Optional Unit V: Sound
C. Characteristics of Sound

1. Intensity

Key Concepts

The terms loudness and intensity are sometimes used interchangeably.

The intensity of sound is a measure of the average rate of sound energy flow per unit area across a surface perpendicular to the direction of propagation.

intensity = ^{energy / time
                   area}
              = ^{power
                   area}

One watt per square metre is too large to measure the small but wide range of sound intensity levels that the human ear is sensitive to. Awkward numbers result.

Special units have been designed to measure sound intensity (the bel and the decibel).

An increase of 1 bel represents a 10 fold increase in intensity.

The intensity in bels is found by where I is the intensity of interest in W/m², and I_o = 10^-12 W/m² (which is assumed to be the threshold of human hearing).

The intensity in decibels is .

The bel and decibel scales are logarithmic.

An increase of 10 dB represents an increase in sound intensity by one order of magnitude. (i.e., 10 dB = 10^-11 W/m², 20 dB = 10^-10 W/m², etc. etc.)

An increase of 3 dB is a doubling of intensity. An increase of 1 dB, representing roughly a 25% increase in intensity, is just barely detectable audibly.

A sound intensity of 0 dB represents the threshold of human hearing. This corresponds to an intensity of 10^-12 W/m²(I_o) . (The threshold of pain is somewhere between 120 dB and 130 dB, depending on an individual's sensitivity to pain.)

To determine the decibel change between two intensities, I₁ and I₂, .

Minimum sound intensity perception in humans varies with frequency. A frequency of about 4 000 Hz yields the lowest threshold for people with acute hearing ability. The human ear is most sensitive to frequencies between 1 000 Hz and 5 000 Hz.

Hearing sensitivity and the range of sounds capable of being produced vary for different kinds of animals.

It is useful to be able to estimate the approximate intensity of sounds emanating from various different sources.

For accurate measurements of sound intensity special instruments can be used. They are placed at standard distances (usually 1 metre) from the source which is being measured.

Sound intensity decreases as distance increases from the source. An inverse square law relationship is observed.

Extremely loud sounds, as from a sudden explosion, may cause the eardrum to rupture.

Sound intensities over 90 dB (or even lower levels for prolonged, repeated periods of exposure) can cause permanent damage to the ear. Higher intensities can only be withstood for shorter periods of time before hearing damage results. This damage is usually irreparable. Damage begins to cause deterioration in the audibility of certain frequencies, then continues until total deafness occurs.

A "ringing" sensation in the ears (as one might experience after a loud rock concert) is an indication that some permanent hearing loss may have occurred. Continued exposure to such noise may further aggravate the hearing damage.

There are very few pain receptors in the ears to warn us that gradual hearing loss may be occurring due to loud noise.

It is recommended (or required in certain circumstances) that ear protectors be used when one is exposed to noisy environments.

Hearing protectors can not protect adequately against intense sounds which travel through the skeleton into the bones in the middle ear. (i.e., with vibrating power tools, some heavy machinery, etc.)

Hearing loss tends to become more pronounced with aging as well as with environment. Studies show that people in certain highly developed parts of the world are susceptible to greater amounts of hearing loss with age.

Hearing aids and new medical advances can assist people who have experienced some hearing loss. Further research into human hearing can lead to new advances which will further benefit those who suffer from hearing loss.

Noise pollution represents a serious threat to the quality of life. Industrialization and the use of new technologies have added to the problem. Noise pollution is not just an urban problem. In rural areas, noise from farm implements and other heavy machinery can also lead to hearing loss.

Noise pollution has been attributed to higher levels of stress in humans. Similar stress might be experienced by other animals. Noise pollution may be contributing to various forms of ecological damage.

Learning Outcomes

Students will increase their abilities to:

1. Define the following terms: intensity, bel, decibel, logarithmic scale.

2. State the units commonly used to measure sound intensity.

3. Illustrate the relationship between sound intensity values expressed in W/m² and in multiples of 10 on the dB scale.

4. Compare logarithmic and arithmetic scales.

5. State that 0 dB has been arbitrarily set to correspond to the threshold of human hearing.

6. State that the threshold of pain lies somewhere between 120 and 130 dB.

7. Explain that minimum threshold levels of human hearing vary with frequency.

8. Estimate the approximate intensity of sounds emanating from various different sources.

9. Recognize that sound intensity diminishes with increased distance.

10. Explain why instruments which measure sound intensity must be placed at some standard distance away from a sound source.

11. Interpret inverse square law relationships, as illustrated by the dependence of distance from the source on sound intensity.

12. Solve problems relating to the intensity of sound.

13. Explain why extremely loud sounds may cause a perforation of the eardrum.

14. Explain that repeated exposure to loud sounds may cause permanent hearing damage.

15. Explain that the intensity of a sound, as well as its duration, determine the extent of hearing damage that could occur from it.

16. Realize that hearing damage is usually irreparable.

17. Recognize that partial hearing loss first begins with certain frequencies, then extends to other frequencies until total deafness occurs.

18. Realize that a "ringing" sensation in the ears may be an indication that permanent hearing damage has occurred.

19. Realize that there are few pain receptors in the ears to warn us when hearing damage may be taking place.

20. Recognize that permanent hearing loss is usually irreparable.

21. Explain that hearing protectors can be used to help protect the ears from loud noises.

22. Recognize that hearing protectors can not protect the ear adequately from sounds which travel through the skeleton into the bones of the middle ear.

23. Explain that hearing loss tends to become more pronounced with age.

24. Explain that in certain developed parts of the world hearing loss among people tends to be more severe.

25. Explain some things which may be of assistance to people who have experienced hearing loss.

26. Appreciate the role that both science and technology play in helping people who have experienced hearing loss.

27. Recognize that noise pollution poses a serious threat to the quality of life.

28. Suggest a variety of factors which are contributing to noise pollution in both urban and rural environments.

29. Explain how the term "quality of life" relates to the issue of noise pollution.

30. Suggest various ways in which the amount of noise pollution experienced in any given situation might be reduced.

Teaching Suggestions, Activities and Demonstrations

1. Allow students to recognize that the bel and decibel scales were devised because of the sensitivity of the human ear to fairly low intensities, which would represent very small numbers when expressed in W/m². Also, the ear is responsive to sound intensities over a range of about 14 orders of magnitude (but roughly a 25% increase in intensity is required to perceive a change).

2. Give a practical example which illustrates that hearing sensitivity varies for different kinds of animals. Demonstrate this, or invite hunters and trappers in to the classroom to share their experiences.

3. Brainstorm to suggest several ways in which people could protect themselves from high intensity sounds.

4. Explain that hearing sensitivity and the range of sounds capable of being produced vary for different kinds of animals.

5. Allow students to suggest and research an hypothesis to explain why hearing loss among people tends to be more severe in certain developed parts of the world.

6. Develop an argument to support or reject the following thesis statement: "Noise pollution may be contributing to various forms of ecological damage." This might be an interesting topic for students to debate.

7. Examine the impact of noise pollution on animals or on people in northern communities.

8. Illustrate how a change between two intensities can be determined using

   

   e.g. if a source producing 50 dB doubles in intensity, the sound intensity becomes

   50 dB + 10 log(I₁/I₂) = 50 dB + 10 log 2
        = 50 dB + 3.01 dB
        = 53 dB

   This shows that doubling the intensity produces a 3 dB increase.