Sound

Georgia Perimeter College

Objectives

  1. Demonstrate an understanding of how sound travels as waves.
  2. Demonstrate an understanding of how the intensity of sound and the units in which it is measured.
  3. Demonstrate an understanding of how the behavior of sound waves is affected by differences in the medium (such as altitude and temperature).
  4. Demonstrate an understanding of sound, related to everyday experiences (such as guitar strings, thunder, echoes, sonic booms, etc.).
This section addresses, in whole or in part, the following Georgia GPS standard(s):
  • S8P2. Students will be familiar with the forms and transformations of energy.
    c. Compare and contrast the different forms of energy (heat, light, electricity, mechanical motion, sound) and their characteristics.
  • S8P4. Students will explore the wave nature of sound and electromagnetic radiation.
  • d. Describe how the behavior of waves is affected by medium (such as air, water, solids).
    e. Relate the properties of sound to everyday experiences.

This section addresses, in whole or in part, the following Benchmarks for Scientific Literacy:
  • Something can be "seen" when light waves emitted or reflected by it enter the eye - just as something can be "heard" when sound waves from it enter the ear.
  • Vibrations in materials set up wavelike disturbances that spread away from the source. Sound and earthquake waves are examples. These and other waves move at different speeds in different materials.

This section addresses, in whole or in part, the following National Science Education Standards:
  • Energy is a property of many substances and is associated with heat, light, electricity, mechanical motion, sound, nuclei, and the nature of a chemical. Energy is transferred in many ways.
  • Waves, including sound and seismic waves, waves on water, and light waves, have energy and can transfer energy when they interact with matter.

Sound

Sound Waves

Sound travels in the form of waves.

Sound waves are compression waves associated with the potential and kinetic energy of air molecules.

Example: A vibrating guitar string forces surrounding air molecules to be compressed and expanded. This creates a pressure disturbance consisting of an alternating pattern of compressions and rarefactions.

The pressure disturbance then travels from particle to particle through the medium (such as the air), transporting energy as it moves. The amount of energy that is transferred to the medium is dependent upon the amplitude of the sound waves.

The more energy put into the plucking of the guitar string, the larger the wave amplitude.


The Intensity of Sound

The amount of energy that is transported past a given area of the medium per unit of time is known as the intensity of the sound wave.

The greater the amplitude, the greater the rate at which energy is transported through it, and the more intense that the sound wave is.

The intensity of a sound wave is measured in Watts/meter2.

As a sound wave carries its energy through a two-dimensional or three-dimensional medium, the intensity of the sound wave decreases with increasing distance from the source. The decrease in intensity with increasing distance is explained by the fact that the wave is being distributed over a greater surface area.

The intensity varies inversely with the square of the distance from the source. So if the distance from the source is doubled (increased by a factor of 2), then the intensity is quartered (decreased by a factor of 4).


The Decibel Scale

The humans ear is capable of detecting sound waves of extremely low intensity. The faintest sound that the typical human ear can detect has an intensity of 1 x 10-12 W/m2.

The human ear can detect a large range of intensities of sound. Because of this, the scale used to measure the intensity of sound is a scale based on multiples of 10.

The scale used to measure sound intensity is referred to as the decibel scale.

The threshold of hearing is assigned a sound level of 0 decibels (dB). This sound corresponds to an intensity of 1 x 10-12 W/m2.

A sound that is 10 times more intense (1 x 10-11 W/m2) is assigned a sound level of 10 dB.
A sound which is 100 times more intense (1x 10-10 W/m2) is assigned a sound level of 20 dB.

Follow this link to see a list of some common sounds with an estimate of their intensity and decibel level.


Sound Wave Interactions

Waves that meet each other or an object in the environment may interact. There are several types of interactions that sound waves may experience.

Reflection occurs when a wave bounces back after striking a barrier. Reflected sound waves are called echoes. Reflected light waves allow us to see objects.

Diffraction is the bending of waves around a barrier or through an opening. The amount of diffraction a wave experiences depends on two factors: the wavelength of the wave and the size of the barrier or opening the wave encounters. Sound travels around corners because it has relatively larger wavelengths than light. We can hear sounds around corners. We can't see around corners because light has a very small wavelength.


The Speed of Sound

In 1660, Vincenzo Viviani and Giovanni Alfonso Borelli measured the velocity of sound by timing the difference between the flash and the sound of a canon. They determined the speed of sound to be 350 meters per second.

The currently accepted value is 331.29 meters per second at 0 degrees Celsius (32 degrees F).

Light travels much faster than sound. This accounts for why you see a flash of lightning, several seconds before you hear the thunder, unless the storm is very close to you.

By counting the number of seconds between the lightning flash and the sound of the thunder, you can tell roughly how far away the storm is. Every five seconds represents one mile. This results from the fact that light travels at about 186,000 miles per second, and sound travels at only about 1,100 feet per second (or about 0.21 miles per second). If the storm is a mile away, the lightning flash reaches you virtually instantaneously, but it takes about 5 seconds for the sound of the thunder to reach you.

The speed of sound, or Mach 1.0, is dependent on air pressure and temperature.
As altitude changes, so do air pressure and temperature. Air is less dense at higher altitudes, and so it's easier for sound waves to travel. That affects the speed of sound.
Therefore, the speed of sound is different at different altitudes.

The speed of sound at sea level is 760 miles per hour (1,220 km/hr).

Speed of Sound at different altitudes

As your altitude increases, air pressure and temperature decrease, and so does the speed of sound. When air particles are warmer, they move and carry sound waves faster, but when they are cold, they travel slowly, carrying sound waves slowly.

Mach 1.0 is the speed of sound in air. A plane flying Mach 2.0 is flying twice as fast as the speed of sound.

The speed of sound is not a constant. It depends on altitude (or more precisely, the temperature at that altitude).

Example:
A plane flying Mach 1.0 at sea level is flying about 761 miles per hour (1225 km/h, 661 knots).
A plane flying Mach 1.0 at 20,000 feet is traveling about 707 miles per hour (1138 km/hr, 614 knots).
A plane flying Mach 1.0 at 30,000 ft is flying 678 miles per hour (1091 km/h or 589 knots)c.

Speeds below Mach 1 are called subsonic.
Speeds between Mach 0.8-1.2 are called Transonic.
Speeds above Mach 1.2 are called Supersonic.
Speeds above Mach 5 (more than five times the sound barrier) are called hypersonic .


Image courtesy of NASA.


Sonic Booms

Sonic booms are loud explosion-like sounds, that are created when aircraft travel faster than the speed of sound (Mach 1).

Airplanes flying fast enough to create a sonic boom generate shock waves from the nose of their planes. The shock waves represent abrupt changes in air pressure.

Sonic booms can hurt your ears and even break windows. For this reason, supersonic flights are required to fly over open water, above 10,000 feet, and no closer than 15 miles from shore. Any supersonic operations over land must be conducted above 30,000 feet or in specially designated areas.

The intensity of a sonic boom depends on the size of an aircraft. The larger the plane, the more air it displaces, and the stronger the shock waves become. The intensity also depends on the aircraft's altitude. The closer it is to the ground, the less distance the waves will travel before hitting the ground, and the less energy they will have lost. An aircraft's shape and the outside air temperature also influence the formation of shock waves.

The first person to fly an airplane faster than the speed of sound was Chuck Yeager. He did this in 1947 in an Air Force Bell X-1 rocket plane.

In 2002, the Space Shuttle produced a sonic boom when it came in for a landing after a mission.

Fighter jets (like the Blue Angels) break the sound barrier when they rush to perform practice runs or execute battle flights.

blue angels
Blue Angels, 2006, Naval Air Station Atlanta, Marietta, GA.
Photo courtesy of Pamela Gore.

The Concorde is the only passenger jet that routinely flies faster than the speed of sound.


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Content provided by:

  1. Ms. Susan Brooks, Renfroe Middle School
  2. NASAExplores.com website
    http://www.nasaexplores.com/show_912_student_st.php?id=021224105329
  3. NASAExplores.com website
    http://www.nasaexplores.com/show_912_student_st.php?id=021219145024
  4. NASAExplores.com website
    http://www.nasaexplores.com/show_58_teacher_st.php?id=021219144315
  5. NASAExplores.com website
    Sonic Booms
Sound icon from NASA

Page created by Pamela J.W. Gore
Georgia Perimeter College,
Clarkston, GA

Page created November 23 - December 20, 2006