Sound pressure level meter

Conducting a noise survey Calibrating and using a sound pressure level meter... 

Exposure monitoring is another element of HCP. The sound level and exposure time should be measured. It is very important that sound levels measured are typical of those encountered by the worker. Proper survey techniques include sound-pressure-level (SPL) meters that should be vigorously apphed at monitored workplaces. They measure the smallest pressure changes initiated by the vibrating source and transmitted through the air. 

A weighted sound-pressure level obtained by use of metering characteristics and the weighting A, B, or C as specified in American National Standards Institute (ANSI) Standard S1.4, Specifications for Sound Level Meters. If the frequency weighting employed is not indicated, A-weighting is usually implied. 

Sound pressure refers to a pressure change arriving at some location. The sound pressure level Lp in decibels is the pressure level relative to a value of 2xl0 N/m Sound intensity at some location is the average rate at which sound energy moves through a unit area normal to the direction of propagation. Units for sound intensity are joules per square meter per second (J/m /s). Sound intensity level Li in decibels (dB) is the sound intensity relative to 10 W/m. ... 

The majority of noise control work undertaken by the safety adviser will involve the measurement and possibly the analysis of noise. It is therefore important that the use and the limitations of sound pressure level measurements and sound level meters are understood. Errors in measurement technique or interpretation could lead to costly mistakes or over-specifying in remedial measures. 

For the operation of sound level meters of different types the safety adviser should refer to the manufacturer s instruction book. The prime requirement for any instrument for noise measurement is that it should not be more sophisticated than necessary and it should be easy to use and calibrate. A typical sound level meter for use by the safety adviser should have the facility for measuring dBA and octave band sound pressure levels. 

Figure 10.1 The effect of the linear (L), A and C weighting filters of a sound level meter on actual sound pressure level...

Noise is measured using a sound level meter which reads sound pressure levels in dB(A) and the peak... 

A sound level meter must be calibrated on a regular schedule if it is to provide meaningful data. Most equipment is battery operated the batteries must be fresh and capable of supplying the instrument with sufficient power. A battery check is followed by a field caUbrator check. The cahbrator produces a pure tone at a known sound pressure level, which will allow appropriate adjustments to the meter. 

Referring again to Fig. A-14, the sound intensity level measured by the casing of the ventilation fan was 94 dB(A). It would not normally be possible to measure the output from this fan accurately, using a sound level meter, in this situation because of the relatively high sound pressure level in this area due to other sources. 

The sensor is the element of an instrument directly influenced by the measured quantity. In temperature measurement the thermal mass (capacity) of the sensor usually determines the meter s dynamics. The same applies to thermal anemometers. In IR analyzers used for concentration measurement, the volume of the flow cell and the sample flow rate are the critical factors. Some instruments, like sound-level meters, respond very fast, and follow the pressure changes up to several kHz. 

Blast pressure gages, heat flux transducers, and sound level meters are to be located at the probable head location of the operator and at representative positions where transient personnel may be located. 

There are many sound level meters on the market, but all work in a similar manner. The basic hand-held set Figure 20.6) consists of a microphone, an amplifier with a weighting network and a read-out device in the form of a meter or digital presentation. The microphone converts the fluctuating sound pressure into a voltage which is amplified and weighted (A, B or... 

Several characteristic parameters and measurements apply to sound and noise. A source s sound intensity, measured in joules per square meter per second or watts per square meter, is the average rate at which sound energy is transmitted through a unit area that is normal to the direction the sound is traveling. Sound intensity is commonly expressed by its level, usually in terms of 10" watts/m. Sound pressure refers to the root-mean-square values of the pressme changes above and below atmospheric pressure, which are used to measure continuous noise. Soimd pressure is measured in Newtons per square meter (N/m ) or pascals (Pa), where 1 N/m = 1 Pa. 

Peak pressure is the highest pressure level reached by the sound wave, and assessments of this will be needed where there is exposure to impact or explosive noise. A meter capable of carrying out the measurement must be specially selected, because of the damping event of needle-based measuring which will consistently produce under-reading. A similar effect can be found in standard electronic circuitry. 

Traditional acoustic instrumentation, such as sound level meters, detects the sound pressure using a single microphone that responds to the pressure fluctuations incident upon the microphone. Since pressure is a scalar quantity, there is no simple and accurate way that such instrumentation can determine the amount of sound energy radiated by a large source unless the source is tested in a specially built room, such as an echoic or reverberation room, or in the open air away from sound reflecting surfaces. This imposes severe limitations on the usefiilness of sound pressure levd measurements taken near large equipment that cannot be moved to special acoustic rooms. 

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