Sound

Prepared by: Victor R. Oribe

Nature of Sound

Sound waves are produced by longitudinal vibrations of molecules in gases and other media.

This means that as a wave travels through a medium, the molecules of the medium oscillate to and fro.

The energy of this vibration is carried by the involved waves, which is brought to our brains for interpretation as sound.

Sound waves that travel in air come in contact with our eardrums causing it to vibrate.

Can sound waves travel in solid?

Can sound waves travel in liquid?

If yes give some situations or examples.

Transmission of Sound Waves

The capacity of matter to transmit sound depends upon its elasticity.

Elastic solid transmit sound well.

Inelastic solid like cloth, rubber, plastic and sponge transmit sound poorly

Liquids, in general, are better conductors of sound than gases.

Solid is considered as the best sound transmitter.

Why is solid a better sound transmitter than liquid and gas?

Waves are transmitted from one place to another by the vibration of particles of the medium.

We know that particles in a solid are closer to each other than the particles in liquid and in gas.

Therefore, sound waves are transmitted faster and more efficiently in solid than in liquid and in gas.

The Velocity of Sound

Thunder is heard few second after the flask of lightning.

An observer standing at a distance from a gun will see the flash of light before hearing the sound of the gunshot

An observer standing at a distance from a fireworks display will see the flash of beautiful lights before hearing the sound of fireworks explosions.

These show that sound travels slower than light.

The velocity of sound varies in different media.

It depends on the elasticity and density of the medium.

In general, the more elastic the medium, the faster the sound can travel.

The sound can travel faster in a solid medium than in liquid or gas because the elasticity of solid is greater than that of liquid and gases.

Assignment:

1. Why sound not transmitted in a vacuum?

2. A distant lightning can be seen several seconds before the accompanying thunder is heard. Why?

Properties of Sound Waves

1. Intensity and Loudness

The quantitative description of the intensity of sound is determined by a sound level scale called the decibel scale.

Sound intensity is the amount of sound energy flowing each second through a unit area perpendicular to the direction of propagation.

2. Reflection of Sound WavesA sound is reflected when it strikes another body.

The reflected wave is called an echo.

The reflecting surface has to be at least 17 meters away from the source in order for the echo to be distinct from the original sound.

If the reflecting surface is less 17 meters, the reflected sound wave just blends with and strengthens the original sound.

The reflection of sound wave is used in determining ocean depths and the altitude of an airplane. A sound wave is sent out

from a ship toward the sea floor.

A wave is reflected from the bottom of the sea and the reflected wave is detected by a receiving device.

Using the recorded elapsed time and velocity of sound in water, the ocean depth may be computed.

The airplane altitude, a sonic altimeter is used.

Sonic Altimeter is consists of a sound emitter and a record that measures the time interval between the emission of sound and reception of the echo.

The product of the time interval and speed of sound in air at a given temperature divided by 2 is the plane’s altitude.

Reflected waves can also cause inconvenience to listeners.

Echoes from smooth curved walls or ceiling can increase the intensity of sound in certain parts of the room.

In some cases the problem is due to reverberation (repeated echoes).

2. Refraction of Sound WavesLike all types of waves, sound also undergoes refraction when a change in medium or change in property of medium takes place.

On a clear sunny day, the earth’s surface is hot.

This causes the temperature of the air just above the surface of the earth to rise.

The temperature of the lower layer of air is higher than the layers above, hence sound should be traveling faster near the surface of the earth.

This difference in the speed causes the refraction of sound waves away from the surface.

On a clear night the layer of air nearer the earth’s surface gets cold faster; hence, the lower layer of air is colder.

As a result, sound travels faster at the higher layer than at the lower layer and is refracted toward the earth’s surface.

The sound traveling with air in the same direction is bent toward the earth’s surface.

The sound traveling against the moving air is bent away from the earth’s surface.

Questions:1. What is an echo?2. When an echo considered useful?3. What happened to the sound waves when

they pass from one medium to another of different density?

4. Why do sound waves benda. away from the surface of the earth

during daytimeb. toward the earth’s surface during

nighttime?5. Differentiate intensity from loudness.

Kinds of Sound Waves

1. Audible Sound Waves

Sound waves that can be detected by human ear.

A sound waves with frequencies in the range of 20 to 20,000 Hz can be detected by human ear.

This range varies from one individual to another.

2. Ultrasonic WavesSound waves with a frequencies above 20,000Hz are called ultrasonic waves.

These waves can also reach human ear but are not detected.

This ultrasonic sound can be detected by some animals

Dogs can hear sounds as high as 50,000 Hz, while bats can hear as high as 100,000 Hz

Ultrasonic Waves in the field of Medicine

Ultrasonic waves are used to probe human organs instead of X-rays.

Unlike X-rays, ultrasonic waves does not damage human tissue.

3. Infrasonic WavesSound waves with frequencies below 20 Hz.

Common source of this waves are vibrating heavy machines, earthquake, thunder, and volcanic eruption.

These waves are audible but can cause damage to the human body.

How Much Have You Learned?

1. Differentiate ultrasonic from infrasonic.

2. How is ultrasonic waves utilized in the field of medicine?

Detection of Sound

Human and animals have a very sensitive detector of sound – the Ear.

The cross-section of human ear is almost similar to that of other vertebrates.

The ear, which is the peripheral auditory system, is divided into three parts:

The outer ear called the pinna, collects the sound waves and concentrates them into ear canal to a limited extent.

This canal transmits the sound waves to the eardrum.

It also protects the eardrum from shock and intrusion by external objects.

Air vibration set the eardrum membrane in motion which in turn causes the three little bones (hammer, anvil, and stirrup) to move.

These three little bones convert the small amplitude vibrations of the eardrum into larger-amplitude oscillations and transfer them to the inner ear through the oval window.

Behind this oval window is a snail-shaped, liquid-filled organ called cochlea to where auditory nerves are connected.

The larger amplitude oscillations create travelling waves that are transformed into impulses sent to the brain through the auditory nerves.

The brain relates and interprets the sound heard to those previously experienced.

Hearing Damage1. Noise with an intensity level above 120dB is considered as an undesirable pollutant in our environment.

2. Exposure to loud sounds (80dB or above) for a long period can cause temporary hearing loss.

3. Overexposure to amplified rock music and noise produced by machineries in factories and other industrial establishment has been mostly the cause of hearing damage.

4. Our human ear has a built-in muscular control in the middle ear.

This limit the potential damage to the ear caused by sustained loud sound.

But our ear cannot react rapidly to protect itself against sudden bursts of short but very loud sounds like gunshots or bomb explosions.

If the sound produced is too loud, it can force an ear mechanism to go beyond its elastic limit, in this case, the hearing damage becomes permanent.

5. Permanent hearing loss may also be caused by physical damage to ear due to diseases, drugs with side effects, or the natural aging process.

6. Infection of the middle ear due to unhygienic practices can result to hearing damage.

7. Hearing damage can also occur to a fetus if the pregnant mother suffered from measles or rubella within the first trimester of pregnancy.

Rubella can cause malformation of the parts of the fetus’ inner ear.

8. Drugs such as streptomycin have been found to cause permanent hearing loss to small children.

Ringing in the ears while the child is under medication is a symptom of such hearing damage.

Because of this, the use of this drug has been carefully controlled.

9. A person may also permanently lose his or her sense of hearing due to normal aging of the auditory system.