ASA Lay Language Papers
161st Acoustical Society of America Meeting

Advanced Acoustics Video Demonstrations for High School and Higher Education Curriculums: Longitudinal Wave Motion Videos

Robert Astrom; BSEE,
BSS Acoustics of Music, INCE, ASA-TCAA
Astrom Acoustics
8045 Clarks Chapel
Athens, OH 45701

Popular version of paper 5zED3
Presented at the 161st ASA Meeting, Seattle, Washington

Acoustics is one of the many disciplines of physics typically taught in high schools and higher education institutions. Acoustics deals with the study of periodic motion, specifically periodic motion as it pertains to the way waves or vibrations travel in solids and fluids such as liquids and gasses. As with the majority of science based classes, acoustics is most effectively taught by using demonstrations and hands on experiments as opposed to only reading and written homework. The hands on experiments allows the student to engage in active or discovery learning as opposed to the more common empty vessel learning style where the students listen to lectures and are told what to memorize. Some experiments are too costly or dangerous to be a feasible addition to acoustics education curriculums in many institutions due to budget cutbacks and safety considerations. To allow students the opportunity to experience demonstrations such as this the author has begun the production of a series of videos, which are to be distributed at no cost to acoustics education curriculums, thus providing them with a way to show these types of demonstrations and experiments at no cost and without risk of injury.

One of the most difficult concepts encountered in acoustics for both educators to teach and students to understand is the concept of longitudinal wave motion and propagation. There are two types of wave motion in acoustics; longitudinal and transverse. Transverse waves travel from side to side perpendicular to the object physically deforming the material by bending. Longitudinal waves travel forward and backwards within the medium. A good example of transverse wave motion is waves on water. The peaks and troughs move along the top of the water at high and low displacements. This motion is quite easy to see and thus understand. Sound in air travels as a longitudinal wave, which cannot be easily seen. The waves instead of having high and low displacements have areas of compression (pushed together) and rarefaction (pulled apart), which correspond to regions of high and low pressure. In addition almost all diagrams of longitudinal waves given in textbooks or drawn on the board in class are depicted as transverse waves inside a tube, which only exacerbates the students confusion as to the difference between the two types of waves. Demonstrations that clearly illustrate the concept of longitudinal waves would be of great benefit to the education of physics students. For this reason, longitudinal wave motion of sound has been selected as the first group of video demonstrations to be produced.

One example of an extremely dangerous demonstration is a Rubens tube. In essence a Rubens tube is a very large pipe-bomb if designed or used incorrectly. The Rubens tube built for this project consists of a 6 foot long 4 inch diameter metal tube with a speaker on one end, a row of tiny holes drilled across the top and at the far end the tube is capped off and a gas inlet is installed. The tube is filled with propane gas and as the propane escapes through the holes it is lit to produce a row of flames across the top of the tube. Single frequency tones are played through the speaker and when the tone played matches one of the resonance frequencies of the tube a sine wave appears in the flames.

Another demonstration that might be considered too expensive to be feasible for some programs is a piston driven slinky. This experiment can cost a few hundred dollars if purchased. For the video series a piston driven slinky was constructed for a cost of approximately $200. When the slinky is driven at a resonance frequency a standing wave will appear. The standing wave will have nodes and antinodes. The nodes are points on the slinky where there is no motion and the antinodes are location of maximum displacement or motion. One additional benefit of a video demonstration of this experiment is that the video can be slowed down so the student can see the slinky in slow motion.

The final section of the video series deals with room acoustics and how rectangular rooms actually act like tubes that are sealed on both ends. The students are encouraged to calculate the resonant frequencies of their classroom and use a speaker to produce a tone at the frequencies they calculated. This allows them to, in essence, stand inside the tube to hear for themselves what is really happening in the tubes they have been learning about.

The video series includes demonstrations for instructional purposes and test questions including Video Quiz Questions to evaluate how well the students understood the material presented. The videos are intended to be a teaching tool for educators to utilize in the classroom as opposed to a standalone class. This project is being undertaken to advance the understanding of and interest in acoustics. As such the videos will be provided at no charge to any educator wishing to use them.