Outline for Exploratorium

 

Intro

 

Characteristics of Sound

§        Frequency, Wavelength, Amplitude, and Velocity

 

How humans hear sound?

§        Vibrations

§        Pressure

 

Experiments

§        Bells

o       Demonstrates how sound waves work

§        Cups

o       Able to see the vibrations of sound waves

 

Conclusion

 
 

Sound Waves

            In life, people receive sound waves in various ways.  Sound can be used for communication through speech and music.  Also, it can be used to achieve information about properties of the surrounding environment.  For example, ships use sonar to locate objects in the sea.  In this report, this group will discuss sound waves and how they work in relation to other objects.

            Sound is a disturbance of mechanical energy that propagates through matter as a wave.  Sound is characterized by the properties of sound waves, which include frequency,

wavelength, amplitude, and velocity.  The frequency of a sound wave is the rate at which the waves pass a given point.  The wavelength is the distance from one crest to another of a wave.  The amplitude of a sound wave is the same thing as its loudness.  Finally, a sound wave’s velocity is 344 meters/second in air.  The relationship between velocity, wavelength, and frequency is velocity= wavelength * frequency.   Sound waves also have a couple other characteristics including the decibel and its pitch.  The decibel is a common measurement of loudness.  It is a complex unit that varies as the ratio of the logarithms of loudness.  Its pitch that we experience is determined by its wavelength or its frequency.  The shorter the wavelength, the higher the frequency becomes, and the higher the pitch that we hear.
 Humans perceive sound by the sense of hearing.  We commonly mean the vibrations that travel through air and can be heard.  However, scientists use a wider definition of sound that includes low and high frequency vibrations that cannot be heard by humans. Vibrations can travel through all forms of matter: gases, liquids, and solids.  Through these mediums, the speed of sound can be calculated.  In general, the speed of sound is proportional to the square root of the ratio of the stiffness of the medium and its density.Also, sound waves give off pressure.  Sound pressure is the pressure deviation from the local ambient pressure caused by a sound wave.  It can be measured using a microphone in air and a hydrophone in water.The SI unit for sound pressure is the Pascal.  In a soundwave, the complementary variable to sound pressure is the acoustic particle velocity.  For small amplitudes, sound pressure and particle velocity are linearly related and their ratio is the acoustic impedance.

This depends on both characteristics of the wave and the medium.  Sound pressure is often measured as a level on a logarithmic decibel scale.

Sound pressure level is defined as:

 
L_\mathrm{p}=10\, \log_{10}\left(\frac{{p}^2}{{p_0}^2}\right) =20\, \log_{10}\left(\frac{p}{p_0}\right)\mbox{ dB}

In this formula, p is the root-mean-square sound pressure and p0 is a reference sound pressure.

Examples of sound pressure and sound pressure levels include:

Source of sound

sound pressure

sound pressure level

 

Pascal

dB re 20 µPa

threshold of pain

100

134

hearing damage during short-term effect

20

approx. 120

jet, 100 m distant

6 - 200

110 - 140

jack hammer, 1 m distant / discotheque

2

approx. 100

hearing damage during long-term effect

6×10−1

approx. 90

major road, 10 m distant

2×10−1 - 6×10−1

80 - 90

passenger car, 10 m distant

2×10−2 - 2×10−1

60 - 80

TV set at home level, 1 m distant

2×10−2

ca. 60

normal talking, 1 m distant

2×10−3 - 2×10−2

40 - 60

very calm room

2×10−4 - 6×10−4

20 - 30

leaves noise, calm breathing

6×10−5

10

auditory threshold at 2 kHz

2×10−5

0
Through studying the characteristics of sound waves, we were able to experiment them and test their results.  The Bells experiment shows how sound waves work by using vibrations.  You can see the vibrations on the steel plate by putting sand on the plate.  There are two plates; one of them is round and the other is square.  They each show a different pattern of vibration.The patterns can be changed by bowing at different points along the edge to produce a node of vibration.  The pressure of the bow against the plate depends on the frequency.  When you first strike the bow against the plate, use short, gentle motions rapidly.  It may take quite a few strokes before the plate begins to vibrate.  Any change in the pressure or angle of the bow will affect the result.

When you get it to work, the plate will start to “sing”.  Then, you need to lengthen the strokes and increase the pressure.  When it is loud, stop bowing and let it ring.  As it fades, the sand will show a star pattern with 4, 6, 8, or 12 points.  The square plate shows specific lines with nodes at specific points.  Bowing the plate at the center of an edge causes it to ring at its loudest pitch.  Bowing it closer to the end causes higher-pitched ringing.

            Also, the group is going to try an experiment using glass cups filled with water.  Water gives off sound waves by vibrations through the water.  If you tap the side of a cup, you will see vibrations through the cup of water.  Somehow, the group will link the cups together and tap at one end of the list.  Hopefully, the vibrations continue through the line of cups.  This will show how fast waves can travel.  If you use a stop watch, you can determine the speed of the sound or vibration.

            Through these experiments, the group will learn how sound waves actually work and their affect on other objects.  It will expand their knowledge and help them in the future.  Also, we get a better understanding of how sound waves affect our life.  We can take sound waves for granted, and this experiment shows us all the different uses of it.  Ship, fishes, and bats all use some sort of sonar to detect other objects.  We need sound waves to hear things like in class or at home.  Sound affects our life everyday.  We now know how they work and their affect on things.
 

References:

 

http://www.fi.edu/fellows/fellow2/apr99/soundvib.html

 

http://en.wikipedia.org/wiki/Sound_waves

 

http://www.school-for-champions.com/science/sound.htm

 

http://www.school-for-champions.com/science/waves.htm

 
Bells Main Page
Bells Construction Page
Exhibit Study Guide

Glasses Main Page
Glasses Construction Page
Exhibit Study Guide