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Measuring Sound
Sound is measured on the decibel scale. The sound is measured by sound meters that are designed to respond to sound close to the same way that a human ear responds to sound. There are many different types of devices that are used to measure sound. Despite their differences, there are three basic parts that all machines use. Each machine uses a microphone, a processing unit, and a read out or display.
Illustration 2 g01159764
Most sound measurement devices have the ability to measure sound using different weighting networks. The human ear responds differently to different frequencies. To a human, low frequency sounds seem quieter, even if they are the same decibel level as a higher frequency sound. Weighting networks are used to make the sound measuring machine display sound levels that are closer to how they are percieved. The most widely used is the A weighting. The A weighting is thought to most accurately model how the human ear responds to sound. Noise that causes hearing loss most closely correlates to the A weighting. The C weighting correlates well with the sound the human ear encounters when earplugs are worn. Illustration 2 shows how the weighting systems remove, or in some cases, add sound to the base reading from the meter.Most sounds that you will want to measure fluctuate over time. If the sound varies rapidly, the needle on an analog gauge will fluctuate rapidly, making it difficult to get an accurate reading. In order to deal with sounds that fluctuate at different rates, two response times were standardized. You may select "F" for fast, or "S" for slow. The fast position will provide a time constant of 125 milliseconds. This provides a quick responding display that works well for slowly fluctuating sound. Switching to the slow position provides a time constant of 1 second. This will smooth out the reading when the sound fluctuates very rapidly. To measure an isolated impulse of sound, there is an I position. The 35 millisecond time constant is short enough to measure these impulses.When you are taking sound measurements, make sure that your body is not acting as a shield or a deflector of sound. Experiments have shown that at 400 Hz, reflections from a person can cause errors as great as 6 dB if you are measuring less than one meter from your body.How To Reduce Sound
There are two basic guidelines to follow in order to lower the sound levels in a given situation.
The first step is to try to eliminate the sound at the source. This can be done by applying acoustical materials to the surface of a machine, by damping the source of the sound, by reducing the frequency of the sound, or other means.
The second is to try and block the sound transmission path. This can be done by placing an enclosure or acoustic screens around the machine. Be sure to mount the enclosure on vibration isolators in order to prevent transmission of vibration and sound. Noise is