The students will be able to:
Intensity and Loudness
The intensity of a sound is the power of the sound in Watts divided by the area the sound covers in square meters. The loudness of a sound relates the intensity of any given sound to the intensity at the threshold of hearing. It is measured in decibels (dB). The threshold of human hearing has an intensity of about .0000000000001 watts per meter squared and corresponds to 0 decibels. The threshold of pain for humans is 1 Watt per meter squared and corresponds to 120 dB. A whisper is between 20 and 30 dB, noisy conversation is about 50 dB, a vacuum cleaner is about 70 dB, a lawn mower is about 90 dB and a car horn at 1 m is about 110 dB. Humans are particularly sensitive to sounds ranging in frequency between 2000 and 5000 Hertz. The dBA scale has a filter so the sound level meter is less sensitive to low and high frequency sounds just like human hearing. It is also better adapted to measuring hearing damage and speech interference in humans.
"Noise" is any unwanted sound or extraneous sounds (sound without any function). Noise affects the natural environment. For example, extra sounds cause "auditory masking" which reduces an animal's ability to detect communications and predators. Some birds sing at a higher pitch in noisier areas. Because female birds of certain species prefer males singing at a lower pitch since it denotes maturity, this may lead to reduced bird populations. Other birds may just sing louder in noisy areas. Chinese frogs have even shifted their calls to the ultrasonic range (above the frequencies humans can hear) so they can locate each other during mating season. Gleaning bats (those that pluck insects from leaves) will not hunt in noisy areas. Noise also "stresses" wildlife making them less resistant to disease. Noise definitely affects aquatic environments. Sound travels much faster and farther underwater, which means a noise source can have a much greater radius of impact than it would have on land. Studies have also shown that low frequency noise in the oceans, largely related to increased commercial shipping, has increased by as much as 10 dB since the 1960s.
Sounds can also adversely affect people. Loud or prolonged sounds have been shown to cause hearing impairment, hypertension, sleep disturbance, annoyance, and ischemic heart disease (heart disease linked to reduced blood flow to the heart). Other complications include possible immune system changes and birth defects. Road noise has been shown to constrict arteries and elevate blood pressure. It may even lead to heart attacks. Noise levels of 50 dB at night increase cortisol (a stress hormone) production which causes arteries to constrict and elevates blood pressure.
Effects of Atmospheric Conditions
Atmospheric absorption of sound varies with environmental conditions such as relative humidity, atmospheric pressure, temperature, and wind. Lower humidity absorbs more sound, especially at higher frequencies, because of "molecular relaxation" in the gases in the air (a level of 10% humidity absorbs the most). A substantial change in atmospheric pressure, equivalent to thousands of feet of elevation gain, has a small influence on noise level for most sources, but substantially affects the received levels of those sounds.
More importantly, wind and temperature can significantly affect how sound waves travel. Wind may cause sounds to be louder downwind, since the molecules through which sound waves propagate are being moved downwind, instead of spreading out equally in all three dimensions. Vertical temperature gradients can cause sound to either refract away from the ground (when warm air near the ground is beneath cooler air above) or towards the ground (when cool air near the ground is trapped by a layer of warm air above, i.e., an inversion).
Review sound waves, particularly:
Students analyze sound level data. Use the data provided with this unit plan or other sound data (including where, when and the conditions recorded under, and typical sounds like birdsong). The students can take a sound meter to different areas around the school to collect data. Students should record the time and conditions at which they sampled the data. Find examples of sound recordings and spectrograms at the NPS Natural Sounds and Night Skies website.
Using reliable books, articles, and websites, students research how sounds affect people and the natural environment. They can examine both positive and negative effects of sounds of differing loudness, intensity, and duration. Students also investigate methods by which sound intensity can be reduced.
Students go to the place where they will be examining the effects of sound, such as a nearby park. Students bring sound level meters (preferably capable of measuring dBA) to record sound intensities. Students will listen and record all sounds heard over a 15 minute period. Students will listen and record only intrinsic sounds for 10 minutes (those sounds typical of the park's daily operations), which may be natural and cultural (like sound of a blacksmith's hammer at Herbert Hoover National Historic Site). Students listen and record extrinsic sounds (not typical of the place) such as nearby traffic, for 10 minutes. Record observations about weather conditions and characteristics of the place while recording the data. Discuss which sounds contribute to the park's purpose and which are disruptive or not consistent with visitors' enjoyment of the park. Students may also want to determine which animals are native to the park and determine how the various sounds may affect them.
Use the students' data and research to assess how the sound levels and intensities may be impacting the place they visited. Compare the data to those already collected by others. Consider how the sound levels may affect the natural residents of the park or human visitors. Research more about impacts on the native species. Discuss the impacts orally and write an outline or paper on the probable effects of different sounds on the residents and visitors, human or animal. Could the park experience be enhanced by eliminating or reducing certain sounds? If so, which sounds and how?
Engage in or observe discussions with students, checking for student's understanding of:
Use student data, conclusions, and recommendations to decide whether students can:
Sounds that are natural to a park are considered natural resources. Birdsong, the bubbling of Hoover Creek, and the sounds of a blacksmith at work are sounds typical of Herbert Hoover National Historic Site. These sounds, both natural and cultural, were sounds Herbert Hoover heard as a boy in West Branch, Iowa. Protecting and preserving them is part of the mission of the National Park Service. Visitors to the park may hear these sounds as well as the encroaching noise from the 21st century like traffic on Interstate 80.
Students studying sound wave properties such as intensity and loudness must also see the effects sounds have on our environment and even how we can reduce any unwanted effects. Herbert Hoover National Historic Site and other national parks present the perfect opportunity to take the students' classroom knowledge of sound properties one step further. Students can measure sound properties and then see how they affect the environment and people. Students may even take an active role in preserving and protecting the National Parks by recommending ways to reduce extrinsic noise. Their recommendations should show an understanding of sound properties as well as of the character of the park.
Students may research how: