MEM 634

ERGONOMIC DESIGN

LAB ASSIGNMENT 4

MOHD SUHAIMI BIN STALAHA

2009399813

Date of submission: 2 November 2011

1.0 TITLE

Castle Sound Level Meter

2.0 OBJECTIVES

2.1 To determine the sound level of a source from different distance.

2.2 To measure the internationally standardized characteristics weightings.

2.3 To draw the chart of frequency spectrum.

3.0 THEORY

3.1 The Basic Sound Level Meter

A sound level meter is an instrument designed to respond to sound in approximately the same way as

the human ear and to give objective, reproducible measurements of sound pressure level. There are

many different sound measuring systems available. Although different in detail, each system consists of

a microphone, a processing section and a read-out unit. The microphone converts the sound signal to an

equivalent electrical signal. The most suitable type of microphone for sound level meters is the

condenser microphone, which combines precision with stability and reliability. The electrical signal

produced by the microphone is quite small and so it is amplified by a preamplifier before being

processed. Several different types of processing may be performed on the signal. The signal may pass

through a weighting network. It is relatively simple to build an electronic circuit whose sensitivity varies

with frequency in the same way as the human ear, thus simulating the equal loudness contours. This has

resulted in three different internationally standardized characteristics termed the "A", "B" and "C"

weightings. The last stage of a sound level meter is the read-out unit which displays the sound level in

dB, or some other derived unit such as dB(A) (which means that the measured sound level has been A-

weighted). The signal may also be available at output sockets, in ether AC or DC form, for connection to

external instruments such as level or tape recorders to provide a record and/or for further processing.

3.2 Calibration.

Sound level meters should be calibrated in order to provide precise and accurate results. This is best

done by placing a portable acoustic calibrator, such as a sound level calibrator or a piston-phone,

directly over the microphone. These calibrators provide a precisely defined sound pressure level to

which the sound level meter can be adjusted.

3.3 Detector Response.

Most sounds that need to be measured fluctuate in level. To measure the sound properly we want to be

able to measure these variations as accurately as possible. However, if the sound level fluctuates too

rapidly, analogue displays (such as a moving coil meter) change so randomly that it is impossible to get a

meaningful reading. For this reason, two detector response characteristics were standardized. These are

known as "F" (for Fast) and "S" (for Slow). "F" has a time constant of 125 milliseconds and provides a fast

reacting display response enabling us to follow and measure not too rapidly fluctuating sound levels. "S"

with a time constant of 1 second gives a slower response which helps average-out the display

fluctuations on an analogue meter, which would otherwise be impossible to read using the "F" time

constant. Many modern sound level meters have digital display, which largely overcome the problem of

fluctuating displays, by indicating the maximum RMS value measured within the preceding second.

Selection of the appropriate detector characteristic is then often dictated by the standard upon which

the measurements are to be based.

3.4 The Impulse Sound Level Meter.

If the sound to be measured consists of isolated impulses or contains a high proportion of impact noise,

then the normal "F" and "S" time responses of the simple sound level meter are not sufficiently short to

give a measurement which is representative of the subjective human response. For such measurements,

sound level meters having a standardized "I" (Impulse) characteristic are needed. The "I" characteristic

has a time constant of 35 milliseconds, which is short enough to enable detection and display of

transient noise, in a way which takes into account the human perception of impulsive sounds. Although

the perceived loudness of short duration sound is lower than that of steady continuous sound, the risk

of damage to hearing is not necessarily reduced. For this reason, some sound level meters include a

circuit for measuring the peak value of the sound, independent of its duration. A Hold Circuit is also

incorporated to store either the peak value or the maximum RMS value. Some standards require the

peak value to be measured while others ask for a measurement using the "I" time constant. In either

case the Hold circuit makes reading the measurement easy.

3.5 Energy Parameters.

As sound is a form of energy the hearing damage potential of a given sound environment depends not

only on its level, but also its duration. So to assess the hearing damage potential of a sound

environment, both the sound level and the duration of exposure must be measured and combined to

provide a determination of the energy received. For constant sound levels, this is easy, but if the sound

level varies, the level must be sampled repeatedly over a well-defined sampling period. Based on these

samples, it is then possible to calculate a single value known as the Equivalent Continuous Sound Level

or Leq which has the same energy content and consequently the same hearing damage potential as the

varying sound level. For an A-weighted Leq the symbol LAeq is used. In addition to determining the hearing

damage potential of a sound, Leq measurements are also used for many other types of noise

measurements, for example community noise-annoyance assessments. The formulas for Leq

determination are

where T is the duration of sound exposure time, and ti is the time within which the Li levels occur.

4.0 EQUIPMENT

Sound level meter and calibrator, sound source, measuring tape, markers.

5.0 METHODS

5.1 Do the battery test and performance check of the Castle Sound Level Meter.

5.2 Set the sound power source in the center of the lab and, using the wideband spectrum,

adjust the output to a convenient level.

5.3 Measure the noise level in dB (A) at 1m, 2m and 4m from the center of the source.

5.4 Determine the linear frequency spectrum at 2m from the center of the source.

5.5 Do not adjust the output setting! But do either part a) or part b)

a) relocate the sound source to a more reverberant space such as stair well or foyer

OR

b) relocate the sound source to a less reverberant space such as in the open well away

from reflecting surfaces.

5.6 Measure the noise level in dB (A) at 1m, 2m and 4m from the center of the source.

6.0 RESULTS

6.1 Sound Pressure Level

Distance 1m 2m 4m

Sound Pressure Level,

dB(A) in classroom

Sound Pressure Level,

dB(A) in alternate

space

6.2 Frequency Spectrum at 2m

Octave Band

Frequency, Hz

125 250 500 1000 2000 4000 8000

Measured Sound

Level, dB

A-Weighting

Calculated Sound

Level, dB(A)

Overall A weighted value - _______________________

6.3 Chart of Frequency Spectrum

125 250 500 1K 2K 4K 8K

Frequency, Hz

7.0 CONCLUSION

While the noise levels in most laboratories are below the threshold level that damages hearing, laboratory noise can be fairly loud. The operation of large analyzers (e.g., chemistry analyzer), fume hoods, biosafety cabinets, incubators, cell washers, tissue homogenizers, and stirrer motors, all contribute to the noise level. Other sources of noise in laboratories include fans and compressors for cryostats, refrigerators, refrigerated centrifuges, and freezers. As an example, a high speed refrigerated centrifuge alone can generate noise levels as high as 65 dBA. To provide some further context, a whisper registers approximately 30 dBA; normal conversation about 50 to 60 dBA; a ringing phone 80 dBA; and a power mower 90 dBA. If noise levels exceed 80 dBA, people must speak very loudly to be heard, while at noise levels of 85 dBA, people have to shout to communicate with coworkers who are an arm’s length away. Using a sound level meter, students should monitor the noise levels generated by various pieces of lab equipment to identify equipment that has excessive noise levels. Most manufacturers have set limits on noise-producing equipment (i.e., less than 85 dBA). When equipment exceeds these limits (i.e., > 85 dBA), personal noise measurement, engineering controls, posting of warning signs, and hearing protection options should be evaluated and implemented. The key is to identify lab equipment that is producing excess noise in the work area and implement controls to keep personal full shift noise levels below the OSHA Permissible Exposure Limits (PELs). 8.0 REFFERENCES

8.1 http://www.ehow.com

8.2 http://www.doityourself.com

8.3 http://clas.wayne.edu

8.4 http://www.ohlearning.com

8.5 http://aldebaran.feld.cvut.cz

8.6 http://www.osha.gov