Empty bottles create interesting tones. Why? Place one to your lips and blow. Hear that resonance? Not music, but an interesting quality nonetheless.

The sound of a beefed-up car engine—a 1969 GTO or 2008 Mustang GT—is music to some people. May not be melodic in a musical sense, but try convincing a gearhead that their professionally tuned 302 cubic inch, V-8 chromed engine doesn’t sound like a symphony and you may receive a sparkplug to the side of your head. But what these gearheads don’t like is unwanted whines or tones in their carefully tuned sound.

Oddly enough, bottles, some musical instruments and cars utilize the same device to control sound frequencies and improve performance.

Dr. Dan Ludwigsen, associate professor of Physics, and several Kettering students are currently examining the acoustics of Helmholtz resonators, which act as sound filters and amplifiers. The primary focus of Ludwigsen’s research efforts deals with standard estimates of the resonance frequency of Helmholtz resonators, which are based on the geometry of the cavity and neck.

For example, a classic example of a resonator is a soda bottle. The air in the bottle neck oscillates as a unit of mass and compresses the air in the cavity, which behaves like a spring. As a result, if one knows the geometry of the bottle and its neck, predicting the frequency at which the “mass” oscillates on the “spring” is fairly easy.

Ludwigsen is specifically looking for an improved correction factor that takes into account any sound that exists beyond the opening in the intake duct, or in the case of the bottle, the neck. Ultimately, what he hopes to achieve is a correction factor for the cavity that’s related to the springiness of air in the neck. The aim is to predict more accurately the frequencies filtered or amplified by a resonator.

To reach this goal, he engages students in several research activities that take place in the Acoustics Laboratory on the second floor of the Academic Building.

For example, senior Pierre Phou worked on a research project in late 2006 that required him to construct a Helmholtz resonator that would measure the ratio of pressure contained inside the cavity and outside of it. Phou constructed his device with a moveable piston that changes volume and with alternate neck lengths, both of which alter resonance frequency. He then placed small microphones inside the resonator to measure the ratio of pressure inside and outside the device, and used a subwoofer to drive the device.

When Phou headed back to his co-op employer—Valeo Climate Controls in Auburn Hills, Mich.—Sophomore Linda Hunt of Shelby Township, Mich., later picked up the project and began examining this question: why do changes in the volume of the Helmholtz resonator affect frequency?

But do this, Ludwigsen said, one must include a correction factor related to the open ends of the neck. Hunt’s work on the project revealed the possibility of an improved correction factor that takes into account the sound wave driving the opening (in the intake duct, for example). The jury is still out in terms of the results of her work. According to Ludwigsen, more refinement is necessary, but they are finding promising results from tests in the lab.

Although the project is rather complex, Hunt finds the research challenging and a compelling compliment to her academic experience.

“I started this project with Dr. Ludwigsen during my on-campus work term in the winter of 2008 because it is an area of interest of his and I was eager to get a feel for what research is all about,” Hunt said.  “He provided me with a lot of tools and started me with the basics, but I started asking questions that we did not have answers for, so we started researching them.  The plan is to write describe our work in a series of papers and perhaps submit them to the Acoustical Society's conference scheduled for November.  I have really enjoyed the project because it is something a lot different then I have ever done and there is never a dull day.  I have collected the same set of data four or five times and made improvements to my setup each time to produce more accurate results.  In collecting and analyzing data this way, we’re able to figure out if there are any issues with the data and we can then go back and correct what we did wrong. I like being able to drive it how we want or we decide,” she added.  

Perhaps an easier way for a reader to understand this complex project is to view it in terms of musical instruments, which provide more examples of resonator applications.

The djembe is a skin-covered hand drum in the shape of a large goblet that is played with bare hands. According to several sources, the name of this African drum translates to “everyone gather together,” which also defines the purpose of the instrument. The drum shell is covered by a drumhead composed of rawhide, which is also secured to the instrument with metal rings, rope and skin. The djembe originated in West Africa and is a critical part of the regions musical tradition and culture.

“The instrument uses the Helmholtz resonance as part of its playing style and tone,” Ludwigsen said. He demonstrated the various tones produced by the drum in the Acoustics Laboratory. Specifically, the drum uses a rounded shape with an extended tube for the body which forms the Helmholtz resonator.

The notes produced by the drum are called the bass, tone and slap. As one can imaging, the slap has a high, sharp sound and the tone is more rounded and full. The bass is the lowest tone and players manipulate these sounds by positioning the drum between their thighs and angling the instrument when striking the skin.

In addition, the air induction system of a car’s engine often adds to the noise level, according to the Society of Automotive Engineers International (www.sae.org). This is an issue since vehicle noise quality is one consideration a buyer might examine when choosing a car. Helmholtz resonators are often used to reduce noise in vehicle induction and exhaust systems. They’re especially helpful in the elimination of particular frequencies that cause annoying tones or whines.

Ludwigsen noted that several other students, some of whom have recently graduated, provided an extensive amount of help to this project. These people include Wes Haveman ‘02 and Chaz Ott ’07. These students specifically worked on the djembe with Dr. Dan Russell, associate professor of Applied Physics.

The majority of work for this project takes place in Kettering’s Applied Physics Acoustics Laboratory, which the University recently renovated to create more space. Ludwigsen also has a website that explains the theory and helps visualize what is happening with Helmholtz resonators and other acoustical phenomena that readers can access. The site is http://www.kettering.edu/~dludwigs/researchport/other.html.  For more information on this project, contact Dr. Dan Ludwigsen at (810) 762-7488 or via email at dludwigs@kettering.edu.

Written by Gary J. Erwin
810-762-9538
gerwin@kettering.edu