PhysicsLAB Resource Lesson
Introduction to Sound

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Sound waves are mechanical, compression waves which are in general longitudinal in nature - meaning that the particles vibrate parallel to the direction of the wave's velocity.

Acoustics Animations © Dr. Dan Russell, 1999
The above animation was created using a modified version of the
® Notebook "Sound Waves" by Mats Bengtsson.
The vibrations of a tuning fork provide a simple means of explaining the characteristics of sound waves. When the tines of the fork move in towards each other, the air molecules just outside of the tines are reduced in density, producing a region of low pressure called a rarefaction. Conversely, at that same moment, the molecules between the tines are now at a higher density, producing a region of higher density called a condensation, or compression. As the tines vibrate, these regions of lower and higher density propagate away from the fork, resulting in a compression wave with a frequency that matches that of the tuning fork. A sound's frequency is totally dependent on the motion of the source creating it -- that is, frequency is independent of the medium through which the sound travels.
This periodic nature can be graphed as sinusoidal disturbance, provided the amplitudes are measured in terms of the changes in the density or pressure in the medium. Compressions (also called condensations) are regions of high pressure and would be represented as crests; while, rarefactions, or low pressure, would be represented as troughs. The frequency of sound, as interpreted by our ears, is called the sound's pitch. If only a single disturbance creates a sound wave, such as a ball crashing into a bat, or a tree falling in a forest, that disturbance is not repetitive and does not result in a periodic wave, it would instead be called a pulse.
Amplitudes are measured in terms of the increase or decrease in mass density or air pressure. Normal atmospheric pressure is measured in Pascals (Pa) where 1 atm = 1.01 x 105 Pa = 760 torr (mm Hg).
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When two or more waves travel through the same medium at the same time, their waves actions will interfere with each other the same as any other types of waves.  If the sounds are close to the same frequencies, we can hear this interference pattern in the form of regular beats - pulsating regions of loud (high amplitude, constructive) and soft (low amplitude, destructive) interference. The pitch of these beats is the average of the two original wave frequencies and the number of beats occurring each second is the difference in their frequencies.
The range of human hearing stretches between 20-20000 hertz. Sounds with frequencies below 20 hertz are called infrasonic and those above 20,000 are called ultrasonic. Ultrasonic waves are used in sonar and in medicine to measure sizes and distances to obstacles. Dogs in general can hear as high as 45 khz, while cats and bats can hears frequencies as high as 75 - 100 khz.
The speed of a compression wave travels depends on the medium through which it is traveling. The greater the elasticity of the medium, the faster sound will travel through it. Metals carry sound waves faster than water which transfers them faster than air. Sound travels 10-15 times faster in metals than in gases; while in liquids, it travels 4-5 times faster than in dry air. The speed of sound in dry air can be calculated with the equation vw = 331 + 0.6 T where T is the temperature in Cº. As the humidity in the air builds, more lighter-mass water molecules are present and sound will travel faster than it would at the same temperature dry-air.
In the same medium, all sound waves travel at the same speed. For example, suppose you are at a concert. As the orchestra plays, you hear all of the notes as they are played - except for the time required for the music to reach your location. The waves played by each instrument arrive in sync exactly as they are performed according to the musical score. Would the wind blowing make a difference in the frequencies that you heard? Look at this NextTime question to figure out the answer.
As sound waves travel through a medium, they lose energy to the medium and are damped. The molecules in the medium, as they are forced to vibrate back and forth, generate heat. Consequently, a sound wave can only propagate through a limited distance. In general, low frequency waves travel further than high frequency waves because there is less energy transferred to the medium. Hence the use of low frequencies for fog horns. Although damped waves have decreasing amplitudes, their wavelength and period are unaffected.
The distance that sound wave can travel and be heard is also a property of the air's temperature gradient. When the air near the ground is warmer than the air at higher elevations, then sound refracts (or bends) upward. Similarly, when the air is cooler near the ground (or water surface) than at higher elevations, the sound refracts (or bends) downward.
This reference page contains more detailed information
and several interesting animations on the refraction of sound.
When sounds reflect off of a surface, that reflection is called an echo. If the surface is smooth, then most of the wave's energy is reflection back in a single direction. If the surface is rough, then the reflected waves are scattered in a multitude of directions. We call the multiple, additional reflections heard from these neighboring surfaces reverberations.

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