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THE LAST WORD

Speed freaks

A single Formula 1 car passing

by makes a noise of around

110 decibels. Last year I went

to a Formula 1 Grand Prix and sat

near the start line, where 20 cars

left the grid at once. The noise was

mind-numbing, much louder than a

single car, but not 20 times louder

(or 2200 decibels, an unachievable

figure). Why wasn’t it?

■ There are two things here. First, the sound sources are not synchronised: the cars are making noise independently of one another, and so a burst of loud noise from one might coincide with a drop in noise from another. For these unsynchronised sound sources, the sound pressure (the air compressions that your ear

detects) increases only according to the square root of the number of sources.

Second, the decibel (dB) scale is logarithmic: things don’t just add together neatly. The increase in the decibel level is the logarithm of the ratio of the number of sources (in this example, 20/1) multiplied by 10. The 19 extra cars add only around 13 dB , so the noise from 20 cars will be just 123 dB or thereabouts.

The same is true for violins in an orchestra. The 16 violins in an orchestra produce only four

times as much volume as a single violin: if one violin produces 70 dB, 16 produce 82 dB. Similarly, silencing half of the trumpets – which are obviously much louder than violins – only reduces their volume by a few decibels, which explains why you need so many more violins than trumpets in an orchestra.

This also has implications for traffic-noise control. If the noise emanating from a car engine is roughly the same level as the noise from its tyres then there’s not much point in reducing engine noise by more than about 3 dB without also reducing the tyre noise.Hugh Hunt

University of Cambridge, UK

■ Back in 1990, I measured sound levels at a Formula 1 Grand Prix at Silverstone in the UK for health and safety. I found that the sound level from a single car passing, measured in the pits, was indeed about 110 dB.

The sound level varied widely throughout the race. In the first lap, all the cars passed by my measuring point virtually at once, but at the end all the cars were well spread out around the track. The difference in maximum level between the first lap and later laps where the cars were spread out was variable but around 12 dB, close to what theory predicts.

Loudness to a human observer is another matter altogether and differs from recorded sound levels. The human ear works using ratios, so doubling the sound

power always produces roughly the same increase in loudness, no matter where you start.

A rule of thumb that works pretty well for most people is that an increase of 10 dB corresponds to “twice as loud”, so 20 cars passing at once would be a bit more than twice as loud as one car to a listener such as your questioner – the 13 dB given by the first correspondent.

This just goes to prove that the decibel is a very confusing unit of

measurement. With this in mind I’ve taken on the challenge of explaining decibels for people who don’t know what a logarithm is, at bit.ly/decibels .Tony Woolf

Acoustic consultant

London, UK

This week’s questions

RIPPLE EFFECT

This glass seems to have no ripples in it when viewed from the side, but lots when viewed from above (see photo). Why is this?Liam, age 11

Galway, Ireland

JUMBO JUMPING

We’ve heard that elephants are the only quadrupeds that can’t jump. Is it true? If so, why not? And if it’s not true, how high can they jump?Tad and Lydia Forty

Bath, Avon, UK

NO SOLIDS

I made a sauce for lamb from blueberries and cassis, but made too much so put some in the freezer. It wouldn’t set solid and spent a month being malleable at -7 °C. It tasted OK when we used it later. Why wouldn’t it freeze solid?Tom Lyndhurst

Pinner, Middlesex

Last words past and present, plus questions, at www.last-word.com

“The decibel scale is logarithmic, so things don’t just add together: 19 extra cars only add about 13 dB”

“The answers to this question show that the decibel is a very confusing unit of measurement”