ICSE Class 7 Physics Chapter 03 Sound

Sound

We interact with the world around us mainly through sight and hearing. Sound plays a very important part in our existence. We recognise many living and nonliving things around us through the sounds they produce. We also use sound to communicate with each other. In this chapter we will discuss how sound is produced, how it travels and what distinguishes one sound from another.

Sources of Sound

Sound is produced by vibrations. Vibration means the rapid back and forth movement of any object or particle. Every source of sound vibrates when it produces sound.

Choose a source of sound, such as the speaker of an audio system or a brass bell. Touch it gently when it is producing sound. You will feel the vibrations at once. If you turn off the audio system, the vibrations will stop and so also will the sound. If you press on the vibrating bell from both sides, it will gradually stop vibrating and its sound will die out. You can easily feel this happening. You can experiment with other things too, for example, an electrical buzzer, a guitar string or a television.

Teacher's Note

Sound is produced by vibrations. Every sound we hear - from a ringing bell to music from speakers - begins with something vibrating back and forth.

The Human Voice

The sound that we are most familiar with is that of the human voice. What is the source of this sound? When we speak, sound is produced by an organ called the larynx or voice box. This is a broad part of the trachea (wind pipe), where the trachea meets the pharynx. Inside it are two folds of tissue called vocal cords. These vibrate and produce sound when exhaled air passes through the larynx. The nature of the sound changes with the tightness of the vocal cords. When they are tight, the sound is shrill. When they are loose, the sound is bass. The nature of the sound is also determined by the length of the vocal cords. Women and children sound shriller than men because they have shorter vocal cords than men. The sound produced by the larynx is modified into speech by the tongue, teeth and lips.

Teacher's Note

Our voice comes from the larynx, where vocal cords vibrate as air passes through them. This is why your voice changes when you have a cold and your vocal cords are swollen.

Musical Instruments

Musical instruments are another very familiar source of sound. They can be divided into three broad groups on the basis of the way sound is produced by them. Let us see what these groups are.

Vibrating Strings

In a large number of musical instruments, sound is produced by vibrating strings. The strings are made of animal gut, nylon or metal and are stretched over a hollow structure, which is usually made of wood. Sound is produced by plucking or bowing the strings. The hollow wooden structure makes the sound louder and more melodious.

Instruments in which sound is produced by vibrating strings are called string instruments. Sitar, sarod, santoor, veena and sarangi are traditional Indian string instruments. Piano, harp, cello, fiddle, violin and guitar are traditionally western instruments, though many of them are very popular in India as well. The larynx, which helps us make sounds, also works like a string instrument.

Teacher's Note

When you pluck a guitar string, it vibrates and creates sound - the tighter the string, the higher the pitch of the sound produced.

Vibrating Air Columns

In the second group of musical instruments, sound is produced by the vibrations of a column of air trapped in a 'pipe'. These are called wind instruments. When the pipe is made of wood, as in the flute, shahnai and clarinet, the instrument is called a woodwind instrument. When the pipe is metallic, as in the bugle, trumpet, trombone and saxophone, the instrument may be called a horn or brass. A band playing metallic wind instruments is often called a brass band.

The whistles used by policemen and sports referees also produce sound with the help of a vibrating column of air. With a little practice, you can use the cap of a pen or a small bottle (like the vials used by homoeopaths) as a whistle. Hold the cap vertically, with the open end just touching your lips, and blow. You could also experiment with drinking straws. Cut the straws to different lengths and blow into them. You will notice that shorter straws make sharper (shriller) sounds.

Teacher's Note

Wind instruments work by vibrating air inside a tube - which is why blowing into bottles or straws of different lengths produces different pitches of sound.

Vibrating Membranes

In instruments like drums, tablas, bongos and mridangams, sound is produced by vibrating membranes (skins). The skin or leather is stretched across a hollow, barrel-shaped or bowl-shaped body made of wood, metal or clay. The skin is struck by the hand or a stick.

Instruments which produce a sound when they are struck or shaken are called percussion instruments. These include not only drums, tablas, etc., but also cymbals, rattles, tambourines and xylophones.

Make drums of different sizes by stretching pieces of balloons across the mouths of cans, tins, and cardboard boxes and tubes. Strike them with a pencil or stick. You will notice that smaller drums sound sharper or shriller.

Teacher's Note

Percussion instruments like drums work by striking a stretched membrane that vibrates to produce sound - larger drums vibrate more slowly and produce lower pitches.

Properties of a Sound

The sounds that we hear around us vary widely in nature. They may be loud or soft, shrill (sharp) or bass (dull), pleasant or unpleasant, and so on. Let us discuss these properties of a sound in some detail.

Loudness

The loudness of a sound is also called its intensity. It depends on the power of the source and the distance of the listener from the source. To take a familiar example, a powerful source of sound, such as a loudspeaker, produces a loud sound. However, the loudness of the sound decreases as we move away from the speaker.

What determines the power of a source of sound? Something called the amplitude of its vibrations. The maximum displacement of a body during its vibrations is called the amplitude (of its vibrations). The greater the amplitude the louder is the sound. A powerful source of sound has a large amplitude, while a low-power source has a small amplitude.

You can explore the relation between amplitude and loudness with the help of the drums you made in the preceding activity. When you strike a drum hard, the skin vibrates with greater amplitude and the sound is loud. When you strike it gently, the skin vibrates with a smaller amplitude and the sound is softer.

Alternatively, you can make a string instrument by stretching a rubber band across a pencil box, as shown in Figure 3.8. When you pluck the string gently, the amplitude of the vibrations will be small and the sound will be soft. When you pluck harder, the amplitude will be greater and the sound louder.

Why does a sound become softer and finally die out as we move away from its source? We will discuss this in the section on the propagation of sound.

Teacher's Note

Loudness depends on how far the vibrating object moves back and forth - a larger movement (amplitude) creates a louder sound, which is why striking a drum hard produces a louder noise than tapping it gently.

Pitch

The sharpness or dullness of a sound is called its pitch. Sharp or shrill sounds have a high pitch, while dull or heavy sounds have a low pitch. Thus, a child's voice is of high pitch, while the sound of a large drum is of low pitch. Again, male voices on the average have a lower pitch than female voices. We should be careful not to confuse between loudness and pitch. The examples given in Table 3.1 should make the distinction clear.

What is the relation between the vibrations of a source of sound and the pitch of the sound it produces? The number of vibrations which a source makes in one second is called its frequency. The pitch of the sound it produces depends on this frequency. Vibrations of high frequency produce sounds of high pitch, while vibrations of low frequency produce sounds of low pitch.

The unit of frequency is the hertz, written in short as Hz. If a source vibrates 200 times in 1 s, its frequency is 200 Hz. The sound produced by a vibrating source has the same frequency as that of the source.

Teacher's Note

Pitch is about how high or low a sound is - it depends on how fast something vibrates. A mosquito's wings beat very fast, creating a high-pitched sound, while a bass drum vibrates slowly and produces a low pitch.

Use the string instrument you made in the preceding activity to find out how you can vary the pitch of the sound produced by it. Alter the tautness of the rubber band by positioning the eraser differently or using two erasers. The pitch of the sound will change with the tightness of the rubber band. A taut string vibrates with a higher frequency than a loose string, so it produces a shriller sound.

You can also alter the pitch by moving the eraser. This will change the length of the rubber band. Shorter strings produce sounds of higher pitch than longer strings. Yet another way of altering the pitch is by changing the thickness of the rubber band. Thin strings produce sounds of higher pitch than thick strings.

How Musicians Alter Pitch

When musicians tune string instruments, they alter the pitch of the sound by rotating knoblike things around which the strings are wound. This changes the tightness of the strings, and hence, the pitch. The strings are also of different thicknesses. The thinner ones produce sounds of higher pitch. While playing, musicians press different parts of the strings to alter their lengths.

In the case of wind instruments, musicians change the frequency by blocking different parts of the pipe. This alters the length of the vibrating air column. The shorter the air column, the higher is the frequency.

Quality

The quality of a sound is also called its tone or timbre. There are two ways in which this property is commonly described.

1. Quality is the pleasant or unpleasant nature of a sound. Musical sounds have a pleasant quality, while noises, such as those produced by vehicles or machinery, have an unpleasant quality.

2. Quality is that property of a sound by which we identify its source.

The sound produced by a source does not usually have only one frequency. There are several frequencies. The lowest frequency is called the fundamental frequency. This determines the pitch of the sound. When we speak of the 'frequency of a sound', we always mean this frequency. The other frequencies present are integral multiples of the fundamental frequency. In other words, if the fundamental frequency is f, the other frequencies present will be 2f, 3f, and so on. The frequencies present in a sound are called harmonics. The fundamental frequency is the first harmonic. The frequencies which are present along with the fundamental are called overtones.

Every source emits sounds with its own fixed pattern of overtones. This is what helps us recognise different sources. In general, richer or more pleasant sounds have a greater number of overtones mixed in a higher proportion.

Teacher's Note

Quality or timbre is what makes a piano sound different from a guitar even when they play the same note - it's the unique mixture of vibrations each instrument produces.

This is a preview of the first 3 pages. To get the complete book, click below.