Sound (Test -2) - Class 9 Science, 21-02-2018 - Class 9 MCQ

Sonar stands for Sound navigation and ranging. Sonar is a device that is used to find the depth of the sea,or to locate underwater objects like enemy's submarine,sunken ships,shoal of fish,etc.

Wave motion is activity that carries energy from one place to another without actually moving any matter.In wave motion, energy—the ability to perform work, or to exert force over distance—is transmitted from one place to another without actually moving any matter along the wave. In some types of waves, such as those on the ocean, it might seem as though matter itself has been displaced; that is, it appears that the water has actually moved from its original position. In fact, this is not the case: molecules of water in an ocean wave move up and down, but they do not actually travel with the wave itself. Only the energy is moved.

Electromagnetic waves differ from mechanical waves in that they do not require a medium to propagate. This means that electromagnetic waves can travel not only through air and solid materials, but also through the vacuum of space.

Distance between two consecutive crests is called wavelength.

Therefore, wavelength= 100 m.

Here,wave velocity = 20 ms^-1 

 frequency = wave velocity/ wavelength.

=> frequency= 20/100 = 0.2 Hz.

Frequency is defined as the number of vibrations or waves produced per second.

If a source of sound has a frequency of 546 Hz, it means that it vibrates 546 times in one second.

When a number of echoes of the original sound are heard,each echo being fainter than the preceding one,such multiple echoes are called reverberations. In halls and auditorium reverberation is reduced by using sound absorbing substances.Sound absorbing panels are kept near the stage.

Wave motion is activity that carries energy from one place to another without actually moving any matter.In wave motion, energy—the ability to perform work, or to exert force over distance—is transmitted from one place to another without actually moving any matter along the wave. In some types of waves, such as those on the ocean, it might seem as though matter itself has been displaced; that is, it appears that the water has actually moved from its original position. In fact, this is not the case: molecules of water in an ocean wave move up and down, but they do not actually travel with the wave itself. Only the energy is moved.

Electromagnetic waves differ from mechanical waves in that they do not require a medium to propagate. This means that electromagnetic waves can travel not only through air and solid materials, but also through the vacuum of space.

The distance between crest and trough is half of wavelength. Loudness of sound 120 dB means its intensity of 1W/m². Ultrasound is produced by electronic oscillator. The speed of the wave depends on properties of medium.

Frequency of wave = wave speed / wavelength.

Frequency =344ms⁻¹/0.02m = 17200 Hz

Bat and porpoises use ultrasound for navigation and location of food in the dark. They contain ultrasound producing organs and receiver that help them to judge the position and location of food and predator.

Music and TV sounds that we hear have noise level below 90 decibels.

In human ear, the sound is amplified in the middle ear by three bone connected with tympanic membrane. The amplified vibration is transmitted to inner ear and is used to produce sensation of hearing.

The speed of sound is faster in solid medium in comparison to liquid and gaseous medium. Aluminium is solid and metal, so the speed of sound will be maximum in it. Speed of sound in Aluminium at 25 degree celcius is 6420 ms^-1

The sound of high pitch cannot be produced by vibrating string of thicker diameter. This is because thick strings vibrate with lower frequency. For example,thin strings of a guitar or sitar give out sound of higher pitch as they vibrate with a greater frequency than the thick strings.

The angle of incidence is equal to the angle of reflection. This law of reflection is applicable to both sound and light waves.

An object moving at a speed greater than that of sound is moving with supersonic speed and not ultrasonic speed.

Wave motion transfers energy from one point to another, which displace particles of the transmission medium–that is, with little or no associated mass transport. Waves consist, instead, of oscillations or vibrations (of a physical quantity), around almost fixed locations.

Hertz (Hz) is SI unit of frequency. Frequency is the number of vibration per second.

Frequency of sound is the number of vibration per second.

Here, frequency = 100Hz. One minute = 60 seconds.

So, number of vibrations in one minute =60×10060×100= 6000 Hz.

The good directional property of ultrasound waves are used for scanning the objects. Ultrasound travels in straight line and do not change its direction.

Crest is the point of maximum positive displacemnt (elevation) of a transverse wave.

Trough is the point of maximum negative displacement (depression) of a transverse wave .

A compression is that part of a longitudinal wave in which the particles of the medium are closer to one another than they normally are and there is a momentary decrease in the volume of the medium.

A rarefraction is that part of a longitudinal wave in which the particles of the medium are farther apart than that in the normal position and there is a momentary increase in the volume of the medium.

A wave can be described as a disturbance that travels through a medium from one location to another location. 

Mechanical Waves are waves which propagate through a material medium (solid, liquid, or gas) at a wave speed which depends on the elastic and inertial properties of that medium.

A transverse wave is a moving wave that consists of oscillations occurring perpendicular (right angled) to the direction of energy transfer (or the propagation of the wave). If a transverse wave is moving in the positive x-direction, its oscillations are in up and down directions that lie in the y–z plane.

Oscillatory motion can be termed as the repeated motion in which an object repeats the same movement over and over,i.e., to and fro or back and forth motion.

Frequency = 220Hz

Wave speed = 440 ms^-1.

Since, wavelength= wave speed / frequency

=>Wavelength = 440/220 = 2 m.

Speed of sound = 346 ms^-1 

Time taken =5sec.5sec.

Since,

distance  =speed×time.

⟹2d=346×5

⟹2d=1730

⟹d=865m

It has been found that the greater the amplitude of the vibration of the source ,the greater is the intensity (and hence,loudness) of sound and vice versa.

Speed greater than the speed of sound is called supersonic speed.

Pitch of a sound depends upon the frequency (or wavelength) of vibration.

Timbre is the characteristic of a musical sound that enables us to distinguish between two sounds of the same pitch and loudness produced by two different sources.It depends upon the waveform of the sound.

A longitudinal wave is a wave in which particles of the medium move in a direction parallel to the direction that the wave moves.Longitudinal wave needs a material medium to travel ,i.e, solids,liquids and gases.

Speed of sound increases with temperature.So,sound travel faster in summer than winter season.

At a constant temperature, the gas pressure has no effect on the speed of sound, since the density will increase, and since pressure and density (also proportional to pressure) have equal but opposite effects on the speed of sound, and the two contributions cancel out exactly.

Rolling thunder is repeated claps of thunder in quick succession. The sound of same loudness and pitch can be distinguished by timbre. The frequency of woman voice is more than man’s voice.

Sound waves in air (and any fluid medium) are longitudinal waves because particles of the medium through which the sound is transported vibrate parallel to the direction that the sound wave moves.
Waves that, like sound, vibrate in the same direction as their direction of propagation are longitudinal waves, while waves that vibrate at right angles to their direction of propagation are transverse waves. Radio waves are one kind of transverse wave.

An aeroplane or a supersonic jet moving at supersonic speed produces shock waves in air.These shock waves carry a large amount of energy. 
A mechanical wave is a wave that is an oscillation of matter, and therefore transfers energy through a medium.Any kind of wave (mechanical or electromagnetic) has a certain energy. Mechanical waves can be produced only in media which possess elasticity and inertia.

Light and Radio waves are electromagnetic waves that does not require any medium of propagation.

The maximum displacement of the particles of a medium from their mean positions during the propagation of a wave is called the amplitude of the wave.

The time required to produce one complete wave (or wave cycle) is called the time period of the wave.

The frequency of a wave is defined as the number of waves produced per second.

The distance travelled by a wave in one second is called the velocity of the wave.

During thunderstorm, we observe lightening first and hears the sound of thunder later, although both occurs simultaneously because light travels much faster than sound.

SI unit of wavelength is metre (m) and velocity is ms^-1.