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Measure sound speaks to how to measure noise and how is sound measured.
This is a very complicated subject. We will reduce it to it's basic terms, to make for easy understanding.
There is constant sound at some level around us at all times. Most annoying sound emanates from mechanical machinery and not from natural sources.
Noise can be a very real and annoying thing as we actually feel the noise waves. There are many different sources, even computer fans. This can get aggravating if your CPU is up at ear level. Noise cancellation is a full time business for sound experts.
To measure sound, speaks of level of sound in decibels. The level measured, when doing sound system measurement is an indication of how loud a sound is. The sound we want to measure is that sound which is caused by the building mechanical systems.
A background sound level check can be done with the mechanical systems off. Another set of readings can be done with the mechanical equipment on. This differential noise measurement provides for a base sound level reading.
To measure sound is done using a sound meter in an All Pass mode where all the discernible frequencies are included. Sound may also be measured in the Calibrated Band Pass mode, where the decibel level is measured at nine different frequency ranges, 31.5 Hz, 63 Hz, 125 Hz, 250 Hz, 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz and 8000 Hz.
This is the normal discernible hearing range for most people.
Noise levels or sound levels are also expressed in different ways but the most common and most used method is the noise criteria curve. This curve is based on how the human ear reacts to a sound level and it's corresponding frequency.
To get a little technical for a moment, the ear reacts to sound pressure, therefore the NC curve is a varying pressure curve, where a decibel of sound measurement is referenced to a pressure level of .0002 microbar. A microbar is approximately 1 millionth of the normal atmospheric curve.
The NC curve chart is made of several sound pressures ranging from NC 10 to NC 70. The decibel level for each frequency, 31.5 Hz, 63 Hz, 125 Hz, 250 Hz, 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz and 8000 Hz, is plotted on the noise criteria curve. The graph curve closest to the parallel plotted curve indicates the noise criteria level.
The measurement of sound power can be stated in decibel levels or in noise criteria.
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