CBSE Class 7 Science Advanced Physics Sound Notes Set 01

Sound

Sources of Sound

Vehicles rattling on the road, a television, a music player, a roaring lion, a barking dog and people talking all give out sound. Anything that produces sound is called a source of sound; sources may be living or non-living. Making sound always costs energy, and different sources give sounds that are different from one another. To compare them we need a few terms, which are developed below.

Production of Sound

Activity 1: Stretch a rubber band tightly between two fixed points and pluck it in the middle. You hear a sound and, at the same time, you can see the band moving rapidly back and forth.

Activity 2: A tuning fork is a U-shaped steel rod mounted on a handle. The handle is the stem and the two arms are the prongs. Hang a small pith ball from a stand, strike the tuning fork with a hammer and bring a prong against the ball (Fig. 5.1). The ball is thrown away again and again, and you also hear a note if you hold your ear near the fork.

Both activities lead to the same conclusion: sound is produced by vibrating bodies.

Vibratory (Oscillatory) Motion

The rubber band and the prongs of the fork both keep moving to and fro about a fixed rest position, called the mean or equilibrium position. Such to-and-fro motion is called vibratory or oscillatory motion.

1. One complete to-and-fro movement is one vibration (one oscillation).
2. The time taken for one vibration is the time period, T. It is measured in seconds.
3. The number of vibrations made in one second is the frequency, f.

Units of Frequency

Larger units are the kilohertz and megahertz: 1 kHz = 1000 Hz and 1 MHz = 10⁶ Hz.

Amplitude is the greatest distance the vibrating body moves from its rest position on one side. It is measured in cm or m.

Experiment to Find the Frequency of Oscillation

A simple pendulum is a small heavy bob hung from a fixed support by a light thread that cannot stretch. In practice a metal ball is tied to a thread and the thread is fixed to a cork held in a retort stand (Fig. 5.2).

Steps:
1. Keep a stop watch ready. Pull the bob aside and let it go.
2. When the bob reaches an extreme position, start the stop watch and count the oscillations.
3. After 20 oscillations stop the watch and note the time t₁.
4. Repeat once more to get t₂.
5. Take the average \( t = \dfrac{t_1 + t_2}{2} \).
6. Time period \( T = \dfrac{t}{20} \) and frequency \( f = \dfrac{1}{T} \).

Record the readings in a table with the columns t₁, t₂, t = (t₁ + t₂)/2, T = t/20 and f = 1/T.

Ultrasonic and Infrasonic Sound

The human ear does not respond to every frequency. Vibrations below 20 Hz are called infrasonic (subsonic) and vibrations above 20 000 Hz are called ultrasonic; we hear neither. Dogs and bats can respond to ultrasonic sound. So a source can be heard only if it makes more than 20 and fewer than 20 000 vibrations each second. This range, 20 Hz to 20 000 Hz, is the audible (sonic) range.

A simple pendulum makes far fewer than 20 swings in a second, so its motion is infrasonic and we cannot hear it.

Uses of Ultrasonic Vibrations

1. Bats send out ultrasonic sound while flying (the book gives the range as 50 Hz to 80 000 Hz). The sound bounces off insects and the bat finds its prey from the echo. Dolphins hunt in the same way.
2. Sonar uses ultrasound to measure the depth of the sea bed.
3. Ultrasound scanning (sonography) produces images of internal organs and of a baby in the womb.
4. Ultrasound can homogenise milk.
5. It can reveal cracks and flaws inside metal objects.
6. A Galton whistle gives ultrasound of about 40 000 Hz. Dogs respond to it, but people cannot hear it, so it is used to train dogs.

Propagation of Sound

Propagation through Solids

Activity 1: Let a friend tap one end of a classroom table with a metal scale while you press your ear to the other end. You hear the tap through the wood, so solids carry sound.

Activity 2: Hold one end of a long metal rod near your ear and have a friend strike the other end. You hear two separate sounds, one that came through the rod and one that came through the air. The one through the rod arrives first, so sound is faster in a solid than in air. The sound that came through air is louder and clearer.

Propagation through Liquids

Fill a metal container with water and knock two coins together under water; you hear the click. Now press your ear to the outside of the container and repeat. The click is louder and clearer. So liquids also carry sound.

How a Sound Wave Travels: Compressions and Rarefactions

A vibrating object, such as a prong of a tuning fork, moves forward and backward. When it moves forward it pushes the air in front of it together, making a region of high pressure called a compression (C). When it moves back it leaves a region of low pressure called a rarefaction (R). These alternate compressions and rarefactions move away from the source (Fig. 5.3) and make up the sound wave.

The air particles themselves only vibrate to and fro along the same line in which the wave is moving, so sound waves are longitudinal. Pressure follows density: a denser region has higher pressure. Hence sound travelling in a medium can be pictured as a travelling pattern of pressure (or density) changes.

Can Sound Travel through a Vacuum?

Sound travels through solids, liquids and gases. To test a vacuum, hang an electric bell inside a glass jar that has an outlet at the bottom (Fig. 5.4). The bell is heard when it is switched on. Connect the outlet to a vacuum pump and remove air slowly: the sound gets fainter and finally cannot be heard. Without a medium there is nothing to carry the sound, so sound needs a material medium and cannot travel through a vacuum.

Experiment to Find the Speed of Sound in Air

Choose an open field where two persons P₁ and P₂, about 1 km apart, can see each other. You need a cracker, a stop watch and a measuring chain.
1. P₁ fires the cracker.
2. P₂ starts the stop watch the moment the flash is seen.
3. P₂ stops the watch when the bang is heard; the reading is the time taken by the sound.
4. Measure the distance between the two places with the chain.

\[ \text{Speed of sound in air} = \frac{\text{distance between the two places}}{\text{time interval}} \]

Note: light takes almost no time to cover the distance because its speed (3 × 10⁸ m s⁻¹) is enormous compared with that of sound (about 330 m s⁻¹), so its travel time is ignored. For the same reason lightning is seen before thunder is heard, even though both start together.

Formation of Thunder

During lightning about 75% of the electrical energy goes into heating the gases in and around the flash. Within a very short time the nearby air reaches around 10 000 K. The sudden heating sets up a pressure wave of compressions and rarefactions, which we hear as the loud, violent sound called thunder.

To estimate how far away the cloud is, start a stop watch when you see the flash and stop it when you hear the thunder; let the time gap be t seconds. Taking the speed of sound as 330 m s⁻¹,

\[ \text{distance of the cloud} = \text{speed of sound} \times t = (330 \times t)\ \text{m} \]

Reflection of Sound

Activity: Stand a smooth, hard wooden plank ABCD upright on a table. Clamp two pipes P and Q on the table at different slopes, with a ticking clock S near the open end of P (Fig. 5.5). The ticking runs along P, strikes the plank and is reflected along Q. Put your ear at the open end of Q and move Q until the ticking is heard best. Measuring the angles that the two pipes make with the normal NF shows they are equal: the angle of incidence i equals the angle of reflection r. Changing the slope of P and re-adjusting Q gives the same result each time. This bouncing back of sound from a surface is reflection of sound.

Laws of reflection of sound: (1) angle of incidence = angle of reflection; (2) the incident sound, the reflected sound and the normal lie in one plane.

Do All Materials Reflect Sound?

No. Hard, smooth surfaces such as metal sheets, walls and plywood reflect sound well. Soft, loosely woven or porous materials such as cloth, thermocol, gunny bags and carpets absorb it. This is why the walls of cinema halls and auditoriums are given a rough covering, and why seats, stairs, doors and windows carry sound-absorbing material or thick curtains: reflected sound would spoil the quality of what people hear.

Echo

If you clap while standing more than about 17 m from a large wall or building, you hear the clap a second time. A sound that is reflected from a hard surface and heard again a short time after the original is an echo.

The ear keeps an impression of a sound for about 0.1 s; this is persistence of hearing. So the reflected sound must reach us at least 0.1 s after the original. Let d be the distance from the listener to the reflecting surface. The sound goes to the surface and back, so the distance covered is 2d. With the speed of sound 330 m s⁻¹:

\[ 330 = \frac{2d}{0.1} \quad\Rightarrow\quad d = \frac{330 \times 0.1}{2} = 16.5\ \text{m} \]

Thus the minimum distance to the reflector for a clear echo in air is about 16.5 m (taken as about 17 m).

Practical Applications of Reflection of Sound

1. Megaphone: a horn-shaped tube. Sound from the narrow end is reflected repeatedly by the walls of the horn and leaves as a beam, so it travels farther.
2. Sonar (Sound Navigation and Ranging): a ship sends ultrasound down to the sea bed; it reflects and is picked up by a detector on the ship. If the round trip takes t seconds and the speed of sound in water is v, then \( v = \dfrac{2d}{t} \), so the depth is \( d = \dfrac{vt}{2} \).
3. Toy telephone: join two paper or plastic cups with a long taut thread. Speak into one cup while a friend holds the other to the ear; the sound travels along the thread.
4. Hearing aids, loudspeakers and the curved sound boards in halls also depend on reflection of sound.

Different Kinds of Sound

We can easily tell apart a boy's voice from a girl's, a flute from a violin, and a cat from a dog. The sounds differ in frequency (pitch), loudness and quality, and this helps us identify the source.

Loudness (Intensity)

Loudness is the property that lets us tell a faint sound from a strong one. It grows with the amplitude of vibration, and the amplitude depends on the energy supplied: the more energy used to set a body vibrating, the larger the amplitude and the louder the sound.

Measuring Loudness

Loudness is measured in decibels (dB); 1 decibel is one tenth of a bel. The book states that people can sense sounds from about 10 dB to 180 dB. Sound above about 90 dB is painful and harms health, while 50 to 60 dB is a normal level.

Factors Affecting Loudness

1. Amplitude: a larger amplitude gives a louder sound.
2. Surface area of the vibrating body: a large table top gives a louder sound than a small one.
3. Distance from the source: loudness falls as you move away, so the front bench hears more than the last bench.

Pitch

Pitch is the quality of a sound by which we call it shrill or flat. A woman's voice is shriller than a man's because its pitch is higher. Pitch depends only on frequency: the higher the frequency, the higher the pitch. It does not depend on the energy or amplitude. In a violin the vibrating length of the string is changed to change the pitch, and a flute changes the length of the vibrating air column.

Experiment: Pitch and Length of an Air Column

Fill a burette to about three quarters with water and blow across its mouth: a high-pitched note is heard. Let some water run out through the tap so the air column becomes longer and blow again: the note is lower. Repeating with still less water gives lower and lower notes. A longer air column vibrates with a smaller frequency, so the pitch falls.

Quality (Timbre) of Sound

Two persons can be told apart by voice even when their vocal cords have the same length and thickness. A violin and a sitar sound different even with strings of equal length and thickness. The property responsible is quality or timbre. It separates two sounds that have the same pitch and the same loudness.

Sound Produced by Human Beings

In humans sound is made by the vocal cords, which are stretched across the larynx with a narrow slit between them. Air pushed up from the lungs through the slit makes the cords vibrate. Tight, thin cords give a high frequency (shrill) voice, while loose, thick cords give a low frequency voice. Muscles attached to the cords control their tension.

Activity: Cut a rubber band into two equal strips, lay one over the other, stretch them and blow between them. Stretching them more raises the pitch. Using a thinner rubber band gives a still higher pitch.

Sound Produced by Animals

Mammals produce sound with vocal cords. Birds use the syrinx, a ring of cartilage at the start of the windpipe; some birds have a two-part syrinx and can sing two notes at once. Insects such as mosquitoes and houseflies make a sound by beating their wings, and we hear it because the wing beat is faster than 20 times a second.

Hearing

The compressions and rarefactions from the source reach the ear and make the ear drum vibrate. These vibrations are passed on through the small hinged bones of the middle ear to the inner ear, where the auditory nerve turns them into signals that go to the hearing centre of the brain (Fig. 5.6).

Music and Noise

A sound that feels pleasant to the ear is music and one that feels unpleasant is noise. The same sound may be pleasant to one person and irritating to another, so the line between the two is not sharp.

Musical Instruments

Noise Pollution

Harsh, loud and unwanted sounds in our surroundings make up noise pollution. It has grown with the use of machines at home, at work and in factories. It depends on how loud the sound is and for how long it lasts at a place. Sources include televisions, radios, vacuum cleaners, crackers, loudspeaker announcements, factories and vehicles. Its effects are loss of hearing, lower efficiency at work and mental tension.

Ways to reduce noise pollution:
1. Fit silencers in automobiles.
2. Use loudspeakers less.
3. Build factories away from residential areas.
4. Keep the volume of TV, radio and music players low.

Points to Remember

1. Sound is a form of energy produced by vibrating bodies.
2. Frequency is the number of vibrations per second; time period T is the time for one vibration; \( f = \dfrac{1}{T} \). Amplitude is the maximum displacement from the rest position.
3. The SI unit of frequency is the hertz (Hz).
4. Below 20 Hz the vibrations are infrasonic; above 20 000 Hz they are ultrasonic.
5. Speed of sound: solids (V_s) > liquids (V_l) > gases (V_g).
6. Speed = distance travelled ÷ time taken.
7. A reflected sound heard after a short gap is an echo.
8. Loudness increases with amplitude and is measured in decibels.
9. Pitch (shrillness) depends on frequency.
10. Quality (timbre) separates two sounds of the same pitch and loudness.