ch26_esit

=Sound=

26.1 The Origin of Sound
All sounds are produced by the vibrations of material objects. Whether is is a person's vocal cords vibrating in order to produce the sound their voice makes, or the strings vibrating a guitar, sound is always produced by vibrations. However, the original vibration of the material object is not what directly causes sound. The vibration of the object will always trigger a vibration of something larger around it. For example, in a person's voice, the vibrating vocal chords will cause the air in the throat and mouth to vibrate. This vibration will then cause a disturbance in a nearby medium, which in most cases is air. Sound always takes the form of longitudinal waves and under ordinary conditions, the frequency of the vibrating source is equal to the frequency of sound waves produced. A **pitch** is how we describe our subjective impression about the frequency of a sound. A high pitched sound will have a high frequency and a low pitched sound will have a low frequency. As people grow older, their sense of sound and hearing range shrinks. A young person normally can hear between 20 to 20000 hertz. **Infrasonic** waves have frequencies below 20 hertz. **Ultrasonic** waves have frequencies above 20000 hertz. People cannot hear these types of waves.

26.2 Sound in Air
A sound producing, vibrating material object will create a longitudinal wave. This wave will be composed of **compressions** and **rarefactions**. Compressions are the higher pressured areas of the wave, and are drawn as the crests. Rarefactions are the lower pressured areas of the waves, drawn at the troughs. These waves will travel outward in all directions from the source. For an example of compressions and rarefactions, think about a slinky. When a pulse is sent through the slinky, it travels through it, and the areas that are bunched up on the slinky are the compressions and the areas that are less packed together are the rarefactions.

26.3 Media That Transmit Sound
You usually hear sound through air, but solids and liquids can also act as mediums for sound. In fact, both solids and liquids conduct sound better than air. Sound travels best in solids, and better in liquids than in air. For example, when your ear is submerged underwater and their is a sound produced underwater, you will hear that sound more clearly than hearing the sound out of water. Sound always requires a medium. For exmaple, in a vacuum, no sound can be heard, because there is nothing to compress and expand. The bell won't be heard in the vacuum.

26.4 Speed of Sound
The speed of sound in dry air at 0 degrees celcius is approximately 330 meters per second. This is about one-millionth the speed of light. If you are ever at a baseball game, you may notice that you hear the sound of the batter hitting the ball after you see it happen. This is evidence that sound travels slower than light. There are factors, however, that can influence the speed of sound in the air. A higher temperature, as well as increased water vapor, will both increase the speed of sound in air. This is because the faster moving molecules will alow the wave to transmit the pulse in less time because they will bump into eachother more often. The speed of sound in air will increase by .60 m/s for every degree increase in the temperature above 0 degrees celcius. The speed of sound in a 20 degree room will be about 340 m/s. Another factor that plays a part in the speed of sound is the elasticity of the material. However, many people misunderstand the concept of elasticity. **Elasticity** is defined as the ability of a material to change shape in response to an applied force, then resum its initial shape once the distorting force is removed. Therefore, according to this definition, for example, steel, is much more elastic than silly putty. Putty is very inelastic because it cannot change into its original shape after it is distorted easily. The more elastic an object is, the easier it can transmit sound, because the atoms are packed tightly together and respond quickly to each other's motion.

26.5 Loudness
The intensity of a sound is proportional to the square of the amplitude of a sound wave. We describe this intensity as loudness. Loudness is subjective but is related to intensity. The unit used for measuring intensity is the decibel (dB). The loudness of a sound is proportional with the logarithm of the intensity of the sound. As the loudness doubles, the intensity increases by a factor of 10. 0 dB represents the **threshold of hearing**, which is the quietest sound that a person can typically hear. With an increase of 10 dB, the intensity will increase by a factor of 10, and with an increase of 20 dB, the intensity will increase by a factor of 100. Also, since the loudness that you perceive doubles with an increase of a fator of 10 in the intensity, the loudness will double with a 10 dB increase. The intensity of sound is objective, and is measured with instruments such as an oscilloscope.

26.6 Forced Vibration
As explained earlier in the chapter, the vibration of the strings on a guitar will cause the soundboard to vibrate as well, which is the cause of the loud sound a guitar can make. Without the soundboard, the vibrating strings alone on a guitar will produce a very faint sound, sometimes barely even audible. This is an example of **forced vibration**. When the strings vibrate, the soundboard is forced into vibration, putting more air in motion.

26.7 Natural Frequency
When any type of elastic material is disturbed, the object will vibrate at its own, distinct set of, or single frequency. These frequencies in objects are described as the object's **natural frequency**. Every object has their own unique sound that is produced when it is put into vibration, and this sound will always remain constant for that object.

26.8 Resonance
For example, consider a wine glass. When you tap the wine glass, it makes its own unique sound, which is its natural frequency. It has been proven that when a person sings out the same pitch as the wine glass' natural frequency, the glass will begin to vibrate, sometimes so much that it breaks. This is because when the singer hits just the right note, the frequency of the sound he is making will constructively interfere with the frequency of the glass.
 * Resonance** can be defined as the phenomenon that happens when the frequency of a forced vibration on an object matches that object's natural frequency, which causes a dramatic increase in amplitude. The term resonance literally means to re-sound. Elasticity is a primary factor in whether or not that material has the ability to resonate. A force to pull the material back to its original position and a sufficient amount of energy to keep it vibrating is necessary for something to achieve resonance.

26.9 Interference
All waves, including sound waves can interfere with eachother. When the crests of one wave match up with the crests of another wave, **constructive interference** occurs, and the amplitude is increased. When the crests of one wave match up with the troughs of another wave, **destructive interference** occurs, and there is a decrease in amplitude. In sound waves,the compressions correspond to the crest of a wave, and the rarefactions correspond to the trough.

26.10 Beats
When two sounds of slightly different frequency are sounded together, the loudness of the two sounds combined becomes loud, then faint, then loud, then faint, etc. This fluctuation in the loudness of sound is called **beats**. For example, a beat will occur when a 200hz tone and a 202hz tone are sounded together. As the two tones are sounded together, since they are traveling at different frequencies, their crests and troughs will not be constantly overlapped. You hear the louder sound when the crests overlap, resulting in an increase in amplitude, and the very faint, hardly audible sound is heard when the two tones are out of step and a crest and a trough are overlapping. An excellent example of beats can be shown in this link: http://www.explorelearning.com/index.cfm?method=cResource.dspView&ResourceID=48