National Society of Black Physicists

NSBP/ASA Classroom Acoustics Student Poster Contest

The National Society of Black Physicists and the Acoustical Society of America are co-sponsoring a student poster contest for the 2009 NSBP/NSHP conference.  In addition to the $500 prize, winners will receive student support to attend the 2009 ASA conference in Portland next November.

Posters entered for these awards can be in any area of acoustics. However, NSBP has developed a simple protocol for making background noise, signal-to-noise ratio, and reverberation time in a classroom. All students, working individually or in a group, are encouraged to make these measurements and present their results as participation in this contest.

Students entering a poster for this contest should submit a normal abstract. The poster session will be on Saturday, February 14, 2009. 

The contest judges will be looking to see if the students can converse with the judges and show insight into the physics of the processes, if the students did careful work (calibration, experimental methodology and analysis), if they completed extra work above and beyond the minimum, i.e., did they do the measurements in libraries, dorm rooms, local K-12 schools, with and without absorbing materials in the rooms, as well as the clarity and artfulness of the poster itself.

General Background on Classroom Acoustics
Acoustics is a branch of physics concerned with the study of sound (mechanical waves in gases, liquids, and solids). It is a rich field with multiple branches including aeroacoustics, architectural acoustics, bioacoustics, medical acoustics, musical acoustics, physical acoustics, psychoacoustics, seismology, underwater acoustics and several others (__________, 2007). Because mechanical waves are so ubiquitous, acoustics is by nature an inter-disciplinary field, drawing people from widely differing backgrounds. But any scientist that studies acoustics is called an acoustician.

A full acoustics course is not commonly offered in the usual undergraduate physics curriculum. Acoustics is covered in most general physics textbooks, but often the topic is presented at the end of the semester of the introductory mechanics course, i.e., general physics I, and not picked up again explicitly elsewhere in the standard physics curriculum (Rossing, 2002). Yet there are many jobs and long term career prospects for physicists in acoustics (Hansen, 2004). In a recent article, Busch-Vishniac and West highlighted some of the rich applications of acoustics, and how to attract more students to the field. (Busch-Vishniac and West, 2007).

One critical application of acoustics is in classroom design (Seep et al., 2000). Much research in the fields of education, psychology, audiology, and engineering has been applied to classroom acoustics (Hodgson, 1988; Serra and Biassoni, 1998; Hodgson et al., 1999; Bistafa and Bradley, 2000; Haines et al., 2001; Hodgson and Nosal, 2002; Lundquist et al., 2003; Shield and Dockrell, 2003). The key physical metrics in classroom acoustics, background noise, signal to noise ratio, and reverberation time can affect speaker intelligibility, mood, student concentration and overall learning (Abimbade, 1999; Hodgson, 1999; Hodgson et al., 1999; Polich and Segovia, 1999; Bradley and Lang, 2000; Crandell and Smaldino, 2000; Nelson, 2000; Nelson and Soli, 2000; Siebein et al., 2000; Hodgson, 2002; Hodgson and Nosal, 2002; Nelson et al., 2002; Godfrey, 2003; Shield and Dockrell, 2003; Skarlatos and Manatakis, 2003; Dockrell and Shield, 2004; Dockrell et al., 2004; Hodgson, 2004; Siebein, 2004; Beaman, 2005; Choi and McPherson, 2005; Dreossi and Momensohn-Santos, 2005; Sato et al., 2005; Kennedy et al., 2006; Dockrell and Shield, 2006). As a result, in many jurisdictions there are standards for the maximum background sound level, minimum signal to noise ratio, and maximum reverberation times in classrooms (Lubman, 1997; Malakootian, 2001; Sutherland and Lubman, 2004; Evans, 2005; Kobayashi et al., 2007).

While requiring some specific training to fully understand all the physical concepts, these metrics can easily be measured by any undergraduate physics major (Rossing, 1986; Hansen and Rossing, 1999; Hodgson et al., 1999; Rossing and Hansen, 2001; Knecht et al., 2002; Crandell et al., 2004b; Hansen, 2006). Indeed Busch-Vishniac and West mention how a high school student working at Johns Hopkins surveyed a number of classrooms to see if they met the standard of 35 dB(A) for background noise. This simple study apparently had a large impact on the campus. Apparently there tended to be a large spike in energy in the 16 kHz octave band . After some work with the facilities managers, they discovered that the motion detectors were emitting noise in that band. Although the instructors and facilities managers could not hear the high-pitched tone, owing perhaps to changes in hearing with age, many of the students could, and found it irritating. As a result of this work, the construction contracts at Johns Hopkins have been changed to include acoustical requirements.

The simplicity of these measurements, the social relevance of the science, and our desire to have students explore all areas of physics have encouraged the National Society of Black Physicists and the Acoustical Society of America to co-sponsor a student poster contest for the 2008 NSBP/NSHP conference. Students, working both alone or in groups, can conduct a study and present their results in a poster at the conference. Posters will be judged on the students’ ability to converse with the judges and show insight into the physics of the problem, indications of careful work (calibration, experimental methodology and analysis), and if they put in extra work above and beyond measurements in a single location, i.e., did they do the measurements in libraries, dorm rooms, local K-12 schools, with and without various absorbers in the rooms, etc.

Acoustics Background
Here we will only highlight some of the key concepts for this exercise. For a quick introduction to acoustics refer to any general physics textbook and either of the commonly used acoustics textbooks (Giancoli and Gahala, 2000; Kinser et al., 2000; Rossing, 2001; Long et al., 2005; Raichel, 2006; Egan, 2007).

Sound is what we call our perception of waves emanating from some mechanical vibration. Being a wave, sound can be characterized by its amplitude, frequency and speed. Mechanical vibrations, thus sound sources, are ubiquitous, and exist in all frequency ranges. In any given space and at any given time, the total sound field will contain waves at any number of frequencies and at various intensities. Some of the waves will be detected by human ears, while different sensors will be needed to detect other waves.

The loudness of sound is related to the amplitude of the wave, which is also related to the energy of the wave. Loudness is also related to intensity, a measurable quantity, and also to sound pressure level (SPL), also a measurable quantity. These quantities are stated in units of decibels (dB). If the intensity of sound at some location r, at some time t, and at some frequency f is I, and some reference intensity is Io, the decibel level is defined as . If loudness is measured in terms of sound pressure level, the expression for decibel is , where , is the medium density, and is the speed of sound.

In the normal classroom environment the sound field would consist of the main speaker plus any sounds that may exist in the background. Sound that is normally not intended to be present, not needed, or somehow is not normally monitored is called noise. Noise in the classroom can degrade the quality of the educational experience. Examples of background noises include environmental sounds such as wind and traffic noise, cell phone ringers, alarms and beepers, people talking, various bioacoustical noises, and mechanical noise from devices such as conditioning, fans and blowers, power supplies and motors.

Until very recently classroom acoustics standards in the United States had not yet been formalized (Lubman, 1997). In 1995 the American Speech-Language-Hearing Association recommended that unoccupied classrooms noise levels, averaged over all frequencies in the human audibility range (20 – 20,000 Hz), not exceed 30 dB. In 2002 American National Standards Institute adopted 35 dB as the maximum background noise (Sutherland and Lubman, 2004). In Sweden the standard is also 35 dB (Evans, 2005).

The standard also requires that the signal to noise ratio in a classroom be +15 dB. The signal level is of course related to the room’s amplification scheme, if any. But classrooms that maintain a background noise level below 35 dB will allow speakers’ voices to reach all listeners at the desired +15 dB signal to noise ratio (Nelson et al., 2002).

Acoustic reverberation is the persistence of sound energy in an enclosed space due to reflections from space boundaries and other surfaces. When sound is produced in a space with reflecting surfaces, a large number of echoes build up and then slowly decay as the sound is also absorbed by the walls, air and other contents in the room. Listening to echoes is a very well known phenomenon even in outdoor areas; all that is needed is some kind of sound reflection surface. But in an enclosed space, especially a classroom, reverberation can seriously diminish the intelligibility of intended communication.

Reverberation time is a measure of the sound energy decay. It depends upon sound absorption at the boundaries and interior surfaces and properties of the air in between, i.e., temperature. Long reverberation times indicate strong echoes. In a classroom with long reverberation times students will perceive words as overlapping, and words like "cat", "cab", and "cap" may all sound very similar. If on the other hand the reverberation time is too short, tonal balance and loudness may suffer (Wikipedia contributors, 2007).

In the late 19th century Sabine developed an empirical relationship between reverberation time in a room, its size and the nature of the absorption. He determined that the reverberation time T is directly proportional to the room volume V and indirectly proportional to the absorption surface area A, i.e., . Sabine assumed that acoustic waves propagate along a ray from a point source, and are both absorbed and reflected by surfaces. In a predominately reflecting space, after a large number of reflections, the volume will be filled with sound energy such that the energy density is the same in all directions.

Solving the differential equation for energy density as a function of time leads to the familiar exponential decay formula, where, , is a characteristic time that depends on the volume of the room, the absorptivity of the boundaries, and the speed of sound c, . Converting the sound energy density to sound pressure level (SPL), and taking the log of this equation results in an equation relating decibels as a linear function of time. Defining the reverberation time as the time required for the level to decay by 60 dB below that of the source results in the Sabine reverberation formula.
(metric) or (English units).
Experimentally reverberation time can be measured by introducing some impulse noise and recording the sound level. From there the data can be analyzed to against this simple model.

Alternatives to this simplified model include more complex situations of non-rectilinear shaped spaces, the existence of standing waves, and the spatial distribution of absorptivities (António et al., 2002; Kostek and Neubauer, 2002; Stauskis, 2003; Nosal et al., 2004; Wang et al., 2005).

Key things to note are that reverberation time may be frequency dependent, i.e., sound at one frequency may be easily absorbed while others more reflected. Additionally the geometry of the enclosure may allow for standing waves.

See the full version of this paper for a protocol for making background noise, signal-to-noise, and reverberation measurements.


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