# Sound

## What is Sound?

Sound, the art of objects making noise and creating vibration through the air. Sound can be made by any object, from a violin playing a solo to the wind blowing, sound is created everywhere. "Most sounds you hear are transmitted through the air, but can also travel in solids and liquids"(Conceptual Physics, pg.392). The medium through which sound travels affects the tone of the sound and the speed it travels. An example of this is clapping under water. The sound takes longer when it travels through the air. [1] [2]
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 courtesy of: up your architecture.

## Vocabulary

1. Pitch- The apparent predominant frequency sounded by an acoustical source.
2. Infrasonic- Sound waves with frequencies below 20 hertz.
3. Ultrasonic- Sound waves with frequencies above 20,000 hertz.
4. Hertz- The SI unit of frequency, equal to one cycle per second.
5. Compression- A pulse of compressed air.
6. Rarefaction- A decrease in density and pressure in a medium, such as air, caused by the passage of a sound wave.
7. Forced Vibration- forcing an object to vibrate. Ex. a tuning fork is not loud when its by itself, but place it on a table an it is suddenly loud. Its because the tuning fork is making or forcing the wood in the table to vibrate.
8. Natural Frequency- A frequency at which an elastic object, once energized, will vibrate. Ex. think of bells, a smaller bell is higher in frequency than that of a big bell, and it rings at a higher pitch also.
9. Resonance- This happens when the frequency of forced vibrations on an object matches the objects natural frequency, and has an increase in amplitude results. Ex. swinging on a swing set in rhythm with its natural frequency produces larger amplitudes.
10. Beats- A variation in loudness of sound caused by interference when two tones of slightly different frequencies are sounded together. Ex. take two tuning forks and hit them on a table to cause a vibration, one fork vibrates at a frequency different from the other, but the vibration of the forks will in moments be in the same beat and then out of beat, then back into beat and so on.
11. Sound Waves- a longitudinal wave in an elastic medium, especially a wave producing an audible sensation. [4]

# Courtesy of: National Aeronautics and Space Administration

What is the speed of sound? Sound takes awhile before it gets to our ears, but no one person hardly notices. The speed of "sound" is actually the speed of transmission or movement of a small disturbance through a medium. The picture to the right explains how the speed of sound goes through-out the air and how fast it goes. The letter "a" in the picture resembles the speed of sound. The equation helps us determine how fast it is going, depending on what the day is like. The equation is a = sqrt ( y* R* T), which "a" is the speed of sound or medium, which then is equal to the square root of the type of gas (y) or how much heat there is, times R, the gas constant, times T (temperature of that day). The speed of sound changes depending on the weather or how hot or cold it is outside. Water vapor and high temperature increases the speed of sound in the air, while cold air decreases. When the temperature is high or hot outside, it makes the sound more clear so sound waves are easily carried and comes to the ear faster, while when is cold or snowy, the sound takes awhile to get to the ear because cold air blocks the sound which is why it takes a little longer to hear the sound. Sound is not easily vibrated through the air in cold/snowy days as it is on hot/clear days. By the way the higher the pitch the higher the frequency, which is the same for if the pitch is low the frequency is low. Pitch and frequency is closely related. Obviously the louder the sound is that you make, the father it goes through the air. Sound waves also help determine what the speed of sound is. The sound wave are the waves to which the sound is coming from. The picture to the right explains how the sound waves keep going and going and the calculation helps us with how fast the waves are going through the air. We can calculate the speed of sound through any material, which in steel can travel about fifteen times faster than air, and four time faster in water than air. The speed of sound travels faster than you know.

## Forced Vibration and Natural Frequency

Have you ever wondered why when you put your cell phone on a table and when it vibrates its really loud compared to when its not on the table? Its because when the cell phone vibrates it forces the table to vibrate as well. This is called forced vibration. This is caused when an object of some type vibrates and hits or is sitting on another object which then starts vibrating the other object its sitting on by force, which also makes the sound of the vibrate louder because two things are vibrating. Forced vibration is found in a lot of things because otherwise some things wouldnt be loud enough to hear without it. Like in a acoustic guitar, the strings wound be faint without the wooden body because therer would be no way of hearing a sound in front of large audiences, even small audiences. Wooden bodies to an instrument is always needed to hear the instrument clearly.
Natural frequency is a vibration or frequency to one object. Like a smaller bell has a higher natural frequency than a larger bell because the smaller it is the louder it is compared to something big and heavy, which would be at an lower frequency.

## The Doppler Effect

The Doppler Effect, also known as the Doppler Shift, was first coined by Austrian physicist Christian Doppler, who first proposed the effect back in 1842. The Doppler Effect shows that there is a change in the frequency of emitted waves produced by the motion of an emitting source relative to an observer. [8] Take for example, the classic case of the Doppler Effect. Pretend that you, the observer, hears a train coming in the distance. Since the train is in motion when it approaches you, the waves emitted by the train whistle get "stacked" on top of one another. Therefore, it seems to be louder than when the train moves away from the observer. This also explains why you can hear the train for a longer period as it moves away from the observer, verses approaching the observer. In Christian Doppler's experiment to prove the Doppler Shift, he had an orchestra play on note on a train and then asked the conductor to go past him as fast as he could. In his experiments, as well as noting the change in volume and time, there was also a change in pitch as the train passed him.
 Image courtesy of Wikimedia Commons

## Animals and their Frequencies

Did you ever know that animals had their own frequencies or had their own way making sound? It is quite interesting! Like elephants can produce a sound frequency so low that us humans or people cant hear them, but their herds can. Then there are these birds that communicate with their wings,, instead of their beaks or mouths.

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