ME458 Lab 1 8/17/2000 page 1
ME458 Noise Control
Laboratory Exercise #1
Outdoor Noise Measurements and Hemispherical Divergence
Objectives:
1. Measure sound levels outdoors
2. Verify free field behavior of point source (hemispherical divergence)
3. Sum octave band measurements to determine overall sound levels
Background: In this experiment, you will gain an understanding of how the sound
pressure level generated by a noisy machine varies as one moves away from it. A
spherical or point sound source should exhibit the inverse square law (6 dB decay per
doubling of distance). To behave as an ideal spherical source, a source should be
compact (small in dimension compared to a wavelength), in a free field (no reflecting
surfaces other than the ground plane) and the ground should be a perfect reflector (no
attenuation of sound energy). While these criteria are seldom perfectly met, most
sources will "look like" a point source if you are far enough away (> one wavelength, or
twice the largest dimension of the source).
Procedure:
Find two noise sources around campus that approximate these conditions and measure
how the SPL varies with distance. Set the SLM for "all pass" (no weighting filter), slow
averaging. Calibrate your SLM with the pistonphone before and after taking all
measurements. Record your data and observations on the attached data sheet.
•=Calibrate your microphone
•=Sketch the position of the machine relative to other obstacles around it.
•=Record the background noise level (if it is possible to turn off the machine)
•=Measure the SPL versus distance from the machine (in individual octave bands and
overall level). A logarithmic distance scale is recommended (such as 1,2,4,8,16).
•=Plot the SPL vs. distance for the overall level and at least three octave bands (63, 500,
4000 Hz for example). I recommend a spreadsheet program, like Excel for doing the
plots. Use a logarthmic scale for range.
•=At one (at least) measurement point, verify that the individual octave band levels
add up to the overall measured sound level
•=Comment on your results - if you did not observe a -6 dB/doubling behavior,
explain why.
Suggested noise sources - a car horn, boom box tuned to static, tape player playing a
pure tone, lawn mower, chain saw, portable power generator, etc.
