Three decibels is double
- A loud machineAn earEight hours, every day
A road drill, a bottling line, a busy school kitchen. Noise at work does its harm slowly: the tiny hairs inside the ear that pick up sound get flattened a little each loud day, and they do not grow back. About 15,000 people in Britain have hearing problems their work caused or made worse. Nobody noticed on any one day.
- 85 dB1 time the energy of 85 dB88 dB2 times the energy of 85 dB91 dB4 times the energy of 85 dB94 dB8 times the energy of 85 dB
Loudness is measured in decibels, written dB. The scale is odd: it is not a ruler. Every 3 dB more is double the sound energy. So 88 dB carries twice the energy of 85, and 91 carries four times. A small step on the number is a big step in the ear.
- 88 dB85 dB
A machine runs at 88 dB. Another runs at 85. How does the louder one compare?
Twice the sound energy.
Right. Three decibels is a doubling. The ear hears it as only a little louder, which is exactly why the number is dangerous to trust by feel.A little more, about 3 per cent.
Not yet. Decibels do not work like percentages. Three more is not three per cent more; it is a hundred per cent more energy.The same. Three decibels is too small to matter.
Not yet. You might barely notice the difference by ear. The ear's hairs notice it fully: the energy reaching them has doubled. - 80 dBoffer protection85 dBcut the noise; protection worn87 dBthe limit, in the ear
The Control of Noise at Work Regulations, 2005. British law sets three lines, measured as an average over a working day. At 80 dB, the employer must offer ear protection and explain the risk. At 85, the employer must cut the noise where that is reasonable, provide protection, and make sure it is worn. 87 dB inside the ear, with protection counted, is the limit nothing may pass.
Move the control to set how loud the work is. Eighty-five decibels for eight hours is one day's dose at the upper line. Every 3 dB more halves the time it takes to reach it.
Time before the day's dose is reached8 hoursA working day8 hours85 dBAt 85 dB, a day's dose at the 85 dB line is reached in 8 hours. That is a whole working day.
Time before the day's dose is reached4 hoursA working day8 hours88 dBAt 88 dB, a day's dose at the 85 dB line is reached in 4 hours. The sound carries 2 times the energy of 85 dB, so the time is cut to a half.
Time before the day's dose is reached2 hoursA working day8 hours91 dBAt 91 dB, a day's dose at the 85 dB line is reached in 2 hours. The sound carries 4 times the energy of 85 dB, so the time is cut to a quarter.
Time before the day's dose is reached1 hourA working day8 hours94 dBAt 94 dB, a day's dose at the 85 dB line is reached in 1 hour. The rest of the shift at that level is over the line.
Time before the day's dose is reached0.5 hoursA working day8 hours97 dBAt 97 dB, a day's dose at the 85 dB line is reached in 30 minutes. The rest of the shift at that level is over the line.
Time before the day's dose is reached0.25 hoursA working day8 hours100 dBAt 100 dB, a day's dose at the 85 dB line is reached in 15 minutes. The rest of the shift at that level is over the line.
At 94 dB, how long before a day's dose is reached?
Four hours. Nine more is half the time.
Not yet. Three more is half the time. Nine more is three halvings, so an eighth of the time: one hour, not four.One hour. Nine decibels is three halvings: 8 hours, 4, 2, 1.
Right. A worker on a 94 dB machine has had a full day's dose by the first break. The other seven hours are over the line, every day.About five hours. Ninety-four is a bit more than 85.
Not yet. Nine decibels is not a bit more; it is eight times the energy. Eight hours divided by eight is one hour.Move the control.
- 103 dBGrinder8 hours85 dB
A grinder runs at 103 dB. Using the rule, how many minutes until a day's dose is reached?
minutesCount how many threes from 85 to 103, then halve eight hours that many times.
Right. Eighteen decibels is six halvings, and 8 hours halved six times is 7.5 minutes. A grinder for a quarter of an hour is two days' dose, which is why that job comes with ear defenders that are not optional. Slide the marker to how loud a chainsaw is at the operator's ear, in decibels.
Slide the marker to your guess, then lock it. Nothing is scored.
you said · 40 dBactually · 110 dB- 1Quiet office45 dB
- 2Talking60 dB
- 3Busy traffic85 dB
- 4Jet engine, close140 dB
A chainsaw, at the ear of the person holding it: —
About 110 dB, and at that level the day's dose is reached in under two minutes. Guesses land low because 110 does not sound far from 85 on a ruler, and on this scale it is more than two hundred times the energy.- Each loud daya little lossTwenty years ongone for good
The harm is slow and it does not come back. A worker on a loud line loses a little hearing every year and notices nothing, because the loss is gradual and the brain fills in. After twenty years, conversation in a noisy room is gone. This is lesson one's slow harm, measured in decibels.
- 2 hours94 dB6 hours70 dB
A worker runs a 94 dB machine for two hours, then spends six hours in an office at 70 dB. Is the day over the 85 dB line?
Yes, twice over. At 94 dB a day's dose takes one hour, and this worker had two.
Right. The dose adds up across the day. Two hours at 94 is two full days' worth, and six quiet hours add almost nothing and take nothing away.No. Two hours is a short time.
Not yet. Short for the clock, long for the ear. At 94 dB the whole day's dose arrives in one hour; two hours is double it.No. The six quiet hours balance the two loud ones.
Not yet. Quiet does not undo loud. The dose only adds: 70 dB for six hours adds a tiny amount, and the two hours at 94 were already over the line.
Lesson complete
Three decibels more is twice the sound energy, so it halves the time a day's dose allows.