Sound
Sound is produced when an object vibrates. When something vibrates, it transfers kinetic energy to the molecules of the medium surrounding it. This kinetic energy causes the molecules in the medium to vibrate, transferring kinetic energy to new molecules. This transfer of kinetic energy through a medium causes a series of compressions (areas where the molecules are crowded together) and rarefactions (areas where the molecules are spread out). These compressions and rarefactions move through the medium, away from the original vibration. When a series of compressions and rarefactions move through a medium, it is called a longitudinal wave. Therefore, sound is a longitudinal wave.
When you speak, air moves from your lungs past your vocal cords, causing them to vibrate. Your vocal cords consist of two flaps of tissue that can move closer to or farther away from each other. When they move closer together, they cause a compression in the air between them. When they move farther apart, they cause a rarefaction. The continuous vibration as air moves over your vocal cords and causes a series of compressions and rarefactions, in other words, sound. A stereo speaker operates in the same manner. The speaker vibrates back and forth, causing the air in front of it to vibrate. This vibration is the sound coming from your stereo.
The speed of the sound waves depends on several factors. These factors are the temperature, elasticity, and density of the medium through which the sound waves travel. Sound travels slower at lower temperatures. For example, the speed of sound in air at 32 °F (0°C) is 0.2 mi/sec (0.32 km/s), compared to its speed in air at 77°F (25°C), 0.21 mi/sec (0.35 m/s). Sound travels most rapidly in solids and slowest in gases. The molecules in a solid bounce back faster when vibrated, in other words, are more elastic, than liquids or gases. The speed of sound is slowest in a material with a higher density. For example, solid nickel is denser than solid iron. The speed of sound in nickel is 3 mi/sec (4.8 km/s), while the speed of sound in iron is 3.2 mi/sec (5.1 km/s). Therefore, the speed of sound would be greatest in a high temperature, low density solid.
Since sound is actually a series of longitudinal waves, sound displays the wave properties of amplitude, frequency, and wavelength. The amplitude of a wave is the height of a wave crest above its origin. The amplitude determines the loudness of a sound. The frequency is the number of wave crests that travel past a point per second. The wavelength is the distance between successive wave crests. Frequency and wavelength are inversely proportional to each other and they determine the pitch of a sound.
The pitch of a sound is how high or low the sound is. As an object vibrates faster, it will create a sound of higher pitch. In other words, the pitch depends on the frequency of the sound waves. The frequency of sound waves is measured as the number of waves, or cycles, per second, also known as hertz (Hz). One hertz is equal to one cycle per second. The high note of an opera singer may have a frequency of 1,000 Hz, whereas the low sound of electricity "humming" has a frequency of around 60 Hz.
If all vibrating objects produce sound, why is there no audible sound when we wave our hand in the air? The human ear can only hear sounds in a relatively narrow range of frequencies, from about 20-20,000 Hz. A sound with a frequency lower than 20 Hz is called infrasonic, and a sound with a frequency higher than 20,000 Hz is called ultrasonic. The sound produced when you wave your hand is infrasonic, or beyond our hearing capacity. Elephants can hear and produce infrasonic sounds below 20 Hz. Dogs can hear sounds up to 35,000 Hz and cats can hear sounds up to 65,000 Hz. This is why your dog can hear a dog whistle, which produces an ultrasonic sound, and you cannot.
The amplitude of a sound wave determines how loud a sound is, or its intensity. The larger the amplitude, the more energy is carried by the wave, and the higher the intensity of the waves. High-intensity waves are louder than low-intensity waves. The relative intensity of sounds can be measured using the decibel (dB) scale. A sound with an intensity of 0 dB can barely be heard, while a rock concert produces sound with an intensity of around 120 dB. Any sound over 120 dB can cause pain and hearing loss in humans.
Waves can combine with other waves to produce interference. The interference may have an additive effect, called constructive interference, or a deleterious effect, called destructive interference. In sound waves, constructive interference increases its intensity, making the sound louder. Destructive interference decreases the intensity, making the sound softer. Engineers construct auditoriums and concert halls so the sound waves coming from the stage bounce off of the walls, creating constructive interference. These engineers are especially careful to avoid floor plans that cause the sound waves to bounce off of the walls creating destructive interference. Destructive interference can create the absence of sound, or dead spots, in a room. Engineers who design buildings with this phenomenon in mind are called acoustical engineers.
Sound waves can be used for our benefit in ways other than to create pleasing music or soothing sounds such as a babbling brook. Ultrasonic waves are used in Sound Navigation and Ranging, or SONAR, systems to explore deep-sea ocean floors. A research vessel can send sound waves down into the water. The sound travels to the ocean floor and bounces back up to the surface. By using the speed of sound in salt water and the time it takes for the sound to travel to the floor and back, researchers can calculate the depth of the floor at that point. Using this procedure, maps of the ocean floors have been created. The auto focus feature on many cameras also uses SONAR to determine how far away the object to be photographed is from the camera lens.
Ultrasonic waves can also be used to clean delicate objects such as fine jewelry. The object is placed in a liquid and ultrasonic waves are sent through the liquid, creating high-speed vibrations that knock dirt and debris from the surface of the object. Physicians can also use ultrasonic waves to create a picture much like an x ray or to send vibrations to damaged muscle, aiding the healing process.
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