If we stick to the meaning of decibel that appears in the dictionary of the Royal Spanish Academy, the concept of decibel is recognized as a synonym of decibel. It is a unit of acoustic intensity. It is equivalent to 0.1 bels, which is the unit that comes out of the quotient between the pressure that produces a wave and a pressure that is taken as a reference.
A little history of the decibel The bel derives from Bell, the surname of the well-known Scottish American naturalized physicist, Alexander Graham Bell. Its symbol is dB and it does not refer to a quantity but to a ratio between quantities, referring to a logarithmic expression. If we wonder why the need arose to use a ratio or comparison between two quantities instead of a fixed quantity, it is due to the discovery that a listener cannot reliably indicate the intensity of a sound, even if he hears it. more than once. Despite this, he is able to compare this property of different sounds. If used in the field of communications, electricity and acoustics, the decibel expresses the relationship between a given magnitude and another reference magnitude. In relation to the measurement of the intensity of sounds, the lowest threshold of perception that the human being has is usually used as a reference magnitude. In this case, an intensity of 0 dB marks the perception threshold in people. Based on this measurement, we can highlight that a train reaches an intensity of 80 dB. An airplane taking off can reach 130 dB, a Formula 1 car can reach 140 dB and the loudest recorded sound was 200 dB, which would be that of the atomic bomb dropped on Japan during World War II, which can give us sound reference.
Why is the decibel used instead of the bel? The main reason why the decibel is used instead of the bel responds to a specific need that is related to the large number of fields that are subject to measurement with this unit. In the hypothetical case that the bel is used in all cases, the result for many of them would be decimal, which would make it very difficult to have a correct reading. Keep in mind that the decibel or decibel is a logarithmic expression, so it is neither linear nor dimensionless. It should also be noted that neither the decibel nor the bel belong to the International System of Units.
Applications in acoustics Currently, the decibel is the measure used to express the power level or intensity level of the sound. This scale is used because the sensitivity of the human ear to variations in sound intensity has an approximately logarithmic scale, not a linear one. Since the decibel is a relative unit, for acoustic applications, a value of 0 dB is assigned to the threshold of human hearing. Despite this, the true threshold of hearing can vary between different people and for the same person, it depends on the frequency of the sound. As a curious note, it is estimated that the pain threshold for human beings starts at 140 dB.