Decibel is defined as:Ī logarithmic quantity called the power level or field level is used to indicate the ratio of one value of a power or field quantity to another on a logarithmic scale. In terms of power, a thunderclap is roughly 1,000,000,000,000 times more powerful than the smallest audible sound. Because the human ear is so sensitive, anything from a light brushing of your fingertip against your skin to a loud thunderclap can be heard. As a result, decibels are employed to measure a sound’s volume.
In fact, loud noise can be damaging to our hearing. Sound is everywhere around us and can be measured to protect and inform us since some noises are dangerous.
Because a one-decibel variation in loudness between two sounds is the smallest difference perceptible by human hearing, the unit decibel is used. The term bel is derived from Alexander Graham Bell, the telephone’s inventor. One decibel (0.1 bel) = 10 times the power ratio’s common logarithm. The decibel (dB) is a unit used to express the ratio of two physical quantities, commonly acoustic or electric power, or to measure the relative loudness of sounds. Hearing loss can be caused by the degree of noise, the distance a person is from the noise (distance from the noise), and the amount of time they listen to it. Loud noise, in fact, can be quite harmful to one’s hearing. If we were to calculate this using equation (5.3) we would get 87.6 dB SPL - try this for yourself.Sound is everywhere around us, and it can be measured in order to inform and protect us, as some noises are dangerous. 85 dB SPL and then add this to the 84 dB SPL which would give us a total of approximately 87.5 dB SPL. We can add the 80.8 and 83 first to give approx. For example if we have 3 measurements of 80.8, 83 and 84 dB SPL. 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. It is equivalent to a 3 dB increase in the total sound pressure level.įigure 5.2: Nomogram for addition of decibels 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 assumptions of a plane wave) then the first thing we need to do is convert our dB SPLs into intensities as in 5.1. 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. 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?