Waves+and+Sound

= Waves and Sound =

** Essential Questions: **

When asked what a wave is, the first vision to appear may be of ocean waves crashing into a sandy shoreline. However, the study of waves will take you well beyond the images of sandy beaches and crashing waves. __A wave__ is simply the motion of a disturbance caused by some vibrating source. The vibration could be caused by something as simple as a hand moving a string back and forth or as complex as the shifting of the Earth’s tectonic plates.
 * 1) What are waves and what can they do?
 * 2) What are the different types of waves?
 * 3) What are some basic properties of waves?
 * 4) What is a sound wave?
 * 5) How is a sound wave transmitted?
 * 6) What is the Doppler Effect?
 * Section 1: Wave Basics **

Waves are generally broken down into two separate categories: **Mechanical Waves** and **Electromagnetic Waves**.

When a mechanical wave passes through a medium, it has the ability to move the particles of that medium in one of two ways: the particles of the medium can be vibrated perpendicular to the wave motion or parallel to the wave motion.
 * __Mechanical waves__ need a material medium in order to travel. Examples of these types of waves would be sound waves, water waves or waves on a rope or string.
 * __Electromagnetic waves__ do not need a medium in order to travel. They are able to travel through the virtually empty vacuum of space. Examples of these waves include the seven types of electromagnetic radiation: radio waves, microwaves, infrared radiation, visible light, ultraviolet waves, x-rays, and gamma rays.


 * __Transverse wave:__ causes the particles of a medium to be vibrated perpendicular to the wave motion. The diagram below shows a typical transverse wave with wave parts labeled.


 * __Longitudinal wave:__ causes the particles of a medium to be vibrated parallel to the wave motion. Imagine the diagram below as a slinky that has been compressed and released showing the compressions and rarefactions as the wave travels.

In the two previous diagrams you’ll notice that many parts of the different waves were listed. The following table provides the definitions for the wave parts as shown in the diagrams.

f = 1/T || Hertz || v = λ × f  || m/s || Some important equations to note are the frequency / period relationship:
 * **Symbol** || <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">**Term** || <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">**Definition** || <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">**Units** ||
 * <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">A || <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">Amplitude || <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">A wave’s maximum displacement from the rest or equilibrium position – transferred energy is proportional to the square of the amplitude || <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">Meters ||
 * <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">T || <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">Period || <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">The time it takes to execute one complete cycle of motion || <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">Seconds ||
 * <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">f || <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">Frequency || <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">The number of cycles or vibrations per unit time – reciprocal of the period:
 * <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">λ || <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">Wavelength || <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">The shortest distance between two points where the wave pattern repeats itself (e.g. crest to crest or trough to trough) || <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">Meters ||
 * <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">v || <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">Velocity || <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;"><span style="font-family: 'Times New Roman',Times,serif;"> The rate at which a wave moves through a medium or free space.
 * <span style="display: block; font-family: 'Times New Roman',Times,serif; text-align: center;"> || <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">Crest || <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">The highest point above the equilibrium position that a transverse wave reaches || <span style="display: block; font-family: 'Times New Roman',Times,serif; text-align: center;"> ||
 * <span style="display: block; font-family: 'Times New Roman',Times,serif; text-align: center;"> || <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">Trough || <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">The lowest point below the equilibrium position that a transverse wave reaches || <span style="display: block; font-family: 'Times New Roman',Times,serif; text-align: center;"> ||
 * <span style="display: block; font-family: 'Times New Roman',Times,serif; text-align: center;"> || <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">Compression || <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">The region of a longitudinal wave in which the density and pressure are greater than normal || <span style="display: block; font-family: 'Times New Roman',Times,serif; text-align: center;"> ||
 * <span style="display: block; font-family: 'Times New Roman',Times,serif; text-align: center;"> || <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">Rarefaction || <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">The region of a longitudinal wave in which the density and pressure are less than normal || ||

f = 1 / T

As well as the relationship between velocity, wavelength and frequency:

v = f λ

** Wave Interference ** Waves do have the ability to interfere with each other as they are traveling through one another. The three basic types of interference are constructive, destructive and complete destructive.

During __constructive interference__, two wave amplitudes that are on the same side of the equilibrium position add to form larger waves.



During __destructive interference__, wave amplitudes that are on opposite sides of equilibrium add together to form smaller waves.



During __complete destructive interference__, waves of equal amplitudes, but with opposite displacements from the equilibrium position interfere with one another and completely cancel each other resulting in zero displacement.

An important thing to note is that regardless of what type of interference occurs, after the waves pass through each other, they regain their original amplitudes and still move in their original directions.

In section 1 many of the general wave properties were discussed, but in this section focus will be on one particular type of wave: sound.
 * Section 2: Sound Waves **

__Sound waves__ are mechanical, longitudinal waves. Thus they need a medium in order to travel and cannot travel through the vacuum of space.

A sound wave is created, like any other wave, by a vibrating source. If we consider a sound that is traveling through the air: a vibrating source pushes against the air molecules near it causing compressions and rarefactions. This longitudinal wave is transmitted through the neighboring air molecules until eventually the energy would be dissipated. If you are near this vibrating source, then the sound wave may reach your ear and your ear drum would vibrate as a result, allowing you to detect the sound wave.

** Pitch ** A term that is important to sound waves is pitch. Pitch is how we represent the frequency of a sound wave. A high pitch sound has a high frequency, while a low pitch sound has a low frequency. The hearing range of an average human during youth is anywhere between 20 Hz and 20,000 Hz, but as you get older, your hearing deteriorates making it impossible for you to hear certain frequencies. Is your hearing in line with your age? Click on the link below to find out!

@http://www.freemosquitoringtones.org/

** Interference of Sound ** Sound waves still exhibit the same properties as other waves, so sound waves can interfere with each other and create dead zones in rooms where sound is reflecting off of the walls and ceilings and interfering with other sound waves in the room. Engineers must take this into consideration when designing amphitheaters and auditoriums so they don’t inadvertently build a room that has poor acoustics.

** Doppler Effect ** An interesting wave property that is especially apparent with sound is called the Doppler Effect. The Doppler Effect is a frequency shift that is the result of relative motion between the source of waves and an observer:

When a sound source moves toward the observer, the detected frequency is higher. When it moves away, the detected frequency is lower.

//__ Reason: __// When a source is moving toward you, the length of the wave is shorter, but the velocity of the wave is the same. As a result the wave appears to have a higher frequency or pitch.

When a source is moving away, the length of the wave is longer, but the velocity is the same. As a result the wave appears to have a lower frequency or pitch.

The Doppler Effect occurs in __ALL__ waves, including visible light, which serves as an argument for expansion of the universe. We can measure which way the light waves are being shifted and determine whether the distant object is moving towards us or away from us.

The intensity of a sound wave is based on the amplitude of the wave. We would experience intensity as the volume of a sound: the greater the intensity, the greater the volume. High volume sounds are capable of damaging your hearing as decibel scale below indicates.
 * Relative Intensity **

An interesting occurrence on the decibel scale is that a sound that is increased by 10 decibels appears to be twice as loud to our ears. Thus when you jump from 90 – 100 decibels, the sound appears to double in loudness. This means, according to the decibel level, sound volume increases at an exponential rate making sounds at a higher decibel level very dangerous.

<span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: left;">If you would like more reading about sound waves, The Physics Classroom is another site that describes different characteristics of sound waves.

The Physics Classroom - Sound Waves <span style="font-family: 'Times New Roman',Times,serif;">**The Loudest Band?**

This is a question often argued amongst rock musicians and fans around the world. Officially the guiness book of world records recognizes The Who as the loudest band in 1976, 126 decibels measured 32m (over 100 ft) from the stage. Guiness then recognized Manowar as the loudest in 1994 at 129 decibels. Unofficially, bands such as Metallica, KISS, The Rolling Stones, Motorhead, AC/DC and Swans have all claimed to have crossed the 130 decibel mark. Guiness will not officially recognize any of these bands since they do not wish to promote hearing loss. Swans was a noise rock band from New York that has been known to surpass 140 decibels at some shows. The full scoop can be read at:

<span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">@http://news.softpedia.com/news/Searching-for-the-Loudest-Band-in-the-World-105105.shtml or at <span style="font-family: 'Times New Roman',Times,serif;">@http://en.wikipedia.org/wiki/Loudest_band_in_the_world

<span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">**Tacoma Narrows Bridge and Resonance**

The last item to discuss is a property of waves called resonance. This property is most often associated with sound waves, but can sometimes be seen in objects that are not being affected by sound. Resonance is the tendency for an object or a system to oscillate at high amplitudes when it is bombarded by waves that are traveling at the natural frequency of the object. For example: if you are on a swing you need to kick your legs with precisely the right timing to keep yourself in motion. If you kick your legs randomly, you will not be able to stay in motion. The Tacoma Narrows bridge experienced this same phenomonon in 1940 and collapsed just 4 months after it was opened. The wind that blew against the bridge would create eddy currents around the bridge piers that exactly matched the resonant frequency of the structure causing it to oscillate. For the first few months, these winds were never sustained long enough to cause major damage, but once the bridge experienced a prolonged wind, the oscillations of the bridge became so violent that the bridge tore itself apart.

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 * Resources**

Faughn, and Serway. //Holt Physics//. Austin: Holt Rinehart & Winston, 2002.

Hewitt, Paul G.. //Conceptual Physics: The High School Physics Program//. Toronto: Pearson Prentice Hall, 2002. Print.

"Loudest band in the world - Wikipedia, the free encyclopedia." //Wikipedia, the free encyclopedia//. N.p., 13 Nov. 2009. Web. 22 Nov. 2009. <http://en.wikipedia.org/wiki/Loudest_band_in_the_world>.

"Sound is a Mechanical Wave." //The Physics Classroom//. N.p., 1 Jan. 2009. Web. 23 Nov. 2009. <http://www.physicsclassroom.com/Class/sound/u11l1a.cfm>.

Vieru, Tudor. "Searching for the Loudest Band in the World - There is no clear winner just yet - Softpedia." //Latest news - Softpedia//. N.p., 21 Feb. 2009. Web. 22 Nov. 2009. <http://news.softpedia.com/news/Searching-for-the-Loudest-Band-in-the-World-105105.shtml>.