The Physics of Sound

Bennett Heussler ’15
Bennett Heussler ’15

Most people are aware that blowing across the top of a bottle produces a tone, or driving on the highway with an open sunroof yields uncomfortably loud turbulence.  The physics behind daily occurrences similar to these regularly go unnoticed, but not so by Bennett Heussler ’15.  He decided to study what causes these sounds and to reexamine previous experiments related to these observations.

With Peter Millet, the Litchfield Professor of Physics, Heussler is calculating the mechanical impedance, of air rising from a Helmholtz resonator.  In other words, he is measuring how much force the rising air can resist due to its own velocity.  Designed by Herman von Helmholtz in the 19th century, the resonator is like an open bottle.  The original design is a spherical container with a neck that tapers to a small opening. 

As air passes over the opening, it fills the container’s base and collects in the neck.  The formed air mass can be moved up and down from the molecules entering and escaping the bottle, creating audible vibrations. Heussler uses a microphone to find the resonant frequency of differently shaped and sized resonators.  He also shines a laser through the bouncing air mass to see if the movement disrupts light. 

Researchers in California first performed this experiment in 1985.  More sophisticated technologies have developed since then, and Heussler is “reevaluating the experiment with greater accuracy.”  Sensitive recording devices and advanced plotting tools are now used to generate the sound curves.

According to Heussler, this project helped him experience physics in a new way.”  During the academic year, many labs restrict students to a set length of time with a predetermined goal, but “now we’re given the opportunity to explore physics by ourselves and see how it unfolds.” 

Working with sound presents unique challenges.  Since there is ambient noise in his Taylor Science Center research lab, Heussler used a locked-in amplifier. The device cancels out all unwanted sounds and only detects a predetermined frequency.  It is programmed to recognize a single wavelength, or their expected frequency in the experiment, while ignoring unrelated background noise.

Rather than generating sounds, Helmholtz resonators can be tuned to ambient noises to absorb, disruptive sounds. Researchers are also beginning to study how the resonator, when pushed to higher frequencies, could absorb sounds created by other structures like bridges.  The structural integrity of an oscillating system, like a bridge, can be reduced when its resonant frequency is changed or distorted. 

Heussler found that studying the effects of nonlinear domains on Helmholtz resonators can be done more easily than he previously thought.  Nonlinear domains develop when objects are pushed past their designed limits.  Depending on the material, a rod may not return to its original shape after being bent.  Similarly, Millet and Heussler think resonators can become distorted after being pushed to a nonlinear domain. Heussler had only five weeks to study the resonators, but he anticipates performing further research in the fall.

Heussler is a graduate of the Severn School (Md.).

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