡 The Golden Rule

Doubling energy = +3 dB. This means that if you put 10 vacuum cleaners together (10× energy), the noise doesn't become 700 dB but just 80 dB (70 + 10). The logarithmic scale "compresses" enormous differences into humanly readable numbers.

3. How Does the Ear Hear It? The Rule of 3 and 10 (Psychoacoustics)

Rule of 3 dB and 10 dB - physical energy vs psychoacoustic perception

Here lies the biggest secret you need to know before paying for soundproofing. There is a huge difference between what the instrument shows (physical energy) and how our brain "translates" it (psychoacoustics).

🔇 Change of 1 dB

Barely perceptible to humans. You can detect it only under laboratory conditions, with headphones and complete silence.

🔉 Change of 3 dB

The instrument reads that the acoustic energy has halved. Our ear, however, perceives only a slight reduction in loudness. Not enough to feel a noticeable difference at home.

🔊 Change of 10 dB - Where the Magic Happens!

For the human brain to perceive that a noise has "halved", the insulation must drop the level by a full 10 decibels. In reality, to lower sound by 10 dB means we have absorbed 90% of the acoustic energy!

💡 Note: When a material says it reduces noise by "only" 10 dB, don't be disappointed! Those 10 dB mean that to your ear, the noise will literally sound half as loud.

4. Noise Scale: A Comparison Chart for Everyday Sounds

To know what you're up against when the neighbour plays music, here's where everyday sounds fall:

Noise scale - from 10 dB whisper to 140 dB fighter jet

Sound Source	Level (dB)	Effect
Absolute silence (Studio)	10 – 20 dB	You hear your own breathing
Whisper / Library	30 – 40 dB	Calm environment
Normal conversation	60 – 65 dB	Tolerable
Loud TV / Vacuum cleaner	70 – 75 dB	Annoying from next door
Motorcycle on the street	85 – 90 dB	Long-term damage threshold
Jack-hammer / Concert	100 – 110 dB	Ear pain after a short time
Fighter jet	140+ dB	Immediate hearing damage

The Bottom Line: Sound insulation is a tough battle. When you buy a material that claims to reduce noise by "only" 10 dB, don't despair! Those 10 dB mean your ear will literally hear half the noise. Don't judge materials by the logic of a tape measure, because with decibels… appearances are deceiving.

Airborne vs. Impact Noise: How Sound Travels in a Building Soundproofing vs. Sound Absorption: The Big Misunderstanding Sound Meters & Limits: How We Measure Noise at Home STC and Rw Ratings: How to Read Sound Insulation Labels

Sound Insulation: Silence and Noise Protection

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