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Learning objectives
describe the production of sound by vibrating sources
describe the longitudinal nature of sound waves in terms of the processes of compression and rarefaction and deduce that
 a medium is required in order to transmit these waves
the speed of sound differs in air, liquids and solids
describe a direct method for the determination of the speed of sound in air and make necessary calculation
relate the loudness of a sound wave to amplitude and pitch to its frequency
describe how the reflection of sound may produce an echo, and how this may be used for measuring distances
define ultrasound and describe one use of ultrasound, e.g. cleaning, quality control and pre-natal scanning
explain why different instruments produce sounds of different qualityÂ
determine the frequency of sound using a calibrated c.r.o
*describe and explain the Doppler Effect and apply the concept in new situations.
What is Sound Engineering?Â
Nature of Sound
Nature of Sound
Sound is a form of energy that is passed from one point to another.
Sound is produced by vibrating sources placed in a medium.
A vibrating object in a medium produces sound waves through shifting of layers of air particles.
Production of sound
Production of sound
Sound waves are produced when a vibrating object causes small but rapid changes to the air pressure around it.
Consider a tuning fork.
When the tuning fork moves outwards, layers of air are pushed close together so that a compression of the air particles is formed.
When the tuning fork moves inwards, layers of air are pulled apart so that a decompression or rarefaction of the air particles is formed.
Therefore, a series of compressions (high pressure regions) and rarefactions (low pressure regions) move away from the tuning fork at the speed of sound.
However, the air particles themselves just vibrate back and forth about their original positions.
Hence, sound is propagated in the form of longitudinal waves such that the medium particles vibrate backwards and forwards in a direction parallel to the direction of propagation of the wave itself.
Detection of sound
Detection of sound
When sound waves reach the ear, the changing pressures of the air, due to compression and rarefaction, make the eardrum vibrate.
Transmission of sound
Transmission of sound
Sound cannot travel through a vacuum.
It needs a medium to transfer the disturbance of air away from the vibrating source.
Characteristics of soundÂ
Characteristics of soundÂ
Loudness
The loudness of a sound depends on the amplitude of vibration.
The greater the amplitude, the louder the sound.
Pitch
All sounds we hear are quite different. They vary from the sharp squeal of tyres braking to the low thud of a bass drum. The property which distinguishes sounds in this way is known as the pitch.
The pitch of a note depends on the frequency.
The higher the frequency, the higher the pitch.
Quality /Timbre
Different musical instruments produce a different quality (or timbre) of sound.
The quality of a note depends on its waveform.
Intensity of soundÂ
Reflection of sound
Reflection of sound
Sound waves can be reflected by large, hard surfaces like buildings, walls and cliffs. If the reflecting surface is soft, most of the incident sound energy is absorbed and very little sound is reflected. Sound waves obey the law of reflection, that is angle of incidence equals angle of reflection. Echos are produced by the reflections of sounds from hard surfaces.
Application of reflection of sound
Determining the speed of sound
Determining the speed of sound
Direct measurement method
Echo method
Using technology to determine the speed of sound in the laboratory
Speed of sound in different state of matter
Audible Frequencies
Audible Frequencies
The human ear can hear sounds of frequencies in the range of between 20 Hz (lower limit of audibility) and 20 kHz ( upper limit of audibility). This frequency range is referred to as the range of audible frequencies.
Animals have different ranges of audible frequencies. Dogs and bats, for example, have a much higher upper audible limit than human beings.
Infrasound and ultrasound
Infrasound and ultrasound
Infrasound
Infrasound is sound with a frequency lower than human audible frequency. This frequency range is the same one that seismographs use for monitoring earthquakes. Infrasound is characterized by an ability to cover long distances and get around obstacles with little dissipation.
Infrasound sometimes results naturally from ocean waves, avalanches, earthquakes, volcanoes, and meteors. Infrasound detectors are used to detect natural disasters and send warning signals. Infrasound can also be generated by man-made processes such as explosions, both chemical and nuclear.
A number of animals produce and use sounds in the infrasonic range. Whales, elephants rhinoceros, giraffes, and alligators are known to use infrasound to communicate over varying distances of up to many kilometers as in the case of the whale, for instance. It has also been suggested that migrating birds use naturally generated infrasound, from sources such as turbulent airflow over mountain ranges, as a navigational aid.
Ultrasound
Animals, such as dogs, bats, and dolphins, can hear these very high-pitched sounds. Bats use ultrasound to 'see' trees and insects at night.
Ultrasound can be used to clean objects. The fast, energetic ultrasound vibrations are absorbed by dirt which is then shaken free. This method of cleaning is used in industry for fragile equipment or to clean in corners which are otherwise difficult to get into. It is also used by dentists to clean teeth. In hospitals, concentrated beams of ultrasound can be use to break up kidney stones and gall stones without patients needing surgery.
Ultrasound is commonly use by doctors to do medical imaging.
Ultrasound is also used to detect cracks in metal structures. Cracks inside the metal reflect the ultrasound and produce an echo which can be detected.
Doppler's Effect
Doppler's Effect
You hear the high pitch of the siren of the approaching vehicle, and notice that its pitch drops suddenly as the vehicle passes you. That is called the Doppler effect.
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