Decibels turn up everywhere once you start noticing them. They appear on the side of a new lawnmower, in the specifications of a pair of headphones, and in workplace signage around building sites and depots. Most of us nod along without ever being told what the number actually means. This article is a plain-English look at the decibel scale and how some familiar Central Queensland sounds sit on it. It is general interest reading, not guidance for any individual.
What a Decibel Is
A decibel, written dB, is a unit used to describe the intensity of sound. The World Health Organization describes the decibel as the standard measure of sound level, with zero decibels set near the very quietest sound a young, healthy ear can detect. From that starting point the numbers climb as sound becomes more intense, which is why a whisper and a passing road train sit so far apart on the same scale.
Sound level meters are the instruments that produce these readings, and the figure they show depends on where the meter is standing. A reading is always a measurement taken at a particular distance from a particular source, which is why the same mower can produce different numbers from the handle and from the far end of the yard.
Why the Scale Is Not a Straight Line
Here is the part that surprises people. The decibel scale is logarithmic rather than linear, so the numbers do not add up the way everyday counting does. The World Health Organization explains that an increase of 10 decibels represents a tenfold increase in sound energy. In other words, 70 dB carries ten times the sound energy of 60 dB, and 80 dB carries one hundred times the sound energy of 60 dB.
Our ears do not experience that jump as a tenfold change. A widely used rule of thumb, cited by hearing and acoustics bodies including the National Acoustic Laboratories, is that a rise of about 10 decibels is heard as roughly a doubling of loudness. So the physics and the perception tell two different stories from the same number, which is a large part of why the scale confuses people at first.
The A in dB(A)
You will often see levels written as dB(A) rather than plain dB. The A stands for A-weighting, a filter that adjusts the measurement to reflect the frequencies human ears are most responsive to. Safe Work Australia uses A-weighted decibels in its national noise material, alongside C-weighted decibels, written dB(C), for very short, sharp peak sounds such as a sudden impact. The weighting is simply a way of making the meter's reading more representative of what a person hears.
Everyday Sounds on the Scale
Sound level examples published by health and safety authorities give a useful sense of scale. Safe Work Australia and Hearing Australia have both published example figures along these lines. All of them are approximate, because real-world readings shift with distance, surfaces and the equipment itself.
- A quiet library or a still bedroom at night sits at roughly 30 to 40 dB.
- Normal conversation is commonly given as about 60 dB. That is the murmur of a full cafe table on East Street.
- A vacuum cleaner or a busy office is often listed at around 70 dB.
- Traffic beside a main road, of the sort you hear standing near the Bruce Highway, is frequently cited in the 70 to 85 dB range.
- A petrol lawnmower, the Saturday morning soundtrack of half of Rockhampton, is usually quoted at about 90 dB.
- A chainsaw is generally listed at around 110 dB.
Some of Central Queensland's most characteristic sounds have never been formally catalogued, of course. Nobody has published an official figure for a chorus of cicadas in a Frenchville backyard in January, or for the roar that goes up at Browne Park when a try is scored, or for the showgrounds on the busiest night of the Rockhampton Show. It is fair to say they all sit somewhere well up the scale. The event end of that scale is the subject of a separate piece on hearing protection at the Rockhampton Show, rodeos and motorsport events.
Where the Numbers Are Used
Decibel figures also appear in Australian regulation. Safe Work Australia's national exposure standard for noise is expressed as an eight hour equivalent continuous A-weighted level of 85 dB(A), together with a C-weighted peak level of 140 dB(C). Those two numbers are the reference points that underpin workplace noise regulations across the states and territories, and they are the reason noise readings are taken on job sites at all. How those rules sit in an ordinary working day is covered in our general piece on hearing at work in Australia.
Understanding decibels does not change what the world sounds like, but it does make the numbers meaningful. Next time a figure appears on a tool label or a warning sign, you will know it is a logarithmic measure, that it was taken from somewhere specific, and that a step of ten is a much bigger step than it looks.
If you'd like to chat about your hearing, the friendly team at CQ Audiology in Rockhampton is here to help. Call (07) 4848 6528 or book online.
This article is general information, not personal medical advice. For guidance about your hearing, speak with a qualified professional.
Sources: World Health Organization (the decibel scale and its logarithmic nature); Safe Work Australia (A-weighted and C-weighted decibels, published sound level examples, national exposure standard for noise); Hearing Australia (everyday sound level examples); National Acoustic Laboratories (perceived loudness and the 10 decibel rule of thumb).
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