Making (a bit of) sense of decibels

Making sense of decibels

A basic principle of noise measurement which all newbies to the subject should be aware of is that the decibel scale was invented by someone who had just spent a long long weekend taking a wide variety of illegal drugs. It only makes sense to maths geniuses and to the rest of us it is a touch bonkers. It is a logarithmic scale which means 2 plus 2 doesn't always equal four, that would be too easy. 

Oh, and 'dB' is the symbol for decibel, as decibel is far too long for lazy noise people to keep writing out.

One of the important principles to know is that three decibels is doubling the energy of the noise. So 88dB has twice the power of 85dB. That means that as a rough rule we can say that a noise level of 93dB is twice as dangerous as a noise level of 90dB. 

You can see why this is important - what seems to be a small change in the number of decibels can actually make a very large difference in the danger levels faced by employees. Indeed an employee at 88dB would hit their daily exposure limit in four hours compared to one who is at 85dB who would only hit their daily exposure limit in eight hours.

It's worth highlighting that 'double the energy of the noise' is not the same as 'doubling the volume'. We're pretty rubbish at hearing changes in noise energy and to the human ear we need about 10dB to sound like double the volume. That's a good reason why we cannot rely on whether things 'sound a bit louder' as a way of judging noise as if it does sound louder, chances are it's quite a big change in the energy involved.


While we're at it, we may as well get all the bizarre bits out in the open now and another one of the decibel scale's lovely features is that 0dB is not silence, that would be far too simple.

0dB was set by getting a load of young people together and finding what was the quietest sound they could hear on average, which means 0dB is actually some noise. You can think of it as perfect hearing, the kind of hearing a teenager has before the ravages of time start to take hold.

Some people, especially younger women, can hear down to -10dB or even -15dB or so. 

Anechoic chamber

Anechoic chamber

For the curious, the quietest place on earth is an anechoic chamber in Microsoft's Seattle campus where they have got down to -20dB. Second place is the Orfield Laboratory in Minnesota. That's the spikey room thing here which is not some kinky sex dungeon despite its looks. Even in there it is only -9.4dB, which is still some noise and not silence. The longest anyone has been able to stay in there is 45 minutes, but anyone in there over 30 minutes has to sit down or they fall over as sound is also used to orient ourselves.

dB(A) and dB(C)

We then confuse the issue further in measurements by measuring decibels in two different ways, dB(A) and dB(C) but there is a separate page in the FAQ for that.