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| PHYS-498, Independent Study in Acoustics Winter 2005 | |
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| Mapping the Vector Sound Intensity Field around a Tuning Fork, Justin Junell (AP '05)
A tuning fork provides a very nice example of a simple vibrating system with a well defined transition from a complicated near-field to a more simple far-field sound radiation pattern. Theoretical predictions show that in the near-field sound energy circulates in an interesting manner with sound energy propagating away from the fork in some regions and towards the fork in others. In the far-field the sound energy radiates away from the fork in all directions, but with varying magnitude depending on the angular position around the fork. Justin experimentally mapped out the vector sound intensity using two different intensity probes. One was a traditional p-p probe which uses two phase matched microphones to measure the pressure (average pressure from the two microphones) and an estimate of the particle velocity (difference in pressures divided by their separation distance). The magnitude and direction of the vector intensity were obtained from the cross-spectrum of the two pressure microhone signals. The other probe was a new p-u probe which uses a very small microphone to measure the pressure and a tiny dual hot-wire anemometer to measure the particle velocity directly. The vector intensity is obtained from the cross-spectrum of the pressure and particle velocity signals. The tuning fork was driven by an electromagnet coil and a small magnet attached to one of the fork tines, and the fork rotated on a turntable. Justin measured the radial and angular components of the intensity at a number distances from the fork. The measured vector intensity map agreed very well with the theoretical predictions. |
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| PHYS-484, Acoustical Measurements Fall 2002 | |
| Modeling a Small Room for Acoustical Measurements, Dave Ebaugh (EE '02)
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| PHYS-498, Senior Research Winter 2002 | |
| Acoustical Study of the Djembe (African Hand Drum), Wes Haveman (AP/EE '02)
The djembe is a hollow drum, originating from Africa, which is capable of producing three distinct sounds: bass, tone, and slap. Three djembe drums (two handmade and one factory made) were studied in this experiment in an attempt to determine how the vibrational modes of the shell, the resonances of the cavity, and the vibrational modes of the membrane individually contribute and couple together to produce the three distinct djembe sounds. Vibrational modes of the shell were determined via an experimental modal analysis experiment. Frequencies of the various shell modes are dependent on the tightness cables used to provide tension to the drum. There was no clear relationship between the shell modes and the drum sounds. Membrane modes were determined by first identifying the resonance frequencies from a swept-sine analyasis (driving the membrane with a magnetic coil and a small magnet), and then mapping the modes with a microphone and watching the phase changes on a oscilloscope Lissajous pattern (see figure at top right). Frequencies were found to be dependent on membrane tension, but frequency ratios remained relatively constant and seem to be a characterstic of the drum. Cavity modes were also identified - the most important cavity mode is the "Helmholtz" mode which creates the loud bass sound of the drum. Other higher order cavity modes were found to potentially couple with membrane modes. At this point in time we are still trying to identify which membrane/cavity mode combinations are primarily responsible for the distinct tone and slap sounds. Our analysis was frustrated by the discovery that CD recordings by professional players reveal two separate slap sounds (a fifth and an octave above the tone) whereas most of what is written about the djembe (and our playing ability) suggests one slap sound which is approximately a fourth above the tone. This research is continuing.
Preliminary results were presented: |
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| Binaural head transfer functions for virtual localization, Jason Kolenda (AP '02)
After recordings had been made for each angular position, Matlab was used to convolve the impulse reponse functions for a specific direction with a "dry" monaural signal which had no directional clues at all. When listening to the resulting sound file the sound appeared to come from the specified direction. This research is being pursued further by Kim Pregitzer during the spring 2002 term. An updated summary will be included after her work is completed. |   |
| PHYS-583, Applied Acoustics Laboratory Winter 2001 | |
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| Design of a Compound Speaker Design, Blong Xiong (ME '01) and Shawn Lange (ME/EE '01)
Blong and Shawn designed, constructed, and tested a compound woofer system consisting of two 12-inch woofers in parallel. The goal of this project was to design a subwoofer system with good bass response, but requiring significantly less volume than a conventional single speaker system. They designed the system using LEAP software (Loudspeaker Evaluation And Performance), then built the system according to their design. They constructed one side of the box from plexiglass so that the interior of the cavity volume could be seen. Once the speaker was built they tested its low frequency response and found that it performed very close to expectations, providing good bass response from a boxed speaker with very little volume. |
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| Factors influencing sustain time for an electric guitar, Wes Haveman (AP/EE '02) and Michael Blenman (AP '02)
Wes and Mike read a paper[1] which desribed research on a phenomenon observed in electric guitars in which the string becomes relatively dead (greatly reduced sustain) when fretted at certain positions along the neck. They setup an experiment to reproduce some of the results in this paper and found that there are positions along the neck where the sustain time is significantly reduced. Measurements were made for each fret position for each string. Attempts were made to compare deat spot locations, string frequencies at those fret positions, and the resonance frequencies and vibrational mode shapes of the guitar neck. Some correlations were observed, but further research is needed. [1] Helmut Fleischer and Tilmann Zwicker, "Investigating Dead Spots of Electric Guitars," ACUSTICA - acta acustica, 85, 128-135 (1999) |
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| Modal Analysis of a 15-inch loudspeaker, Matt Dilber (EE '01), Greg Lapplander (EE '01), and Chad Hanna (AP/ME '01)
Experimental modal analysis was performed on a 15-inch diameter woofer speaker in hopes of determining the various modes of vibration. In a multi-speaker system a cross-over network is used to send only frequencies within a certain range to each speaker. The upper limit of a speaker's useful frequency range is determined by two primary factors (i) the frequency at which it begins to become directional and no longer radiates sound equally well in all directions; and (ii) the higher frequency modes of vibration for which the speaker does not move as a solid whole, but neighbing regions on the cone vibrate with opposite phase severely reducing the radiation efficiency. A small (0.5g) accelerometer was attached to the loudspeaker and a force hammer was used to tap the speaker at each of approximately 200 points on its conical surface. A measurement of acceleration/force was obtained for each point and a computer software package (STAR Modal) was used to curve-fit the data. It was hoped that several modes of vibration could be found which could be compared to theoretical expectations. Unfortunately the speaker cone was severely damped so that very few peaks were visible on the measured frequency response functions. Only one clean mode shape (shown at right) was obtained. This mode shape, which is a natural resonance of the speaker cone system at 480 Hz would strongly suggest that this 15-inch woofer would not be a very effective radiator of sound at this frequency. |
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| PHYS-583, Applied Acoustics Laboratory Winter 2000 | |
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| Acoustic analysis of McKinnon Theater, Nate Dau (AP '01), Ya-Jiang Bemmen (AP/ME '00), and Eric Kendall (ME '00)
McKinnon Theater is a 400 seat auditorium on the campus of Kettering University, Flint, MI. The theater is used primarily for student assemblies, special speakers and events, small musical performances, plays, and showing movies. The acoustics of the theater have some serious flaws, including a remarkable flutter echo when the curtain in front of the movie screen on stage is open. For our project, we took the opportunity to make detailed measurements of the flutter echo and of the reverberation time of the theater comparing the results when the curtians were opened and closed. Click here for a more detailed summary with images and sound files. |
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| PHYS-583, Applied Acoustics Laboratory Summer 1999 | |
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| Thermoacoustic Refrigerators: A table-top demonstration, Pontus Weibull (AP '99)
An inexpensive (less than $25) tabletop thermoacoustic refrigerator for demonstration purposes was built from a boxed loudspeaker, acrylic tubing and sheet, a roll of 35 mm film, fishing line, an aluminum plug and two home-made thermocouples. Temperature differences of almost 16oC between the top and bottom bottomof the stack were achieved after running the cooler for four minutes. While nowhere near the efficiency of devices described in the literature, this demonstration model effectively illustrates the behavior of a thermoacoustic refrigerator.
A formal writeup of this project was published in the American Journal of Physics:
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| Predicted and measured on-axis response of a boxed loudspeaker, Trent Nobach (EE '00) and Lance Kornoelje (EE '00) On-axis pressure in front of a JL Audio 12w-6 subwoofer loudspeaker was predicted using The software package LinearX LEAP (Loudspeaker Evaluation And Performance) was used to predict the on-axis pressure in front of a JL Audio 12W-6 subwoofer loudspeaker for two different box configurations. The on-axis response as predicted with LEAP is shown at top right. The actual on-axis pressure was measured by way of a sine-swept measurement using a SR785 FFT analyzer, Alesis RA100 amplifier and an AT4041 microphone. The measured response for both boxes is shown at bottom right. The overall trends for both curves are quite similar for predicted and measured response. The main reasons for the differences which are noticeable are due to the fact that LEAP required estimates of some loudspeaker parameters which we were unable to measure or obtain from the manufacturer. |
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| Baseball bat vibrations - comparing experimental modal analysis and finite element analysis results, Paul Pedersen (AP '01) |  |
| Modal Analysis of a snare drum, Andrea Majcher (EE '99) and Brent Kirkwood (EE '99) We conducted an experimental modal analysis of the batter and snare heads of a snare drum (without snare) using a hammer impact fixed response technique. Two of the mode shapes we found are shown at right. (click on the images for larger versions) The left figure shows the lowest (1,0) mode in which the two heads move together in phase. This mode is often modeled as the lower frequency mode of a lumped parameter system consisting of two masses (the two drum heads) connected by a spring (air). The figure on the right shows a higher frequency mode in which the batter head (upper) and the snare head (lower) are both exhibiting the (1,1) mode, though with different amplitudes. Our experimental mode shapes and frequencies agreed rather well with results published in the literature. |
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| Chladni Patterns on a Square Plate, Amber Koa (EE '00) and Zach Lott (EE '00) |  |
For his course project, Dave attempted to compare theoretical and computational models of standing waves and reflected sound paths with the actual sound behavior in an actual room. Hebuilt a 1/10th scale model of a room from half-inch ply-wood and plexiglass. A 4-inch boxed loudspeaker was used to drive the room in order to find its resonance frequencies, and the standing wave patterns were found by scanning the volume of the room with a microphone. Measured frequencies compared very well with theoretical calculations. Dave used CATT Acoustic room acoustics modelling software to predict the paths of the direct sound and the first 100 early reflections for a sound source centered on one wall and an observer in the middle of the room off to one side. He then placed a small loudspeaker at the center of the front wall, and a small microphone at the position of the observer and measured the time delay between the direct sound and the first early reflection from the closest floor. Placing a square of sound absorbing foam on the floor at the location where the sound reflected, he was able to remove this early reflection.
A Binaural Head Measurement System (Aachen Head) was used to measure the impulse response in 10o intervals in the azimuthal plane. The binaural head had microphones imbedded within each ear and measured sound in three-dimensions in the same way that the human hearing system does. The sound source was a recording of two wood blocks being slapped together. The time difference between the signals arriving at the two ears was used to determine the interaural time difference (ITD) as a function of azimuthal angle, as shown above right. The intensity level difference (ILD) between the two ears was also determined, as shown below right. The ITD and ILD are two of the mechanisms which our brains use to determine the location of a sound source.
