Loudness is a measure of sound intensity taking frequency into account, and is called a A-weighted decibel, dB(A), or a phon. This is a standard threshold, but it also depends on frequency. If we were to calculate this using equation (5.3) we would get 87.6 dB SPL - try this for yourself.\)s o is the reference pressure which is 20 micropascals or 0 dB, and s is the observed sound pressure.The human ear has a standard sound threshold of 120 dB, which expressed logarithmically is around 10 12.
When measuring sound pressure level, we use the equation: Sound Pressure Level 20 log 10 (pp ref) dB. 85 dB SPL and then add this to the 84 dB SPL which would give us a total of approximately 87.5 dB SPL. Decibels are not a unit of measure but a logarithmic function which indicates the ratio between two values. The noise it makes is 100,000 times greater than the human hearing threshold. On a construction site, a large sheet of steel is dropped on the ground. The equation dB10logI represents the decibel level, where I is the ratio of the sound to the human hearing threshold. We can add the 80.8 and 83 first to give approx. The intensity of sound is measured on the decibel scale, dB. For example if we have 3 measurements of 80.8, 83 and 84 dB SPL. The scale for measuring intensity is the decibel scale. If we have more than two sound levels to add we can simply break them down into a series of pairs. At the right hand of the scales, if the two sound levels differ by as much as 20dB then the lower sound level makes very little difference to the total sound level. 80+1 = 81 dB SPL).Īt the left hand side of the nomogram, if the two sound levels are equal (difference = zero) then we should add 3 dB (i.e. 1 dB) this is then added to the higher sound level (i.e. So for our previous example, we take the difference between the two sound levels (80 - 74 = 6 dB) and read the lower scale to find the correction (approx. One more observation readily verified by examining Table 17.2 or using I ( p ) 2 v w 2 I ( p ) 2 v w 2 is that each factor of 10 in intensity corresponds to 10 dB. It is equivalent to a 3 dB increase in the total sound pressure level.įigure 5.2: Nomogram for addition of decibels The decibel scale is also easier to relate to because most people are more accustomed to dealing with numbers such as 0, 53, or 120 than numbers such as 1. If we add two unrelated sounds of the same intensity together, Now since we are talking about plane waves, our total sould pressure level = 83.01 dB SPL. So we now have the sound intensity of our combined signal and we can now convert this back to a dB value: If we now add I 1 and I 2 to give I total we have:
If we refer to the two sound intensities as I 1 and I 2 which are both equal, then as we have already seen: I 1 = I 2 = 10 -4 W/m 2
It makes things easier if a logarithmic scale is used this is what the decibel scale is. assumptions of a plane wave) then the first thing we need to do is convert our dB SPLs into intensities as in 5.1. The ratio of intensities between silence and ‘ow that hurts my ears’ is about 1:100 million million. A sound that is 10 times more intense ( 110-11 W/m2) is assigned a sound level of 10 dB. The threshold of hearing is assigned a sound level of 0 decibels (abbreviated 0 dB) this sound corresponds to an intensity of 110-12 W/m2. If we assume that the value in dB SPL is the same as it would be if we measured it in dB IL (i.e. The scale for measuring Sound intensity - Wikipedia is the decibel scale. So, for example suppose we have two independent sound sources producing white-noise and the sound pressure level of each one measured on it's own is 80 dB SPL - our question is, what is the resulting sound pressure level when they are both turned on together?