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Understanding Decibels

Wednesday, 31 July 2019

You may have noticed lots of our amplified telephone describe their volume in decibels, or dB. We've taken a bit of a deep-dive into the science and history to help you on your way to Understanding Decibels.

What are decibels?

We'll be covering some technical information first. If you just want the no-nonsense basics, you can skip straight to the end.

Sound Pressure

Before we get distracted by units, it's worth taking a moment to look at what we are actually trying to measure. Most people know that decibels refer in some way to sound or volume, but they actually refer to something more specific: sound pressure level.

Sound pressure is the local pressure deviation from atmospheric pressure caused by a sound wave. In more simple language it might be referred to as the physical intensity of a sound. When sound pressure is measured on a logarithmic scale, such as with decibels, it is generally referred to as sound pressure level, and in air it can be measured with a microphone.

The Bell System

A decibel is actually one tenth of another unit – the bel – named after the Bell group of telephone companies in North America from the late 19th and early 20th centuries, now AT&T. The bel was developed as a measure of signal loss across telegraph and telephone circuits over a set distance. Even when the bel was first formulated the decibel was almost always used instead, and the bel has since faded from history.

Some confusion emerges from the fact that, despite both relating to telephones, the decibel's origin as a measurement of signal loss is not quite the same thing as its most common modern usage as a measurement of sound pressure level. This does make clear, however, one of the most important things about decibels, which is that they are meaningless without a stated distance.

Some Technical Stuff

As might have become clear, a decibel is not solely a measure of sound. It is actually a unit of measurement to show relative values on a logarithmic scale. In sound pressure level – what we are actually looking at – all values are given relative to 0dB, which is the bottom threshold of human hearing.

The ratio expressed by decibels is between one value and another on a logarithmic scale. This means that as the decibel value grows in a linear manner, the sound pressure it represents actually grows exponentially.

In the Real World

Looking at examples from the real world can a be fascinating experience, and it shows just how quickly the intensity increases up the scale. There are even spaces which measure as significantly quieter than the bottom limit of human hearing:

Decibel Level Sound source Distance
-20dB Microsoft's anechoic chamber Ambient
0dB Human auditory threshold At ear
10dB Leaf rustling Ambient
40dB Normal conversation 1m
60dB Passenger car 10m
85dB Hearing damage from long term exposure At ear
100dB Jack hammer 1m
120dB Vuvuzela horn 1m
130dB Threshold for pain At ear
150dB Jet engine 1m
170dB Rifle being fired 1m

To Cut to the Chase

Starting at 0dB – the bottom threshold for human hearing – decibels measure sound pressure, which might be easier to understand as the intensity of sound. They are on a logarithmic scale which means that a small increase in decibels can represent a very large increase in the sound pressure. For instance, doubling from 10dB to 20dB would actually represent 100 times the sound pressure.

In Practice

The frustrating reality of sound measurements is that many people publish sound pressure levels without any kind of reference distance. If you want to take a scientific approach this makes the values fairly meaningless, but there are still general standards you can compare these kinds of measurements to, particularly with amplified telephones where manufacturers are working to similar standards with objects always used at the ear. To that end, we have put together a quick and easy guide for you to look at phones volumes:

Sound in Decibels (dB) Volume Level
10dB Normal telephone
20dB Double a normal telephone
30dB Four times a normal telephone
40dB Eight times a normal telephone
50dB Sixteen times a normal telephone
60dB Thirty-two times a normal telephone
70dB Sixty-four times a normal telephone

We hope you have found this guide useful! If you have any thoughts, or would like help with anything we have covered, leave a reply below.