By: Akina StaffordForm: 5E

Sound WavesSound waves are longitudinal waves. The

particles carrying the sound vibrate about fixed positions parallel to the direction in which the sound travels.

Production and PropagationSound waves travel through solids,

liquids and gases, i.e. through any medium which contains atoms or molecules.

Sound waves are always generated when an object or medium vibrates. Examples of objects that generate familiar sounds are a drum membrane, a guitar string and our vocal cords. The vibrating object then transmits the vibrations to the surrounding medium and in this way the wave has been generated and starts to propagate.

Example: Drum Membrane

The diagram above shows a drum with its vibrating membrane generating compressions and expansions (rarefactions) in air and therefore, generating sound waves.

How sound is Transmitted

Because sound is transmitted as a compression wave, it can travel only through a medium that contains particles that can be forced closer together or further apart. Sound cannot be transmitted in a vacuum because there are no particles to push closer together or spread out.

Example:

A ringing bell is placed in a jar and air inside the jar is evacuated. Once air is removed from the jar, the sound of the ringing bell is no longer heard. The clapper is seen striking the bell; but the sound which it produces cannot be heard because there are no particles inside of the jar to transport the disturbance through the vacuum.

Wave Parameters

The success of any operation conducted in the ocean environment may depend on the height of the seas, the direction of the seas and the wave period. Waves, in general, are described by wave height, wave length, and wave period. Wave direction is another important aspect used to describe waves.

Pitch

Pitch = Frequency of Sound

The perceived pitch of a sound is just the ear’s response to frequency, i.e., for most practical purposes the pitch is just the frequency. Rapidly vibrating objects produce sounds of high frequency or pitch. Slowly vibrating objects produce sounds of low frequency or pitch.

Example:

The sound from a violin is of higher pitch than that from a double bass because the sound from the violin makes the eardrum vibrate at a faster rate. Sounds from the violin have higher frequencies.

The Violin

Loudness

Sound loudness is a subjected term describing the strength of the ear’s perception of a sound. The loudness of a sound depends on: the amplitude of the vibrator producing it- the greater the amplitude of the vibration, the louder the sound and the ‘amount of energy’ reaching our ears.

Speed of Sound

The speed at which sound propagates (or travels from its source) is directly influenced by both the medium through which it travels and the factors affecting the medium, such as altitude, humidity and temperature for gases like air. To estimate the speed of sound the principle method is used: speed =

distance sound travels time taken

Speed of sound of some materials Medium:

Air (dry)WaterIronConcreteCarbon dioxide

Speed of sound at 0ºC/ms-1:

330 1400 2700 5000 265

Example: Thunder is the sound

made by lightning. The sudden increase of pressure and temperature by the lightning produces rapid expansion of the air surrounding and within a bolt of lightning. In turn, this expansion of air creates a sonic shock wave which produces the sound of thunder.

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Audio FrequenciesSensitivity of the Human Ear:

The human ear can respond to minute pressure variations in the air if they are in the audible frequency range, roughly 20 Hz (Hertz) – 20 kHz (Kilo- Hertz). Contributing to the wide dynamic range of human hearing are protective mechanisms that reduce the ear’s response to very loud sounds.

Reflection of Sound Waves

When Sound waves from a point source strike a plane wall, they produce reflected spherical wave-fronts as if there were an ‘image’ of the sound source at the same distance on the other side of the wall.

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Refraction of Sound Waves

Refraction is the bending of waves when they enter a medium where their speed is different. Refraction is not so an important phenomenon with sound as it is with light where it is responsible for image formation by lenses, the eye, cameras, etc. But bending of sound waves does occur and is an interesting phenomena in sound.

Refraction of Sound

If the air above the earth is warmer than that at the surface, sound will be bent back downward toward the surface by refraction.

Diffraction of Sound Waves

Diffraction is the bending of waves around small obstacles and the spreading out of waves beyond small openings.

* small in this case is compared to wavelength.

Electromagnetic WavesElectromagnetic waves

are formed when an electric field (shown as blue arrows) couples with a magnetic field (shown as red arrows). The electric and magnetic fields of an electromagnetic wave are perpendicular to each other and to the direction of the wave.

Properties of Electromagnetic WavesAll electromagnetic waves:

travel at the speed (c) of 3 * 108 ms-1 in free space

are transverseobey the laws of reflection and refractionshow interference and diffraction effectscan be polarizedare unaffected by electric and magnetic

fields

Example:

Electromagnetic Wave diagram showing relationship of the electric field, the magnetic field, and the direction that the wave is moving. A and B indicate points of maximum and minimum field strength.

Electromagnetic Spectrum

A ‘family of electromagnetic waves’ commonly referred to as the electromagnetic spectrum. The electromagnetic spectrum ranges from the short wavelength (high frequency) gamma rays to the long wavelength (low frequency) radio waves. A picture is shown on next slide of the electromagnetic spectrum.

Use of each type of E.M. WaveWaves:

Gamma-rays

X-rays

Ultraviolet Radiation

Infrared Radiation

Microwaves

Radio Waves

Major Uses:

In medicine- to sterilize instruments, to kill cancerous growths.

To ionize gases, to detect flaws in metals, forgeries, broken bones.

To kill bacteria, used in fluorescent lighting.

In infrared-sensitive cameras to detect diseases, in thermal photocopiers.

Microwave cooking, telecommunication links.

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