Photo Gallery of PHYS-484, Acoustical Measurements

This is a completely hands-on laboratory course covering many aspects of instrumentation and measurement taking skills that are common in the field of Acoustics. During the first six weeks of the course students meet twice a week to perform a number of required laboratory exercises which introduce them to the experimental tools and measurement techniques commonly used in industry, as well as some exercises which illustrate concepts previously learned in classroom courses. During the last four weeks of the course students choose a research project for which they design the experiment and collect and analyze the data. A number of previous student projects are also available.

Sound Level Meter Calibration with 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 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.

Measuring L₁₀, L₅₀, and 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, to compare with the answer provided by a Type 1 sound level meter. 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.

Familiarization with a Frequency Analyzer

Directivity Patterns for Sound Sources

The figure at right shows some of the directivity patterns the students measured. Top Center plot shows the pattern for a single 4-inch boxed loudspeaker at 250 Hz (blue curve that looks like a big circle) showing that at this low frequency the speaker radiates equally well in all directions. The red and black curves in the top center plot are for the same speaker at 7500 Hz and 15,000 Hz, showing that at higher frequencies the speaker becomes very directional. The bottom left plot shows the directivity pattern when four identical speakers are arranged in a dipole configuration. The bottom right plot shows the directivity pattern when four identical speakers are arranged in a lateral quadrupole configuration.