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Ripple effect
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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
“The decibel scale is logarithmic, so things don’t just add together: 19 extra cars only add about 13 dB” 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
Questions and answers should be concise. We reserve the right to edit items for clarity and style. Include a daytime telephone number and email address if you have one. Restrict questions to scientific enquiries about everyday phenomena. The writers of published answers will receive a cheque for £25 (or US$ equivalent). Reed Business Information Ltd reserves all rights to reuse question and answer material submitted by readers in any medium or format.
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
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
■ 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, “The answers to this question show that the but at the end all the cars were decibel is a very confusing well spread out around the track. The difference in maximum level unit of measurement” between the first lap and later laps measurement. With this in mind where the cars were spread out I’ve taken on the challenge of was variable but around 12 dB, explaining decibels for people close to what theory predicts. who don’t know what a logarithm Loudness to a human observer is, at bit.ly/decibels. is another matter altogether and Tony Woolf differs from recorded sound levels. The human ear works using Acoustic consultant London, UK ratios, so doubling the sound
New Scientist retains total editorial control over the content of The Last Word. Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK, by email to [email protected] or visit www.last-word.com (please include a postal address in order to receive payment for answers). For a list of all unanswered questions send an SAE to LWQlist at the above address.
Do Polar Bears Get Lonely? Our latest collection ight serious enquiry, brilliant insight and the hilariously unexpected cted Available from booksellers m/ and at www.newscientist.com/ polarbears
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
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
“The decibel scale is logarithmic, so things don’t just add together: 19 extra cars only add about 13 dB” 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
Questions and answers should be concise. We reserve the right to edit items for clarity and style. Include a daytime telephone number and email address if you have one. Restrict questions to scientific enquiries about everyday phenomena. The writers of published answers will receive a cheque for £25 (or US$ equivalent). Reed Business Information Ltd reserves all rights to reuse question and answer material submitted by readers in any medium or format.
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
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
■ 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, “The answers to this question show that the but at the end all the cars were decibel is a very confusing well spread out around the track. The difference in maximum level unit of measurement” between the first lap and later laps measurement. With this in mind where the cars were spread out I’ve taken on the challenge of was variable but around 12 dB, explaining decibels for people close to what theory predicts. who don’t know what a logarithm Loudness to a human observer is, at bit.ly/decibels. is another matter altogether and Tony Woolf differs from recorded sound levels. The human ear works using Acoustic consultant London, UK ratios, so doubling the sound
New Scientist retains total editorial control over the content of The Last Word. Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK, by email to [email protected] or visit www.last-word.com (please include a postal address in order to receive payment for answers). For a list of all unanswered questions send an SAE to LWQlist at the above address.
Do Polar Bears Get Lonely? Our latest collection ight serious enquiry, brilliant insight and the hilariously unexpected cted Available from booksellers m/ and at www.newscientist.com/ polarbears
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