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acoustics student can judge their classroom. This work, with its direct relevance to student learning, is an extraordinarily good application to bring into the acoustics class, because it can be taught at so many different levels. At the simplest level, it is possible to teach students how to use a sound level meter and to have them survey a classroom to see if it meets the background noise level requirement in the standard of less than 35 dBA. A high school student working with me for six weeks was given this as a project and managed to survey the majority of our classrooms. In the course of this work he found that our newest classroom building nearly met the background noise level requirement, but there tended to be a large spike in energy in the 16 kHz octave band. After some work with the facilities managers, we discovered that the motion detectors were emitting noise in that band. Although the instructors and facilities managers could not hear the high-pitched tone, many of the students could and found it irritating. As a result of this work, the construction contracts at Johns Hopkins University have changed to include acousti- cal requirements. At a higher level of sophistication, one could clearly have students measure both background noise and reverberation time and recommend changes to bring the classrooms into compliance with the classroom acoustics standard.
Clearly, there are innumerable other examples of appli- cations and social issues that could be brought into an acoustics classroom. The examples presented above simply define some of the more obvious classes of acoustics prob- lems that could interest students and be used to reinforce concepts being presented in class.
Hands-on experiences
In virtually all studies of how people learn, there is a strong link demonstrated between hands-on experiences and long-term learning. This is the reason that most science and engineering programs are loaded with laboratory experiences. In some scientific fields, it is quite difficult to introduce stu- dents to appropriate laboratory experiences. Astronomy cours- es might suffer from a lack of appropriate telescopes nearby, for instance, and biology classes might not wish to incur the expense of maintaining a vivarium to enable student labs deal- ing with animal anatomy and behavior.
Acoustics is a wonderful field for providing students with hands-on experiences and active demonstrations. Indeed the ASA, under the direction of Uwe Hansen, who served as Chair of the Committee on Education in Acoustics (2000-06) has provided special sessions for high school stu- dents at its biennial conferences as part of its outreach activ- ities. However, lab exercises and active demonstrations have not been standard components in acoustics classes in the last decade, primarily because of the resources required to devel- op them—space, equipment, teaching assistants, and dedicat- ed time of faculty. Admittedly it is difficult to justify the allo- cation of large amounts of precious resources for classes with relatively small attendance.
Given the fiscal realities of most universities then, one might ask what hands-on experiences can be provided in acoustics without great expense. The answer to this question
provides a rich set of opportunities to reinforce the in-class learning that an undergraduate introductory class in acoustics might present.
Professional quality equipment in acoustics is expensive and fragile—not good qualities for student laboratories. However, much less expensive versions of many common elements of an acoustics lab are now available, and these can provide a reasonable hands-on experience for students with- out breaking the bank. For instance, there are many type 2 sound level meters available on the market that provide vari- ous filter networks (such as A-weighting) as a standard and that come complete with interfaces to a computer so that data can be downloaded for later analysis. Further, these sound level meters often can serve as a simple microphone and amplifier, eliminating the need to purchase additional equip- ment. Coupled with audio software on a computer, such as Adobe Audition, the sound level meter thus provides signifi- cant laboratory capability. The software, while listed for a cost comparable to inexpensive type 2 sound level meters, is usually available for an educational discount that renders it nearly free.
In addition to the computer and sound level meter, it is now possible to purchase very inexpensive MP3 play- ers/recorders with built in electret microphones and digital interfaces, and inexpensive headphones of reasonable quality. These devices, added to the mix, complete the poor man’s acoustics laboratory. The computer, software, sound level meter, MP3 player/recorder, and headphones can be purchased for a total of less than $1000 with educational discounts, and the space they occupy is hardly more than that needed for a desktop computer. The sound level meter and MP3 play- er/recorder are totally portable and powered by batteries.
There are a number of lab experiences possible that use the computer, software, sound level meter, MP3 play- er/recorder, and headphones. Many of these measurements can be conducted by students working on their own, so that teaching assistance support and faculty-dedicated time are minimized. For instance, students can find the quietest and noisiest places on campus simply by using the sound level meter. They can also consider the spectra and intensity of var- ious noise sources, or estimate the speech intelligibility index (SII) of a space from the background noise. Students can measure reverberation time by making a noise (popping the standard red balloon if available) while recording the sound on the MP3 player/recorder and then using the computer audio software to analyze the decay as a function of time. This method is reminiscent of the one many of us employed in the past to determine reverberation time using a microphone and oscilloscope before the advent of modern equipment to auto- mate the process. By introducing various materials into the room, students can also find the absorption coefficients of materials. Using two sound level meters, students can record sounds that are combined in the software to be roughly equiv- alent to a binaural recording, thus enabling a comparison between monaural and binaural listening conditions as played over headphones. One can also engage the same approach to recording various sounds and asking students to rate them in terms of annoyance.
32 Acoustics Today, April 2007