Sound

What is sound?

• Sound is
• A form of energy made by vibrations.

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• When an object vibrates it causes the air particles around it to move. �
• These particles bump into particles close to them and this continues until they run out of energy.

Try this:

• Put your finger on your neck and say “aah” as loud as you can.

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• Now say it as soft as you can.

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• You can not only hear the sound, but you can feel the vibration inside your throat.

How does sound travel?

• Sound travels through all forms of matter— gases, liquids and solid.�
• These are called the medium.

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• Sound cannot travel through a vacuum.

What is pitch?

• Pitch is how high or how low a sound is.

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• This is a high pitched sound.

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• This is a low pitched sound.

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• Pitch is how high or how low a sound is.

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• This is a high pitched sound.

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• This is a low pitched sound.

How does sound travel?

• Sound passes through the medium as longitudinal waves.
• When the vibrations are fast you hear a �high pitch. When they’re slow, you hear a low pitch.

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High Pitch

Low Pitch

How else can we change sound?

• We can make it louder or softer by changing the amplitude of the height of the wave.
• The higher the amplitude, the louder the sound. The lower the amplitude the softer the sound.

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How do musical instruments create sound?

• It depends on the type of instrument. There are four types. �
• Woodwind
• String
• Percussion
• Brass

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Woodwinds Instruments

• Woodwinds make music by blowing on the top of the instrument or on a reed.

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String Instruments

• Strings make music by plucking or strumming the strings.

Percussion Instruments

• Percussion instruments make music by striking, shaking or scraping them.

Brass Instruments

• Brass instruments make music by buzzing lips while blowing.

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Trumpet

Sound needs a medium to travel :-

Sound is a mechanical wave and needs a medium for propagation. Sound travels through solids, liquids and gases. Sound does not travel in vacuum.

Activity:-

Suspend an electric bell in an air tight bell jar. Connect the bell jar to a vacuum pump. If the switch is pressed, we can hear the sound of the bell. If air is pumped out through the vacuum pump, we cannot hear the sound of the bell. This shows that sound needs a medium to travel and sound cannot travel in vacuum.

Bell jar

Electric bell

Cork

CHARACTERISTICS OF A SOUND WAVE

• The change in density from one maximum value to the minimum value and again to the maximum value makes one complete oscillation.
• The distance between two consecutive compressions or two consecutive rarefactions is called the wavelength, λ.
• The time taken by the wave for one complete oscillation of the density or pressure of the medium is called the time period, T.
• The number of complete oscillations per unit time is called the frequency (ν), 1/ν = T
• The speed v, frequency ν, and wavelength λ, of sound are related by the equation, v = λν.

Reflection of sound :-

Like light, sound gets reflected at the surface of a solid or liquid and follows the laws of reflection.

i) The angle of incidence is equal to the angle of reflection.

ii) The incident ray, the reflected ray and normal at the point of incidence all lie in the same plane.

Activity :- Take two pipes of the same length and arrange them on a

table near a wall or metal plate. Keep a clock near the open end of one

pipe and try to hear the sound of the clock through the other pipe by

adjusting the position of the pipe.

Now measure the angles of incidence

and reflection. Then lift the second

pipe and try to hear the sound.

It will be seen that the angle of

incidence is equal to the angle of

reflection. The incident ray, the

reflected ray and normal all lie in

the same plane.

ECHO

• For hearing a distinct sound, the time interval between the original sound and the reflected one must be at least 0.1 s.
• The minimum distance of the obstacle from the source of sound must be 17.2 m. This distance will change with the temperature of air.
• Echoes may be heard more than once due to successive or multiple reflections. The rolling of thunder is due to the successive reflections of the sound from a number of reflecting surfaces, such as the clouds and the land.

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REVERBERATION�

• The persistence of sound in an auditorium is the result of repeated reflections of sound and is called reverberation.

USES OF MULTIPLE REFLECTION�OF SOUND

• Megaphones or loudhailers, horns, musical instruments such as trumpets and shehanais, are all designed to send sound in a particular direction without spreading it in all directions.

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• Stethoscope is a medical instrument used for listening to sounds produced within the body, chiefly in the heart or lungs. In stethoscopes the sound of the patient’s heartbeat reaches the doctor’s ears by multiple reflection of sound.

Range of Hearing

• The audible range of hearing for average human beings is in the frequency range of 20Hz – 20 kHz.

• Sound waves with frequencies below the audible range are termed “infrasonic” and those above the audible range are termed “ultrasonic”.
• Ultrasound has many medical and industrial applications.
• The SONAR technique is used to determine the depth of the sea and to locate under water hills, valleys, submarines, icebergs, sunken ships etc

Structure of Human Ear

Human Ear

• We are able to hear with the help of an extremely sensitive device called the ear. It allows us to convert pressure variations in air with audible frequencies into electric signals that travel to the brain via the auditory nerve.

Solved Examples:

1). A person clapped his hands near a cliff and heard the echo after 5 s. What is the distance of the cliff from the person if the speed of the sound, v is taken as 346 m s–1?

• Solution:

Given, Speed of sound, v = 346 m s–1 , Time taken for hearing the echo, t = 5 s ,

Distance travelled by the sound = v × t = 346 × 5 = 1730 m. In 5 s sound has to travel twice the distance between the cliff and the person. Hence, the distance between the cliff and the person = 1730/2 = 865m.

2). A sound wave has a frequency of 2 kHz and wave length 35 cm. Calculate the speed of the wave?

• Solution:

Given, Frequency, ν = 2 kHz = 2000 Hz

Wavelength, λ = 35 cm = 0.35 m.We know that speed, v of the wave= λ ν

= 0.35 m × 2000 Hz = 700 m/s.