Sound is a longitudinal wave that travels through a material medium. Sound waves are used to discuss those waves that have frequency to produce hearing in humans. Humans are able to hear sound between the frequencies of 20 to 20,000 Hz. This is called the audible region. Sound below the audible region is called infrasonic and those above the audible region are called ultrasonic.
Sound can travel through any material, but the sound we hear travels through the air. The speed of a sound wave depends on the density of the medium, and the ease with which the medium can be compressed. The speed of sound in air is 332 meters per second at zero degrees Celsius, and increases by about 0.6 m/s for each degree increase in air temperature. Sound can travel through liquids as well as solids. The speed of sound is greater in solids and liquid than in gasses. Sound cannot travel through a vacuum, because there are no particles to vibrate.
Sound waves share the properties of other waves. They reflect off hard surfaces. These reflected sound waves are called echoes. Sound waves can also be diffracted, spreading outwards when passing through narrow places. Two or more sound waves can also interfere with each other causing dead spots.
Sound as Mechanical waves
Since sound waves are disturbances which are transported through a medium by the mechanism of particle to particle interaction, they are therefore called mechanical waves. Mechanical waves are waves which require a medium in order to transport their energy from one location to another. There are three processes that are involved in the creation of sound waves. First, there exist a medium which carries the disturbance from one location to another. Because mechanical waves rely on particle interaction in order to transport their energy, they cannot travel through regions of space that do not have particles. This medium could be the air or any other material. The medium is a series of interconnected and interacting particles. Secondly there must be a source of disturbance. This is usually a vibrating object that disturbs the first particle of the medium. Thirdly the sound wave is transported from one location to another by means of particle to particle interaction. If the medium is air then one air particle is displaced from its resting position, it then exerts a push or pull on its nearest neighbor causing them react to the force by moving from their original position. Mechanical waves cannot travel through a vacuum.
Newton’s Laws of motion and conservation of energy principles are also applied to the motion of waves. Waves including waves such as sound waves, water waves and waves that are formed on a spring transmit energy are classified as mechanical waves. Mechanical waves serve as models for studying waves such as electronic waves and other waves that cannot be observed.
Sound as Longitudinal waves
Sound waves in the air are longitudinal waves because particles of the medium, air, water, or string through which they travel vibrates parallel to the direction which the sound moves. When the air in the medium is disturbed it pushes on the surrounding air molecules forward. This causes the air to compress into a small region of space. When the medium moves in the reverse direction it causes the air pressure to be lowered, causing the air molecules to move back. This back and forth movement is imparted to other air molecules in the medium.
The recognition that sound wave is a longitudinal wave arose from the fact that it propagates in air. An essential characteristic of a longitudinal wave which makes it different from other waves is that the particles of the medium move in a direction parallel to the direction of the source of energy.
Because of the longitudinal motion of the air particles, there are regions in the air where the air particles are compressed together and in other regions the air is spread apart. The compression regions are regions of high pressure, while the regions of low pressure are called rarefactions.
Sound as pressure waves
Since a sound wave consists of repeating pattern of high and low pressure regions moving through a medium, it is sometimes referred to as pressure waves. A sound detector can differentiate between regions of high pressure; this corresponds to the arrival of compressed air, low pressure region corresponding to rarefaction and region of normal pressure. The plot of the fluctuations in air pressure versus time will produce a sine curve. The peak points of the sine curve represent the compression periods, the low points represent the rarefactions, and the zero points correspond to the normal periods
The Properties of Sound and how it is measured
Sound, like other waves travels at a certain speed and has the same properties of frequency and wavelength. Sound travels at a definite speed and travels slower than light.
Pitch and Frequency
Pitch is a human perception resulting from the sensing of acoustic energy. It is the brain’s response to the frequency of sound. The pitch is a pure sound a sinusoidal wave with a single fixed wavelength. The higher the frequency the higher the pitch.
Intensity and the decibel
The intensity of sound is defined as the power transmitted by the sound through a unit of area. This is the rate at which the energy being transported by the wave flows through a unit area perpendicular to the direction of travel of the wave.
Intensity is given as:
Intensity = power/ area.
Intensity is measured in units of watts/ meter squared. The human ear responds to intensities covering the range from 10
to more than 1 W/m
. Intensities are scaled by factors of ten. 10
called 0 bel is the reference point, is barely audible and was named after Alexander Graham Bell. A sound ten times as intense has an intensity of 1 bel or 10
. This is 10 decibels. The threshold of hearing is given as I
= 1.0 × 10
The sound --level is defined as:
How Sound travels (The speed of sound)
Sound like all waves travels at a certain speed and therefore has the properties of frequency and wavelength. The relationship of speed and wavelength is given by
v = f Λ where v is the speed of sound, f is the frequency and Λ is the wave length. The wave length of sound is the distance between adjacent identical parts of a wave. The frequency is the same as that at the source and is the number of waves that passes a point per unit of time. The speed of sound is independent of the frequency.
The speed of sound is dependent on the medium through which it travels. The speed of sound in a medium is determined by a combination of its rigidity and its density. The more rigid the medium the faster the sound travels. The greater the density of the medium, the slower the speed of sound. The speed of sound in air is low because air is compressed. The speed of sound is greater in liquids and solids than it is in gases. This is because liquids and solids are more compressed.
The speed of sound is affected by temperature in a given medium. The speed of sound in the air at sea level is given as
The speed of sound in gases is related to the average speed of molecules in the gas
Using Waves Speed to determine Distance
At normal pressure and a temperature of 20 degrees Celsius a sound wave will travel at approximately 343 m/s or 750 miles / hour. The speed of sound waves is much slower than light waves. Light waves travel at a speed of 300,000,000 m/s or 900,000 times the speed of sound. This difference explains why a light wave is seen before the sound wave (the thunder) is heard. The time delay between the arrival of the light wave and the arrival of the sound wave can be used to calculate distance of a storm. The model distance = v (velocity) (time) can be used to determine the distance that the sound wave has traveled.
The time delay between the production of sound and its reflection of the sound off a barrier can be used to calculate the speed at which sound travels. The time delay between the shout in a canyon and the return of the echo represents the time the shout travels the round trip distance. This time increment can be used to calculate the distance to the canyon wall.
Earthquakes occur as a result of a sudden release of energy in the Earth’s crust that creates seismic waves. These waves are caused by slippage along faults in the Earth’s interior. These faults can be as deep as 400 miles in the interior. Some of the tremors loose their energy before they reach the Earth’s surface. Where there is a major shift in an area where there are faults, a great deal of pressure is released. The rocks move into a new position. When the movement stops, the waves carry the energy in all direction throughout the Earth. The point at which the initial movement of the rock occurs is called the focus or the hypocenter.
Seismic energy travels through the crust in the form of waves. There are two basic kinds of seismic waves. Body waves and surface waves. Body waves travel outward in all directions, including downward, from the quake focus. Surface waves are confined to the upper crust. They travel parallel to the surface like ripples on the surface of a pond, and are usually slower than body waves.
As the earthquake waves move outward from the focus, the energy follows. Therefore the earthquake is strongest at the focus. Earthquake waves are of two types, longitudinal and transverse waves. Transverse waves are referred to as S waves move the Earth from side to side. Longitudinal waves, called P waves push and pull the Earth.
During an earthquake the body waves are the first to arrive. The fastest are the P waves. These P waves moves as an acoustic wave in the air. The S- waves are the second waves to arrive. The S- waves arrive with a sudden jolt. The last waves to arrive are the surface waves. These waves occur with the up and down and back and forth and side to side movements. These movements make the ground appear to roll.
Seismic waves weaken the farther away from the source. The strongest quakes will occur at the epicenter, the point above the focus of the earthquake. The focus can be either a few miles below the ground or can be as deep as 435 miles below. Earthquakes not only present themselves as ground shaking. They can trigger landslides that can cause vast areas to be covered by the sea blocks of the earth’s crust can shift along fault lines either horizontally or vertically. Loose soil or sand can be shaken so hard that individual grains separate, turning the earth into a soft liquid. This situation is referred to as liquefaction.
Since earthquakes are sound waves in the earth it demonstrates how the speed is dependent on the rigidity of the medium. The P waves of an earthquake travel faster in granite than the speed of the S- waves. Both the P waves and S waves travel slower in less rigid material. P waves have speeds of between 4.00 to 7.00 km /s, and s waves have speeds of between 2.00 to 5 km/s. The P waves travel ahead of the S waves as they traveled through the earth. The time between the P-waves and the S --waves is used to determine the distance of the epicenter of the earthquake.
The Doppler Effect
The Doppler effect is an alteration of the observed frequency of a sound due to motion of either the source or the observer. The actual change in frequency is called the Doppler shift. For a stationary observer and a moving source, the observed frequency f is
is the frequency of the source, v
is the speed of the source, and v
is the speed of sound. The minus sign is used for motion toward the observer and the plus sign for motion away. For a stationary source and a moving observer the observed frequency is given as:
where v is the speed of the observer.
In general a Doppler effect is experienced when there is motion between the source of the sound and the observer. If the source of the sound and the observer is moving towards each other in opposite directions the frequency heard by the observer is higher than the frequency of the sound. When the source of the sound and the observer are moving away from each other the frequency heard by the observer is lower than the source frequency.
What causes the Doppler Effect or Shift?
The sound waves emitted by the point of source sound spread out spherically. If the source is stationary, then all the spheres are centered on the same point. Observers on either side of the source will experience the same frequency and the wave length between them are the same. If the source is moving, then each disturbance moves out in the sphere from the point of the source, but the point of the source moves. This movement causes the wavelength to be closer together on one side and farther apart on the other side. The wavelengths are closer together in the direction which the source is moving and farther apart in the opposite direction. If the observer is moving, then the frequency at which they hear the sound changes. If the observer is moving towards the source then the frequency is higher. If the observer is moving away from the source then the frequency is lower.