How loud is safe?
Stand near a whisper, a conversation, busy traffic, a pressure horn or a firecracker. See the sound level in decibels, how it drops as you move away, and how long your ears can safely take it each day.
Core concept
Loudness is measured in decibels (dB): every 10 dB is ten times the sound energy; the level falls about 6 dB each time the distance doubles, and above 85 dB the safe listening time halves for every 3 dB more
Topic: Science around us
The loudness of a sound is measured in decibels (dB). A whisper is about 30 dB, a conversation 60 dB, busy traffic 85 dB, a pressure: How much force presses on an area. A sharp knife has high pressure at its edge. All key words horn close by 110 dB and a firecracker 140 dB. The scale is not like a ruler: each 10 dB more is ten times as much sound energy: What is needed to make things move, heat up, light up or grow. All key words, so 90 dB has a hundred times the energy of 70 dB. Sound spreads out as it travels, so each time you double your distance from the source the level drops by about 6 dB. Loud sound damages the tiny hair cells in the inner ear, and they do not grow back. Up to 85 dB is safe for a whole working day of 8 hours; for every 3 dB above that, the safe time halves.
What you will learn
- Read sound levels in decibels and compare everyday sounds.
- Work out how the level falls as the distance doubles.
- Find the safe daily listening time for a loud sound.
Before you start
- Know that sound is a vibration that travels through air.
Materials
- A free sound-meter app on a phone, if a teacher has one
Equipment
- None
Procedure
- Record busy traffic 2 m away.
- Stand 1 m from a pressure horn and record.
- Move 16 m away from the horn and record.
How this simulation is simplified
- Levels 1 m away: whisper 30 dB, talking 60 dB, busy traffic 85 dB, pressure horn 110 dB, firecracker 140 dB (typical values).
- The level falls 20 × log₁₀(distance in m) dB from its level at 1 m; safe hours = 8 ÷ 2^((level − 85) ÷ 3), at most 24 (a common workplace rule).