Kettering University Acoustics

	Physics 484: Acoustical Measurements
	Acoustics at Kettering	Dan Russell	Dan Ludwigsen	Acoustics Animations

Week #1: Sound Level Measurements During the first week of PHYS-484, Acoustical Measurements students are introduced to sound level meters. They learn how to calibrate a meter, what the differences between A and C weighting scales mean and how the A-weighted overall level succeeds as a sound quality metric (part 1). They also learn how to measure percentage exceeded levels for community noise (part 2), and they verify that sound levels drop by 6dB every time the distance from an omni-directional source doubles (part 3).
Sound Level Meter Calibration and A- and C-Weighting Sound pressure levels, especially A-weighted levels, are used to set local noise ordinances, and play an important role in defining sound quality for many products. In fact, an overall A-weighted sound pressure level is one of the sound quality metrics used to compare industrial products from automobiles to dishwashers. In this part of the experiment, students learn how to calibrate a sound level meter using a pistonphone (124dB at 250Hz) and an oscillator driven calibrator (114dB at 125, 250, 500, 1000, and 2000 Hz). While they are calibrating the meters they also compare the A and C weighting curves. Then they use software on our NeXT workstations to create two sound signals with different octave band spectra but the same total overall sound level. Finally the students use an octave band frequency analyser to measure the A-weighted octave band spectra for the two signals they created. A final comparison of two very different sound files (nicknamed beauty and beast), which have the same octave band spectra, leads to an understanding of the limitations of the A-weighted sound pressure level as a sound quatlity metric.	More Pictures Roll mouse over thumbnail to enlarge image
Measuring L₁₀, L₅₀, L₉₀ for Traffic Noise One way of characterizing environmental sound levels is to measure what are called "percentage exceeded sound levels." L₉₀ is the sound pressure level that is exceeded 90% of the time, and represents the level of background noise. L₁₀ is the sound pressure level exceeded 10% of the time, and represents the peak noise level. A good Type 1 (expensive) sound level meter will display L₉₀ and L₁₀ automatically. The students in the photos are calculating the levels by hand. Standing at the intersection of two streets outside the academic building, they used a sound level meter to record the A-weighted levels every 5 seconds for 30 minutes.
Sound Pressure Level versus Distance from a Source The further you are away from a sound source, the quieter the sound. For an omnidirectional source (one which radiates sound equally well in all directions) the sound pressure drops by half every time the distance from the source doubles. When measuring sound pressure levels, this means that the sound pressure level decreases by 6dB every time the distance from the source doubles. The sound source for this experiment was a small (4" diameter) boxed loudspeaker through which broadband noise was played. The loudspeaker was placed against a cement wall, which acted as a baffle and increased the level in front of the speaker. The thick grass on the ground in front of the loudspeaker minimized the reflections from the ground. The source was set to produce a 120dB A-weighted sound pressure level immediately in front of the speaker. Then the students measured the sound pressure level every 25 cm out to a distance of 4 meters, then every 50cm as far as they could go before the level was indistinguishable from background noise.

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Copyright Daniel O. Ludwigsen, 2004