Of 10 thousand, 0.3 is the log of 2, so this sound has a sound pressure 20 P ref is the sound pressure of the reference level, and p 2
So if you read of a sound pressure level of 86 dB, it means that Personal music systems with in-ear speakersĪre capable of very high sound levels in the ear, and are believed by some toīe responsible for much of the hearing loss Rather young people or in people who have not been exposed to loud music or
(Usually this sensitivity is only found in Nevertheless, this is about the limit of sensitivity of the human ear, in its This is very low: it is 2 ten billionths of an atmosphere. Reference level (for air) is usually chosen as p ref = 20 micropascals (20 μPa), Than a ratio, a reference level must be chosen. So, when it is used to give the sound level for a single sound rather Standard reference levels ('absolute' sound level)Ī ratio. It is difficult to notice the difference between successive pairs.ġ0*log 10(1.07) = 0.3, so to increase the sound levelīy 0.3 dB, the power must be increased by 7%, or the voltage by 3.5%. You may notice that the last is quieter than the first, but (This makes the dB a convenient size unit.)Ġ.3 dB steps. That sound levels that differ by less than 1 dB are hard to distinguish, as That the second of any pair is quieter than its predecessor.ġ0*log 10(1.26) = 1, so to increase the sound level byġ dB, the power must be increased by 26%, or the voltage by 12%.ĭecibel? Sound levels are rarely given with decimal places. Last is quieter than the first, but it is rather less clear to the ear As you listen to these files, you will notice that the One decibel is of the same order as the Just Noticeable Difference (JND)įor sound level. Series, successive samples are reduced by just one decibel. Sound files and animation by John Tann and George This further below, but it's a useful thing to remember when choosing The power does not make a huge difference to the loudness. Line shows a continuous exponential decay with time. The green line shows the voltage as a function of time. Now 1/√2 is approximately 0.7, so -3 dBĬorresponds to reducing the voltage or the pressure to 70% of its original The second sample is the same noise, with the voltage The first sample of sound is white noise (a mix of a broad range ofĪudible frequencies, analogous to white light, which is a mix of all visibleįrequencies). We saw above that halving the power reduces the sound pressure by √2 and Sound files to show the size of a decibel This frequently asked question is a little subtle, so it is discussed here on ourįAQ. (increase of 3 dB)? Or do I double the pressure (increase of 6 dB)? What happens if I add two identical sounds? Do I double the intensity Original power) and you reduce the level by another 3 dB. What happens when you halve the sound power? The log of 2 is 0.3010, so the log Level between two sounds with p 1 and p 2 is therefore: When we convert pressure ratios to decibels. X 2 is just 2 log x, so this introduces a factor of 2 (Similarly,Įlectrical power in a resistor goes as the square of the voltage.) The log of In a sound wave, all else equal, goes as the square of the pressure. Respond proportionally to the sound pressure, p.
Sound is usually measured with microphones and they ( Note also the factor 10 in theĭefinition, which puts the 'deci' in decibel: level difference in bels (named for Alexander Graham Bell) is just log (P 2/P 1).) So far we have not said what power either of the speakers radiates, But note that the decibel describes a ratio:
In discussing sound: they can describe very big ratios using numbers This example shows a feature of decibel scales that is useful Times the power of the first, the difference in dB would be Had 10 times the power of the first, the difference in dB would be If the second produces twice as much power than the first,ġ0 log 2 = 3 dB (to a good approximation). Using the decibel unit, the difference in sound level, between the two is defined to Version of the same sound with power P 2, but everythingĮlse (how far away, frequency) kept the same. (If you have forgotten, go to What is aįor instance, suppose we have two loudspeakers, the first playingĪ sound with power P 1, and another playing a louder Butįirst, to get a taste for logarithmic expressions, let's look at some Phon and to the sone, which measures loudness. The ratio may be power, sound pressure, voltage or The dB is a logarithmic way of describing a ratio. The decibel ( dB) is a logarithmic unit used to Problems using dB for amplifier gain, speaker power, hearing
0 kommentar(er)
