Sound Intensity and Loudness
OverviewOver five days, students learn about the properties of sound waves, particularly sound intensity and loudness. They learn how to measure intensity and loudness, the difference between sound and noise, and when a sound is considered noise pollution. Using data like intensity readings and spectrograms, and simple techniques like listening to and identifying sounds, students research how sounds affect people and the environment.
The students will be able to:
- Understand the relationship between sound intensity and loudness.
- Measure sound level readings in dB and dBA and convert it to intensity as well as understand how these levels are perceived by humans.
- Research how sound affects the natural environment as well as people.
- Distinguish between sound and noise.
- Analyze sound data to show the effects it has on the environment and human visitors.
- Make recommendations about sounds in a particular place by using sound level data and research about the effects of sound.
- Predict the effect of humidity, temperature and wind on sounds.
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).
- Sound level meters that read in dB and dBA
- NPS Natural Sounds and Night Skies website
- Holt Physics, textbook by Serway and Faughn published in 2002 by Holt, Rinehart, and Winston.
- Use of the internet for research.
- Sample data collected at Herbert Hoover NHS in 2012
Review sound waves, particularly:
- Sound travels through a material as a mechanical wave. The wave is a longitudinal, or compressional, wave.
- Sound occurs when energy causes air particles to move closer together and further apart. The closer the particles get or the further apart they get, the greater the sound's amplitude. Sound amplitude causes a sound's loudness and intensity. The bigger the amplitude is, the louder and more intense the sound. Sound intensity is measured in Watts per meters squared.
- Other sound wave properties include the frequency in Hertz (how many waves per second), and wavelength (literally the length of one wave, from compression to compression).
- Humans can only hear sounds between 20 and 20,000 Hertz. Animals have different ranges and may hear sounds we cannot.
- Loudness is the human perception of sound intensity. It is frequently measured in dB which is a scale based on the human threshold of hearing (which is given a measurement of 0 dB on up). The dBA scale mimics the human range of hearing by filtering those high and low frequencies people don't hear as well. Noise is unpleasant or unwanted sounds, and noise pollution is considered to be any sounds that disrupt activities.
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:
- The vocabulary
- The relationship between sound intensity and loudness and how these two exemplify wave properties
- How sound intensity and loudness are measured, how their measurement scales work, and what these measurements tell us about a sound
- The difference between sound and noise
- How sounds affect living things both positively and negatively.
Use student data, conclusions, and recommendations to decide whether students can:
- Relate sound levels to their effects on living things and
- Substantiate the relationship with evidence.
- Understand the character of the park well enough to distinguish between intrinsic park sounds and extrinsic park sounds.
- Understand sound properties thoroughly enough to understand how sound can be mitigated.
- Understand the character of the park well enough that they can choose sound mitigation measures that will enhance the park experience for visitors.
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:
- Conditions such as temperature, humidity and wind affect sound level readings.
- Spectrographs of sounds are made and how they are used to portray different sounds.