Topic+3.3+Sound

=toc Learning Outcomes:=



=Production of Sound:=



=Longitudinal Nature of Sound Waves:=

Visit the link for the effect : @http://faraday.physics.utoronto.ca/IYearLab/Intros/StandingWaves/Flash/long_wave.html



=The journey of Sound :=



=Intensity and pitch :=

Activity:Do you hear what I hear?
Visit the link @http://science.education.nih.gov/supplements/nih3/hearing/guide/lesson3.htm for instructions. Web resource needed: @http://science.education.nih.gov/supplements/nih3/hearing/activities/lesson3.htm Print material needed: mater 3.1, 3.2,3.3,3.4 and 5.2,5.3,5.4,5.5 from @http://science.education.nih.gov/supplements/nih3/hearing/guide/nih_hear_masters.pdf

3.3 Sound has a physical basis
Sound represents vibrational energy. It is created when a medium such as air, wood, metal, or a person’s vocal cords vibrate. Sounds carried as energy are transferred from one molecule to the next in the vibrating medium. To understand sound, consider the analogy in which a stone is dropped into a body of water. This action produces ripples that will spread out in all directions from the point where the stone contacted the water. The ripples become weaker (decrease in intensity) as they get farther away from the origin. So it is with sound. The vibration through a medium proceeds in waves. However, unlike ripples on water, sound waves move away from their point of origin in three dimensions, not just two. Sound waves possess specific characteristics. **Frequency** represents the number of complete wave cycles per unit of time, usually one second (see Figure 5). Frequency is expressed in **hertz (Hz)**, which means cycles per second. Low-frequency sounds are those that vibrate only a few times per second, while high-frequency sounds vibrate many more times per second. The term used to distinguish your perception of higher-frequency sounds from lower-frequency sounds is **pitch**.  The speed of sound is constant for all frequencies, although it does vary with the medium through which it travels. In air, sound travels at a speed of roughly 340 meters per second. Sound travels fastest through metals because the molecules of that medium are packed very closely together. Similarly, sound travels about four times faster in water than in air. It follows that sound travels faster in humid air than dry air; in addition, humid air absorbs more high frequencies than low frequencies, leading to differences in the perception of sound heard through the two media. Finally, temperature can affect the speed of sound in any medium. For instance, the speed of sound in air increases by about 0.6 meters per second for each degree Celsius increase in temperature. The human ear responds to frequencies in the range of 20 Hz to 20,000 Hz (20 kHz), 18 although most speech frequencies lie between 100 and 4,000 Hz. Frequencies above 20,000 Hz are referred to as **ultrasonic**. Though ultrasonic frequencies are outside the range of human perception, many animals can hear these sounds. For instance, dogs can hear sounds at frequencies as high as 50,000 Hz, and bats can hear sounds as high as 100,000 Hz. Other sounds, such as some produced by earthquakes and volcanoes, have frequencies of less than 20 Hz. These sounds, referred to as **infrasonic**or **subsonic**, are also outside the range of human hearing. 
 * Figure 5.** Representation of frequency. The arrow indicates one cycle of the sound wave.

**Figure 6.** The sound spectrum. We all know that sounds can be louder or softer, but what does this mean? Sound is energy, and this energy, when traveling through air, displaces, or vibrates, air molecules. For example, the softest sound humans can hear is a sound that displaces particles of air by one-billionth of a centimeter. 13 The extent to which air particles move from their original resting point determines the **amplitude** of the sound wave (see Figure 7). The greater the amplitude of the sound wave, the greater the intensity, or pressure, of the sound. **Intensity** refers to the overall amplitude of a sound. This distinction in terms is necessary, since nearly all sounds to which we are exposed are complex sounds made up of a combination of sound waves. **Loudness** is our perception of the intensity, frequency, and duration of a sound. 

**Figure 7.** Representation of amplitudes of a wave. The dashed line has a lower amplitude than the solid line. Every 10-dB increase in sound intensity represents a 10-fold increase in sound intensity and a perceived doubling in loudness. Sound intensity is measured in relation to an accepted reference point. One such reference is the threshold at which a sound can be heard. How the intensity of any given sound compares with this standard reference level is given in units known as decibels (dB). The **decibel** is one-tenth of a bel, a unit named after the inventor Alexander Graham Bell. The decibel scale is not a linear one, but rather represents the ratio of the sound to the reference standard. To understand why ratios are necessary, consider the tremendous range of sound intensities we are capable of hearing. Scientists estimate that the human ear is sensitive to about 100,000,000,000,000 (10 14 ) units of intensity. Also consider that a shout is about 1,000,000 (10 6 ) times more powerful than a whisper. Because dealing with such large numbers is cumbersome, the decibel scale is used to simplify comparisons (see Table 1). Every 10-dB increase in sound intensity represents a 10-fold increase in sound intensity and a perceived doubling in loudness. Therefore, a sound at 60 dB is 100 times as intense as a sound at 40 dB but is only perceived as four times as loud. In this way, the predominant range of human hearing is represented on a scale from 0 to 140 dB. The average intensities of some everyday sounds are presented in Table 2. Table 1. The Decibel System ||~ **Intensity Ratio**
 * ~ **Intensity Difference (dB)** ||
 * = 1:1 ||= 0 ||
 * = 2:1 ||= 3 ||
 * = 4:1 ||= 6 ||
 * = 8:1 ||= 9 ||
 * = 10:1 ||= 10 ||
 * = 16:1 ||= 12 ||
 * = 20:1 ||= 13 ||
 * = 100:1 ||= 20 ||
 * = 400:1 ||= 26 ||
 * = 800:1 ||= 29 ||
 * = 1,000:1 ||= 30 ||
 * = 2,000:1 ||= 33 ||
 * = 8,000:1 ||= 39 ||
 * = 10,000:1 ||= 40 ||
 * = 100,000:1 ||= 50 ||
 * = 1,000,000:1 ||= 60 ||
 * = 10,000,000:1 ||= 70 ||
 * = 100,000,000:1 ||= 80 ||
 * = 1,000,000,000:1 ||= 90 ||
 * = 10,000,000,000:1 ||= 100 ||
 * = 100,000,000,000:1 ||= 110 ||
 * = 1,000,000,000,000:1 ||= 120 ||
 * = 10,000,000,000,000:1 ||= 130 ||
 * = 100,000,000,000,000:1 ||= 140 ||

Table 2. **Average Intensities of Everyday Sounds** ||~ **Sound** Even common noises, such as highly amplified music and gas-engine mowers or leaf blowers, can damage human hearing with prolonged exposure. Individuals are often unaware of the damage loud noise does to their hearing. Even common noises, such as highly amplified music and gas-engine mowers or leaf blowers, can damage human hearing with prolonged exposure. Sporting events can also expose individuals to hazardous decibel levels as defined by the Occupational Health and Safety Administration (OSHA). Under OSHA guidelines, the limit of continuous noise exposure for an eight-hour day in an industrial setting is 90 dB. OSHA also prohibits workplace **impact noise** (short bursts of sound) greater than 140 dB. By increasing our awareness of decibel levels of common environmental noises, we can better limit our exposure to hazardous noise levels or take measures to protect our ears.
 * ~ **dB Level** ||
 * < hearing threshold ||= 0 ||
 * < breathing ||= 10 ||
 * < rustling leaves ||= 20 ||
 * < whispering ||= 25 ||
 * < library ||= 30 ||
 * < refrigerator ||= 45 ||
 * < average home ||= 50 ||
 * < normal conversation ||= 60 ||
 * < clothes dryer ||= 60 ||
 * < washing machine ||= 65 ||
 * < car ||= 70 ||
 * < vacuum cleaner ||= 70 ||
 * < busy traffic ||= 75 ||
 * < noisy restaurant ||= 80 ||
 * < outboard motor ||= 80 ||
 * < inside car in city traffic ||= 85 ||
 * < electric shaver ||= 85 ||
 * < screaming child ||= 90 ||
 * < passing motorcycle ||= 90 ||
 * < convertible ride on freeway ||= 95 ||
 * < table saw ||= 95 ||
 * < hand drill ||= 100 ||
 * < tractor ||= 100 ||
 * < diesel truck ||= 100 ||
 * < circular saw ||= 100 ||
 * < jackhammer ||= 100 ||
 * < gas engine mower ||= 105 ||
 * < helicopter ||= 105 ||
 * < chain saw ||= 110 ||
 * < amplified rock concert ||= 90–130 ||
 * < shout into ear at 20 cm ||= 120 ||
 * < car horn ||= 120 ||
 * < siren ||= 120 ||
 * < threshold of pain ||= 120–140 ||
 * < gunshot ||= 140 ||
 * < jet engine ||= 140 ||
 * < 12-gauge shotgun ||= 165 ||
 * < rocket launching ||= 180 ||
 * < loudest audible tone ||= 194 ||