About Sound
Sound is a compression waveform that moves through air or other materials. Sound waves are created by the vibration of some object and are detected by causing a sensor to vibrate. Sound has the standard characteristics of any waveform.
Questions you may have about sound are:
- What is sound?
- What are the characteristics of sound?
- What are some ways sound is detected?
A 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 them from penetrating the material.
Not in vacuum
Sound does not travel through a vacuum or in outer space.
Compression wave
Sound is created by back-and-forth vibrations, thus creating compression waves in the material where sound travels.
Characteristics of sound
A sound wave has characteristics just like any other type of wave. It has an amplitude, a velocity, a wavelength, and a frequency.
Amplitude and wavelength in a waveform.
Note that this illustration shows a transverse wave. It is difficult to show amplitude and wavelength in a compression wave, but the essence is the same.
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.
Velocity of sound
The speed or velocity of sound in air is approximately 330 m/s. The speed varies with the density of air, such that sound travels slower at higher altitudes or on days when the air pressure is lower.
Note: The difference between Speed and Velocity is that Velocity usually includes direction of travel.
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.
Use to measure distance
Since the speed of sound in air is approximately 330 metres per second, you can use that fact to approximate how far lighting is by simply counting the seconds it takes the sound to reach you after the lightning bolt is seen.
If the lightning was about 1 mile away, it would take the sound traveling at 330 metres per second approximately 5 seconds to reach you. The amount of time it takes the light to reach you is negligible, since light travels at 186,000 miles per second.
In different materials
The speed or velocity of sound depends on the density of the material in which it is travelling. The greater the density of the material, the faster the velocity and hence the faster the speed of sound. Sound travels faster in water than in air, and it travels much faster in steel.
In outer space
Since there is no matter in a vacuum or outer space, its density is zero and there is no sound there. Sound requires matter to vibrate.
Wavelength and frequency
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. Since that is so difficult to draw, we usually refer to the frequency of sound, instead of its wavelength.
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, and the sound we hear will be in a higher pitch.
Detecting sound
When sound hits an object, it can cause the object to vibrate. This leads to the method to detect sound.
The main way you detect or sense sounds is through your ears. 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. The sound vibrates a membrane, which creates an electric signal that is amplified and recorded.
