Overview of Sound Waves
by Ron Kurtus
Sound waves can be classified into three groups, according to their frequency ranges. Infrasound consists of frequencies below 20 Hz, audible sound consists of frequencies between 20 Hz and 20,000 Hz (20 kilohertz), and ultrasound consists of frequencies over 20 kHz.
Each type of sound is a series of longitudinal or compression waves that move through air or other materials. Sound does not travel in a vacuum.
Like any waveform, sound has the characteristics of wavelength, frequency, amplitude, and speed or velocity. Sound waves are created by the vibration of some object, like the cone in a radio loudspeaker. The waves are detected when they cause a detector to vibrate.
Questions you may have include:
- What is a description of sound?
- What are the characteristics of sound waves?
- How is sound created and detected?
This lesson will answer those questions. Useful tool: Units Conversion
Sound is a mechanical vibration
Sound is a regular mechanical vibration that travels through matter as a waveform. It consists of longitudinal or compression waves in matter.
Travels through matter
Although it is commonly associated in air, sound will readily travel through many materials, such as water and steel. Some insulating materials absorb much of the sound waves, preventing the waves from penetrating the material.
Does not travel in vacuum
Because sound is the vibration of matter, it does not travel through a vacuum or in outer space.
When you see movies or TV shows about battles in outer space, you should only be able to see an explosion but not hear it. The sounds are added for dramatic effect.
However, note that in outer space, there are actually some widely-spaced atoms and molecules floating around. But since they are so far apart, regular wave motion would not be great enough to detect.
Sound waves are different than light waves
Light and radio waves are electromagnetic waves. They are completely different than sound, which is vibration of matter. Electromagnetic waves are related to electrical and magnetic fields and readily travel through space.
Sound is a longitudinal wave
The rapid back-and-forth vibration of an object creates the longitudinal or compression waves of sound. Longitudinal waves are waves that oscillate in the same path that the sound wave is moving. This is different than the up and down or transverse motion of a water wave.
(See General Wave Motion for more information.)
Transverse wave (water wave)
Longitudinal or compression wave (sound)
The illustrations above show a comparison of a transverse wave—such as a water wave—and the compression wave of sound.
Characteristics of sound
A sound wave has the same characteristics as any other type of waveform. It has wavelength, frequency, velocity and amplitude.
Wavelength is the distance from one crest to another of a wave. Since sound is a compression wave, the wavelength is the distance between maximum compressions.
Speed or velocity
The sound waveform moves at approximately 344 meters/second, 1130 feet/sec. or 770 miles per hour at room temperature of 20oC (70oF).
(See Speed of Sound in a Gas for more information.)
The frequency of sound is the rate at which the waves pass a given point. It is also the rate at which a guitar string or a loudspeaker vibrates.
The relationship between velocity, wavelength and frequency is:
velocity = wavelength x frequency
Since sound is a compression wave, its amplitude corresponds to how much the wave is compressed, as compared to areas of little compression. Thus, it is sometimes called pressure amplitude.
Creating and detecting sounds
Creating and detecting sounds are similar effects, but opposite. They demonstrate the duality of nature.
Whenever an object in air vibrates, it causes longitudinal or compression waves in the air. These waves move away from the object as sound. There are many forms of the vibration, some not so obvious.
The back and forth movement of a loudspeaker cone, guitar string or drum head result in compression waves of sound. When you speak, your vocal cords also vibrate, creating sound.
Blowing across a bottle top can also create sound. In this case, the air inside the bottle goes in a circular motion, resulting in sound waves being formed. Wind blowing through trees can also create sound this indirect way.
Sound can also be created by vibrating an object in a liquid such as water or in a solid such as iron. A train rolling on a steel railroad track will create a sound wave that travels through the tracks. They will then vibrate, creating sound in air that you can hear, while the train may be a great distance away.
When a sound wave strikes an object, it can cause the object to vibrate. This leads to the method to detect sound, which requires changing that vibration into some other type of signal—usually electrical.
The main way you detect or sense sounds is through your ears. The sound waves vibrate your ear drum, which goes to the inner ear and is changed to nerve signals you can sense.
You can also feel sounds. Stand in front of a stereo or hi-fi loudspeaker on at full volume, and you can feel some of the vibrations from the music.
There are mechanical devices that detect sounds, such as the microphone. The sound vibrates a membrane, which creates an electric signal that is amplified and recorded.
(Also see Detecting Sounds for more information.)
Sound consists of longitudinal or compression waves that move through air or other materials. It does not travel in a vacuum. Sound has the characteristics of wavelength, frequency, speed and amplitude. Sound waves are created by the vibration of some object and are detected when they cause a detector to vibrate.
Sound out problems to solve them
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Overview of Sound Waves