Sound — ICSE Class 10 Physics Important Questions

Answer: (b) pitch.

Pitch depends on frequency; a higher frequency gives a shriller (higher-pitched) note and a lower frequency a flatter note.

Answer: (b) decibel (dB). Ordinary conversation is about $60\,\text{dB}$60 dB. Long exposure to sound above about $80\,\text{dB}$80 dB, such as heavy traffic or loudspeakers, is noise pollution: it can cause headaches, stress and loss of hearing. The hertz is the unit of frequency.

Answer: (b) forced vibration.

The fork forces the table top to vibrate at the fork's frequency. The larger vibrating area produces a louder sound, but since the table's natural frequency need not equal the fork's, it is forced (not resonant) vibration.

Answer: (c) $850\,\text{m}$850 m.

First cliff: $d_1=\dfrac{340\times2}{2}=340\,\text{m}$d₁=340×2/2=340 m. Second cliff: $d_2=\dfrac{340\times3}{2}=510\,\text{m}$d₂=340×3/2=510 m. Distance between cliffs $=d_1+d_2=340+510=850\,\text{m}$=d₁+d₂=340+510=850 m.

Answer: (a) Rhythmic marching could match the bridge's natural frequency and cause resonance, building up dangerously large vibrations. Breaking step prevents this, so R correctly explains A.

Natural (free) vibrations: a body vibrates at its own natural frequency with a constant amplitude, with no resistance acting. Example: a simple pendulum swinging in vacuum (or a struck tuning fork, very nearly).

Damped vibrations: the amplitude decreases gradually with time, because friction or air resistance takes away energy; the vibrations finally stop. Example: a pendulum swinging in air slowly comes to rest; the sound of a struck tuning fork dies away.

Forced vibration: the body vibrates under an external periodic force at the frequency of the applied force, which may differ from the body's natural frequency; the amplitude is generally small.

Resonance: a special case of forced vibration in which the frequency of the applied force equals the natural frequency of the body; the amplitude of vibration becomes very large. Thus every resonance is a forced vibration, but not every forced vibration is resonance.

The sound travels to the wall and back, so it covers twice the distance to the wall.

Total distance $=2d=2\times85=170\,\text{m}$=2d=2×85=170 m.

Time $t=0.5\,\text{s}$t=0.5 s.

$v=\dfrac{\text{total distance}}{\text{time}}=\dfrac{2d}{t}=\dfrac{170}{0.5}=340\,\text{m s}^{-1}$v=(total distance)/time=2d/t=170/0.5=340 m s^-1

The speed of sound in air is $340\,\text{m s}^{-1}$340 m s^-1.

(i) Loudness: the characteristic by which a loud sound is distinguished from a faint one. It depends on the amplitude of vibration (and on the area of the vibrating body and distance from the source). Loudness is subjective (it depends on the listener's ear), whereas intensity, the sound energy passing per second through unit area, is objective and can be measured.

(ii) Pitch: the characteristic by which a shrill (high) note is distinguished from a flat (low) note. It depends on the frequency of the sound - higher frequency means higher pitch.

(iii) Quality (timbre): the characteristic that enables us to distinguish between two sounds of the same loudness and pitch produced by different sources. It depends on the waveform (the number and relative intensity of overtones).

The sound travels down to the sea bed and back, so it covers twice the depth.

$2d=v\times t=1500\times3=4500\,\text{m}$2d=v× t=1500×3=4500 m

$d=\dfrac{4500}{2}=2250\,\text{m}$d=4500/2=2250 m

The depth of the sea is $2250\,\text{m}$2250 m.

Sound is used (in SONAR) rather than light because sound waves can travel long distances through water with little absorption and are strongly reflected by the sea bed, whereas light is quickly absorbed and scattered by water and cannot penetrate to such depths.

(a) Resonance is the phenomenon in which a body is set into vibrations with a large amplitude when it is acted upon by a periodic force whose frequency is equal to the natural frequency of the body.

(b) Take two identical tuning forks $A$A and $B$B of the same frequency, each mounted on its own sound box, and place the open ends of the boxes facing each other. Sound fork $A$A with a rubber hammer and then quickly damp it with the hand. Fork $B$B is now found to be vibrating and giving out sound. Because both forks have the same natural frequency, the periodic air pressure variations from $A$A force $B$B into resonant vibration. (Two identical pendulums coupled on a common string behave similarly - energy transfers to the one having the same length/natural frequency.)

(c) Useful: resonance in the air column of musical instruments (flute, sitar) and in a radio-tuning circuit that selects a station. Harmful: resonance can cause a bridge to collapse under rhythmic marching or high winds, and can shatter a glass when a singer's note matches its natural frequency (any one of each).

(a) Conditions for a distinct echo: (i) the minimum distance between the source and the reflecting surface should be $17\,\text{m}$17 m (so the reflected sound returns after at least $0.1\,\text{s}$0.1 s, the limit of persistence of hearing); (ii) the reflecting surface should be large and hard; and the intensity of sound should be sufficient to be heard after reflection.

(b) Let the original distance be $d$d and the speed of sound be $v$v.

First firing: $2d=v\times2.5$2d=v×2.5 ... (1)

After moving $170\,\text{m}$170 m closer, distance $=(d-170)$=(d-170): $2(d-170)=v\times1.5$2(d-170)=v×1.5 ... (2)

Subtract (2) from (1): $2d-2(d-170)=v(2.5-1.5)$2d-2(d-170)=v(2.5-1.5)

$340=v(1.0)\Rightarrow v=340\,\text{m s}^{-1}$340=v(1.0)⇒ v=340 m s^-1.

From (1): $2d=340\times2.5=850$2d=340×2.5=850, so $d=425\,\text{m}$d=425 m.

The speed of sound is $340\,\text{m s}^{-1}$340 m s^-1 and the boy was originally $425\,\text{m}$425 m from the cliff.

(i) The loud sound is produced by resonance (the air column resonates with the tuning fork).

(ii) At that level the natural frequency of the vibrating air column becomes equal to the frequency of the tuning fork, so the air column vibrates with maximum (large) amplitude and the sound is loudest.

(iii) Striking the fork harder increases the amplitude of vibration, so the loudness of the sound increases. The frequency (pitch) stays the same because it is fixed by the fork.

(iv) A tuning fork of higher frequency produces a note of higher pitch (a shriller sound), since pitch increases with frequency.