According to the Occupational Safety and Health Administration, 22 million folks—together with almost all energy plant employees—are uncovered to probably damaging noise at work annually within the U.S. Hearing conservation packages attempt to stop listening to loss and equip employees with the information to safeguard themselves. Understanding the fundamentals of sound and noise propagation may assist engineers consider areas and establish strategies to additional cut back publicity.
Sound and noise propagation analysis is a fancy and specialised topic. It is encountered to some extent in all sorts of industrial, industrial, residential, and transit programs. This article supplies some elementary info for readers who’re unfamiliar with sound and noise propagation. It contains among the terminology and fundamental equations utilized in noise propagation research.
Decibels and Sound Levels
The fundamental unit utilized in noise evaluation is decibel (dB), which is a measure of the sound stage (Figure 1). The sound stage may very well be the sound energy stage, sound stress stage, or sound vibration stage.
|1. Sound monitoring surveys. A tripod-mounted noise meter is proven within the foreground of this picture measuring noise from a pump motor. Courtesy: Bechtel Corp.|
The sound energy stage (PWL) is the quantity of vitality generated from a sound supply just like the wattage of a lightweight bulb. The sound energy stage (in dB) is the sound energy in watts relative to the sound energy reference base of 10–12 watts. The decibel is a ratio (considerably just like %), and within the case of sound energy stage is the same as 10 occasions the log of the sound energy stage ratio. The equation, present in chapter eight of the 2017 ASHRAE Handbook: Fundamentals, I-P Edition, is:
PWL (in dB) = 10 log (P1 / Pref)
the place P1 / Pref is the ratio of the sound energy stage and Pref is the reference base of 10–12 watts. The benefit of utilizing decibels is very massive change ratio of P1 / Pref will be lowered to a small quantity as a result of log (P1 / Pref) is small.
Sound stress stage (SPL) is the stress generated by sound waves as they strike the human ear. It is affected by distance from the sound supply. The sound stress stage (in dB) is the same as 10 occasions the log of the sq. of the sound stress ratio. The equation, additionally discovered within the 2017 ASHRAE handbook, is:
SPL (in dB) = 10 log (P1 / Pref)2
SPL (in dB) = 20 log (P1 / Pref)
the place P1 / Pref is the ratio of the sound stress stage with P1 because the efficient root imply sq. (rms) sound stress and Pref is the reference stress of 20 micropascals (20 µPa). This reference is used as a result of it’s the lowest sound stress which will be heard by a younger, wholesome human ear within the most listening to sensitivity of about 1,000 Hz.
Addition of Decibels
Decibels are logarithmic portions. Therefore, they should be added logarithmically slightly than algebraically. The 2017 ASHRAE handbook supplies the next equation:
dBsum = 10 log (10L1/10 + 10L2/10 + 10L3/10 + … )
the place L1, L2, and L3 are the dB values from a number of sources. For instance, when two sources of 70 dB every are added, the end result can be:
dBsum = 10 log (107 + 107) = 73 dB
Based on the equation above, extra calculations will be carried out to acquire the outcomes proven in Table 1 when summation of two noise sources of various values is taken into account. Similarly, for decibel subtraction, if there are two sources producing 73 dB and if one supply of 70 dB is eliminated, the end result could be:
dBsum = 10 log (107.three – 107) = 70 dB
|Table 1. Summing completely different sound ranges. This desk exhibits the variety of decibels (dB) to be added to the very best sound stage when two completely different sound ranges are mixed. Source: 2017 ASHRAE Handbook: Fundamentals, I-P Edition|
Understanding Noise and Sound
The following are some phrases ceaselessly utilized by sound consultants and transient explanations of what they imply.