Sound Waves
by Ron Kurtus (revised 26 June 2005)
Sound is a series of compression waves that moves through air or other materials. These sound waves are created by the vibration of some object, like a radio loudspeaker. The waves are detected when they cause a detector to vibrate. Your eardrum vibrates from sound waves to allow you to sense them. Sound has the standard characteristics of any waveform.
Questions you may have include:
- What is sound?
- What are the characteristics of sound?
- How is sound created and detected?
This lesson will answer those questions. There is a mini-quiz near the end of the lesson.
Useful tools: Metric-English Conversion | Scientific Calculator.
Sound is waveform in matter
Sound is a waveform that 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.
Some atoms in space
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 different than light waves
Also note that 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 compression wave
The back-and-forth vibration of an object creates the compression waves of sound. The motions of a loudspeaker cone, drumhead and guitar string are good examples of vibration that cause compression waves. 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)
Compression Wave (sound)
The illustration above shows a comparison of a transverse wave such as a water wave and the compression wave sound wave.
Characteristics of sound
A sound wave has characteristics just like any other type of wave, including amplitude, velocity, wavelength and frequency.
Amplitude
The amplitude of a sound wave is the same thing as its loudness. Since sound is a compression wave, its loudness or amplitude would correspond to how much the wave is compressed. It is sometimes called pressure amplitude.
Decibel
A common measurement of loudness is the decibel (dB). It is really 1/10 of a bel, which was named after the inventor of the telephone, Alexander Graham Bell. It is a complex unit that varies as the ratio of the logarithms of loudness.
Decrease in loudness
A sound wave will spread out after it leaves its source, decreasing its amplitude or loudness. The amplitude decreases as the square of the distance from the source. Also, if there is some absorption in the material, the loudness of the sound will decrease as it moves through the substance.
Speed or velocity of sound
The speed or velocity of sound in air is approximately 344 meters/second, 1130 feet/sec. or 770 miles per hour at room temperature of 20oC (70oF). The speed varies with the temperature of air, such that sound travels slower at higher altitudes or on cold days.
Note: The difference between speed and velocity is that velocity usually includes direction the of travel. We'll interchange them here, but in some cases the distinction is important.
A jet plane traveling at the speed of sound would be moving at about 680 mph at sea level. At very high altitudes, the speed required would be much lower.
(See Speed of Sound in a Gas for more information.)
Wavelength
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.
Frequency
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 loud speaker vibrates. Frequency is also called the pitch of a sound. It is called the note in musical sounds.
Relationship
The relationship between velocity, wavelength and frequency is:
velocity = wavelength x frequency
Since the velocity of sound is approximately the same for all wavelengths, frequency is often used to better describe the effects of the different wavelengths.
Pitch
The pitch or note of a sound that we experience is determined by its wavelength or its frequency. The shorter the wavelength, the higher the frequency becomes, and the higher the pitch that we hear.
Creating and detecting sounds
Creating and detecting sounds are similar effects, but opposite. They demonstrate the duality of nature.
Creating sound
Whenever an object in air vibrates, it causes 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.
Detecting sound
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.)
Summary
Sound is a compression waveform created by the vibration of some object. Sound moves through air or other materials. The characteristics of sound are that it has amplitude, wavelength, frequency and velocity. Sound must cause another object to vibrate to be detected.