Sound radiation power is also compared by dB ratings, and for this purpose the 0 dB reference base was arbitrarily chosen to be 0.000,000,000,001 watt, or written in exponent form: 1 × 10 -12 watt. When comparing the sound pressure of two sound waves, their dB difference is, by definition, 20 times the logarithm (to the base 10) of their ratio. Because this wide range of sound pressure is unhandy to work with, the dB scale, instead of being linear, was set up in logarithms to compress it into a more compact scale where not more than 3 digits are required to express pressure levels over the entire audible range. All other sounds are louder than this, the loudest being over 10,000,000 times the PSI intensity of the 0 dB base. It is more conveniently written in exponent form as 3 × 10 -9 PSI. It is more pressure has been assigned a value of 0 dB. This pressure has been assigned a value of 0 dB. It is also the least difference between two sound waves that can normally be detected. The standard reference base selected for the intensity (pressure) of a sound wave is 0.000,000,003 PSI because this is the least sound wave intensity that can be detected by the average human ear. To state that a certain pump has an 80 dB noise level means that the ratio between its sound level and that of the selected zero base (0 dB) is 80 on the dB scale. Whether or not so stated, a dB rating is always a ratio either between two sounds, or between one sound and a fixed reference base. They could be used in mechanical work for comparing two levels of force, torque, work, HP, etc. Decibels could be used in illumination for comparing power level of two sources of light, or the illumination intensity at distances from the light source. It can be, and is, used in other technologies such as electronics for comparing voltage, current, or power levels. Instead of a linear scale, dB ratings were placed on a logarithmic scale to compress the upper end into a more usable and practical range.Ī dB has no assigned value it does not represent a definite quantity of anything it is simply a ratio used to compare the relative intensities of two sound waves, or the relative power levels of two sound sources. The two scales are different but were carefully defined so their relationship to each other would be such that a change of (so many) dB on the power level at the source would result in the same dB change in pressure reading at any distance from the source. The dB scales for expressing and comparing sound power and sound pressure were arbitrarily selected so as to be convenient to use. This is a different scale than the dB power scale on which acoustical wattage is rated. Although two sound waves could be compared by their PSI as read on a sound level meter, it is more convenient to compare them on the dB pressure scale. Discomfort or damage to a listener’s ears is from sound pressure, not from sound power at the source. Sound pressure is the strength of the traveling soundwave, in PSI or other pressure units, at a specified distance from the source of the sound. Two sound sources (pumps) could be compared by their relative acoustical wattage outputs, but can be more conveniently compared by assigning decibel ratings on the dB power scale. Sound power is the acoustical power, sometimes expressed in watts, generated by a source of sound such as a hydraulic pump. The purpose here is to define more clearly how decibels are used to rate noise of hydraulic pumps, and to give examples of calculations. Information in this issue supplements the basic information on noise levels in Design Data Sheet 24. Womack Data Sheet 35: How to Make Noise Calculations with Decibels
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