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Stringed instruments are provided with a hollow box’ so that resonance may take place and a loud sound can be produced.

So as to produce sound of desired frequency. By plucking the string of more thickness, the frequency of sound produced decreases.

If the two strings are plucked at different places in the same way and their lengths be adjusted such as they give notes of the same pitch and loudness, their wave forms will differ. Hence they will be of different quality. 

 The ‘quality’ of a given note is determined not by its frequency and loudness but by the overall effect of the harmonics present in it. The harmonics are multiples of the fundamental or basic frequency of the ‘note’. Depending on the conditions under which vibrations are taking place, sometimes we get one set of harmonics and sometimes another set. The quality of the two notes would then be different even though their fundamental frequencies may be the same.

The range of audibility is in between 20Hz to 20,000Hz.

The sound heard after reflection is known as an echo. To hear the echo distinctly, the obstacle should be at a minimum distance of 17m from the observer.

The impression of sound lasts on the eardrum for 1/10 of a second the second sound travels a distance 332(m/s) x 1/10s = 33.2m. Thus, the minimum distance between the source of sound and the reflecting body should be 33.2/2 = 16.6m.

As the distance 10m is far less than 16.6m, therefore the ear cannot make out when the original sound had died and echo has been received. Thus, no echo is heard.

Echo will not be heard because minimum distance between the source and the reflecting surface should be 17m. 

Speed of sound in air = 350ms^-1

∴ Time in which echo is heard, t = 2d/v = (2 x 850m)/350ms^-1 = 4.86s.

If man had been standing at a distance 12m then 

t = 2d/v = 2 x 12/340 = 24/340 = 0.07sec.

which is less than 0.1sec.

Hence, man can not hear a clear echo.

Given:

d = 68m, v = 340ms-1.

∴ Time taken = (total distance)/speed = 68/340 = 1/5 = 0.2s(second)

so echo is heard after 2t i.e.

0.2 x 2 = 0.4 sec.

Given:

d = 850meter,

t = ?

speed v = 350m/sec

∴ t = 2d/v

⇒ t = (2 x 850)/350 ⇒ t = (2 x 17)/7 = 4.86sec.

When the glass rattles, at that moment, its natural frequency corresponds with frequency of piano note thus, resonance takes place, which makes the glass to vibrate violently. 

Unit for measuring sound level—Decibel.

Mechanical waves are waves which needs material medium for its propagation i.e., cannot travel in vacuum, e.g. sound waves.

The number of vibrations per second taken by a wave is called its frequency. 

Sound is a form of energy. It is that external physical cause which affects the ear to produce the sensation. Sound is produced only when some material body is made to vibrate.

 Tuning fork will resonate with 512 Hz.

As the water level in the pitcher increases, the length of the vibrating air column above the water surface in it decrease and hence the frequency of sound produced increases i.e., the sound gets shriller.

(i) As the bucket is filled the sound gradually increases. 

(ii) Frequency ∝ 1/length; Resonance takes place.

Here we refer to a mechanical wave, its two properties are: 

(i) These waves transmit only energy and momentum through the limited motion of the particles of the medium. 

(ii) While transmitting energy, the medium remains unshifted, i.e., only energy is transmitted while particles of the matter of the medium do not travel with the same.

(i) By decreasing the vibrating length of the string, (ii) By increasing the tension on the string, (iii) By decreasing the radius of the string.

The maximum displacement from mean position in vibration of a wave is called amplitude.

The sounds of two musical instruments of the same pitch and same loudness can be distinguished by their wave forms.

The waves of frequency above 20,000Hz are called ultrasonic waves. The waves of frequency below 20 Hz are called infrasonic waves.

X-rays. These are used for determining fracture of bones, hidden objects in customs at Airports. 

The process by which bats and dolphins detect the presence of an obstacle by hearing the echo of the sound produced by them is called sound ranging. These are used in RADAR.

Sound requires material medium for its propagation. It is produced when a body vibrates. The vibrations produced at a point are transmitted in the medium (air) from one point to the next and so on in the form of mechanical waves which produce compressions and rarefactions in air. These compressions and rarefactions produce vibrations in the ear drum which are perceived by us as sound.

The speed of sound is lowest in gases, higher in liquid and highest in soilds.

Radio waves. These waves are not audible to us because their frequency is 3 × 10⁴Hz to 3 × 10⁹Hz. Which is more than audible range. 

No, sound waves cannot travel in vacuum because there are no material particles which can vibrate and propagate energy.

Two differences between light waves and sound waves.

Resonance is a special case of forced vibration, when a body vibrates under the influence of an external periodic force, with a very large amplitude. Resonance occurs when the frequency of external force is either equal to or is integer multiple of the natural frequency of the vibrating body.

The vibrations of the blade die away after some time due to air resistance (or damping) The frequency of vibration of the blade can be lowered: (i) Increasing the vibrating length of the blade, (ii) Increasing the thickness of blade.

When an oscillatory system is made to oscillate under the action of an externally applied periodic force, it is said to execute a ‘forced vibration’.

Examples: 

(i) The vibrations produced in the board of a guitar when its string is made to vibrate. 

(ii) The vibrations produced in the table top when the stem of a vibrating tuning fork is pressed against it.

When the soldiers walk in step, they produce some fixed frequency. If this frequency corresponds the natural frequency of bridge, the resonance can take place. This in turn will make the bridge to vibrate violently, and hence make it collapse. To avoid such a situation, the soliders are asked not to march in step.

Noise is a harsh and unpleasant sound produced by a body making a succession of irregular and discontinuous disturbances, e.g., a sound produced by a stone thrown on a thin sheet, thunder of lightening, etc.

As the piece of pebble is dropped into the water, waves are seen to be formed on the surface of water which travel outwards from the point, where the pebble strikes the water surface. The cork piece simply tosses up and down without any appreciable forward or backward displacement. From this observation, it is concluded that water (medium) does not move, but the wave (or disturbance) advances.

The reason is that when soldiers are in steps, they all exert forces in the same phase and so strong vibrations of a particular frequency are produced. If this frequency becomes equal to the natural frequency of the bridge, it will start vibrating with large amplitude due to resonance and it may crumble down.

In musical sound the component wave do not change their wavelength hence the frequency and amplitude affect a musical sound like this.

It is a pleasant, continuous and uniform sound which is produced by a vibrating body making periodic and regular vibrations, e.g., sound produced by flute, violin etc.

This can be stopped by mounting the engine on such a structure, say steel springs that absorbs the vibrations of the engine without communicating them to the frame and in this condition the rear view mirror attached to the frame will not vibrate vilently. 

When the frequency of the engine of motor bike corresponds with the natural frequency of rear view mirror, die resonance takes place. Thus, rear view mirror vibrates with a larger amplitude violently. 

This phenomenon is called ‘resonance’. In this phenomenon when a body begins with the natural frequency of the others vibrating body B, then A begins to vibrate violently with the maximum amplitude. The frame of a motor cycle along with the rear view mirror is found to vibrate viotently when it is driven to a particular speed, this happens when the frame has the natural frequency of vibration equal to that of the piston when the engine is driven at that particular, speed.

There are three main characteristics: (i) Intensity or loudness, (ii) Pitch, and (iii) Quality.

(i) The diagram shows damped vibrations. 

(ii) A tuning fork vibrating in air. 

(iii) The amplitude of the wave decreases due to frictional force which the surrounding medium exerts on the vibrating body. 

(iv) After some time the amplitude gradually decreases and finally stops.

 (i) The loudness or softness of a sound is determined by the amplitude (or intensity) of the wave. In this case the amplitude of R is greater than the amplitude of other vibration modes. Hence, the vibration R will produce more sound than the other two. Louder sound corresponds to the wave of larger amplitude. 

(ii) Shrillness or Pitch of a note depends on the wavelength or frequency of wave and here the vibration P has m ore frequency than others. If f is the principle frequency, then 

Frequency (P) = 3f 

Frequency (Q) = 2f 

Frequency (R) = f 

Hence P has more shrillness than others. 

(iii) If the frequency of vibration of R = f, frequency of vibration of Q = 2f, and that of P =3f. 

Ratio fP/fR=3/1 

fP:fR=3:1 As f∝1/λ

hence λP/λR=1:3

(i) The frame of a motor cycle along with the rear view mirror is found to vibrate viotently when it is driven to a particular speed. This happens when the frame has the natural frequency of vibration equal to that of the piston when the engine is driven at that particular speed. 

(ii) This phenomenon is called ‘resonance’. In this phenomenon when a body begins with the natural frequency of the others vibrating body B, then A begins to vibrate violently with the maximum amplitude. 

(iii) This can be stopped by mounting the engine on such a structure, say steel springs that absorbs the vibrations of the engine without communicating them to the frame and in this condition the rear view mirror attached to the frame will not vibrate violently.

Intensity depends on amplitude, pitch on frequency and quality on waveform.

When the vibrations produced by the feet of the person are impressed on the rails, they vibrate with forced vibrations, thereby producing loud sound.