It is all about pitch: An introductory design project in acoustics for 1st year undergraduate engineering students.

This acoustics project evolved responding to a challenge to increase the role of analysis in design for 1st year students early in their engineering careers. Student teams must design three musical instruments each producing a single note meeting frequency and sound level specifications using different physical sound generation processes. They are given background material, example analyses, references, and resources. A key requirement is that they create a spreadsheet with equations guiding the design of each instrument before proceeding with construction. Students experience the entire design process: brainstorm, analyze, create, build, test, iterate, present, demonstrate, and report. This introduces the range of resources available to them. An emphasis is on comparing theory and experiment and explaining reasons for any disagreements. As implemented, this project concentrated over about a two week period, provided an introduction to a major design project continuing for a full semester.

Waterfall low-frequency vibrations and infrasound: implications for avian migration and hazard detection.

Many researchers have suggested that birds may use natural infrasound sources for navigation and hazard avoidance. However, there is a need to define the sound levels and frequencies to characterize potential infrasound sources. This paper summarizes new measurements from Niagara Falls which define a stable, powerful infrasound source that could be detected by birds on a regional scale of over 400 km. Measurements made in the vicinity of Niagara Falls show that exceptional infrasonic pressure levels can occur in the regions of large waterfalls (> 100 Pa at a range of about 500 m). This paper reviews investigator assessments of avian use of infrasound. A review of the results of Cornell researchers on pigeon hearing provides a basis for estimating avian detection ranges of waterfalls. It is possible that migrating birds use sounds from waterfalls as beacons- a component of their "navigation toolbox" as well as infrasound for hazard avoidance.

Infrasonic ray tracing applied to small-scale atmospheric structures: thermal plumes and updrafts/downdrafts.

A ray-tracing program is used to estimate the refraction of infrasound by the vertical structure of the atmosphere in thermal plumes, showing only weak effects, as well as in updrafts and downdrafts, which can act as vertical wave guides. Thermal plumes are ubiquitous features of the daytime atmospheric boundary layer. The effects of thermal plumes on lower frequency sound propagation are minor with the exception of major events, such as volcanoes, forest fires, or industrial explosions where quite strong temperature gradients are involved. On the other hand, when strong, organized vertical flows occur (e.g., in mature thunderstorms and microbursts), there are significant effects. For example, a downdraft surrounded by an updraft focuses sound as it travels upward, and defocuses sound as it travels downward. Such propagation asymmetry may help explain observations that balloonists can hear people on the ground; but conversely, people on the ground cannot hear balloonists aloft. These results are pertinent for those making surface measurements from acoustic sources aloft, as well as for measurements of surface sound sources using elevated receivers.

Infrasonic ray tracing applied to mesoscale atmospheric structures: refraction by hurricanes.

A ray-tracing program is used to estimate the refraction of infrasound by the temperature structure of the atmosphere and by hurricanes represented by a Rankine-combined vortex wind plus a temperature perturbation. Refraction by the hurricane winds is significant, giving rise to regions of focusing, defocusing, and virtual sources. The refraction of infrasound by the temperature anomaly associated with a hurricane is small, probably no larger than that from uncertainties in the wind field. The results are pertinent to interpreting ocean wave generated infrasound in the vicinities of tropical cyclones.