In order to make measurements of noise and sound a method is used that measures the results in units called decibels.
Decibels can be a bit of a mystery to the uninitiated (and sometime those of us who should know better) and so I hope to go over some of the basics to help understand how they relate to noise and noise measurement (this will take more than one post).
Most of the information below has been unscrupulously scrounged from Wikipedia.
The decibel (dB) is a logarithmic unit of measurement that expresses the magnitude of a physical quantity (usually power or intensity) relative to reference level. Since it expresses a ratio of two quantities with the same unit, it is dimensionless.
Being dimensionless means that you can’t describe a quantity of something as being XXdB. It has to be referenced to something. Like radio frequency power can be expressed as XX dBm. Where the ‘m’ means that the number is being referenced to one mili-watt. Likewise for noise measurements it’s common to see them expressed as XX dBA. The ‘A’ part is the reference (we’ll cover the reference in another post). Anyone who quotes a measurement for noise as being XXdB does not understand decibels (this is true for some ‘experts’).
A decibel is one tenth of a bel (B).
The decibel is useful for a wide variety of measurements in science and engineering (e.g., acoustics and electronics) and other disciplines. It confers a number of advantages, such as the ability to conveniently represent very large or small numbers, a logarithmic scaling that roughly corresponds to the human perception of, sound and light, and the ability to carry out multiplication of ratios by simple addition and subtraction.
The decibel is commonly used in acoustics to quantify sound levels relative to a reference level is typically set at the threshold of perception of an average human.
A reason for using the decibel is that the ear is capable of detecting a very large range of sound pressures.
Because the power in a sound wave is proportional to the square of the pressure, the ratio of the maximum power to the minimum power is above one (short scale) trillion. To deal with such a range, logarithmic units are useful: the log of a trillion is 12, so this ratio represents a difference of 120 dB. Since the human ear is not equally sensitive to all the frequencies of sound within the entire spectrum, noise levels at maximum human sensitivity — for example, the higher harmonics of middle A (between 2 and 4kHz) — are factored more heavily into sound descriptions using a process called frequency weighting. More on that later.
No comments:
Post a Comment