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Some of topics included in this course are: Fundamentals of Acoustics, Levels and Decibels, Divergence and Directivity, Hearing, Human Response to Noise, Frequency Analysis, Sound Sources and Fields, Room Acoustics, Sound Power, Noise Barriers, Outdoor Sound Propagation, Helmholtz Resonator and Vibration Control. Key points of this lab manual are: Outdoor Noise Measurements, Hemispherical Divergence, Outdoor Noise Measurements and Hemispherical Divergence, Sound Levels Outdoors, Inverse Square
Typology: Study notes
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Objectives:
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.
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.
Lab 1 – Hemispherical Divergence – Data Sheet
Date: Time: Team Members:
Description of test:
Weather conditions:
Equipment used (include serial numbers):
SLM settings: Meter Range: Response: Fast Slow Weighting: A C Lin
Sketch of test layout:
Measured Divergence Data: Octave Center Frequency - Hz Range (ft)
Overall dBA
Overall dBLin
Homework Problem: (hand in separately)
Find the SPL in dB of a square wave by adding its spectral components. Is there another way to calculate the total SPL of this signal for verification? The fundamental frequency of the square wave is 60 Hz and its amplitude is 1 Pa.
Pressure (Pa) time (seconds)
Hint: the Fourier coefficients of a square wave are:
0 forn 2,4, 6 ...
forn 1,3,5,....
( ) sin 1
∞
=
n
n
n
n
a
n
a
P t a n t