Sound

Hello, and welcome to today's physics lesson! Today, we are going to explore the fascinating world of sound — how it is produced, how it travels, and what makes different sounds unique. By the end of this lesson, you will understand why we hear echoes, why sound cannot travel in space, and how musical instruments create such beautiful melodies.

Let us begin with a simple question: what exactly is sound? Sound is a form of energy that produces the sensation of hearing in our ears. When you hear a school bell ring, a guitar play, or even your own voice, you are experiencing sound energy at work.

Now, here is something crucial — sound is produced due to vibrations. Every source of sound is a vibrating body. When an object vibrates, it moves rapidly back and forth, and this motion creates sound waves that travel to our ears.

Imagine you pluck a guitar string. The string moves to and fro from its rest position — this is called vibratory motion. As long as the string vibrates, you hear sound. The moment the vibrations stop, the sound disappears. This is true for all sounds: when the vibrating body stops, the sound ceases.

Let us try a simple example. Press one end of a ruler firmly on a table and pull down the other end. When you release it, the ruler vibrates rapidly and produces a humming sound. After a few seconds, the ruler stops moving — and the sound stops too. This proves that vibration is essential for sound production.

Where do different sounds come from? Let us explore some common sources of sound.

A tuning fork is a U-shaped metal instrument with two prongs. When you strike one prong against a rubber pad, both prongs begin to vibrate, producing a clear, pure tone. You can actually see these vibrations if you touch a light ball suspended by a thread to the prong — the ball will jump back and forth rapidly.

Musical instruments produce sound through different types of vibrations. In wind instruments like the flute and clarinet, air blown into the pipe causes the air column inside to vibrate. In instruments like the harmonium, thin metal reeds vibrate when air passes through them. Stringed instruments such as the sitar, guitar, and violin produce sound when their strings are plucked, struck, or bowed.

Instruments like the drum and tabla have stretched leather membranes that vibrate when struck, creating rhythmic sounds.

Now, what about us humans? We produce sound through an amazing organ called the larynx, also known as the voice box. Located in your throat, the larynx contains two folds of tissue called vocal cords. When you speak, air from your lungs passes through these cords, causing them to vibrate. You can feel this vibration by placing your fingers gently on your throat while you talk or sing. When you breathe normally, the vocal cords are relaxed and open. But when you produce sound, they tighten and vibrate to create your voice.

Interestingly, not all animals use vocal cords. Bees, for example, do not have voice boxes at all. They create their buzzing sound by moving their wings up and down extremely fast.

Here is a fundamental principle about sound: sound needs a medium to travel.

Sound cannot travel through a vacuum. This means in the empty space of the universe, where there is no air, sound cannot propagate at all.

Imagine two astronauts on the moon. Even if they stood right next to each other and shouted, they would hear nothing. There is no atmosphere on the moon to carry the sound waves. This is why space movies that show explosions with loud booms are not scientifically accurate!

We can demonstrate this with a simple thought experiment. Place an electric bell inside a sealed glass jar connected to a vacuum pump. When you ring the bell with air inside, you hear it clearly. But as the pump removes air from the jar, the sound becomes fainter and fainter. When almost all air is removed, you see the bell's hammer striking the gong, but you hear nothing. The sound simply cannot reach you without a medium to carry it.

Sound can travel through solids, liquids, and gases — but it travels differently in each. In fact, sound travels fastest through solids, slower through liquids, and slowest through gases.

Here is a practical example. If you press your ear against a railway track, you can hear an approaching train long before you hear it through the air. The sound travels through the solid steel much faster — about 5960 m/s — compared to just 330 m/s in air. In water, sound travels at approximately 1500 m/s.

Now let us understand how sound actually moves through a medium. Sound travels in air as a longitudinal wave.

What does this mean? In a longitudinal wave, the particles of the medium vibrate back and forth in the same direction that the sound is traveling. Imagine a line of people standing close together. If the first person pushes the second, who pushes the third, and so on, a wave of motion passes through the line. Each person moves slightly but stays in roughly the same place. Similarly, air particles vibrate about their mean positions, passing energy from one particle to the next.

As sound travels, it creates regions of compression — where particles are pushed close together — and regions of rarefaction — where particles spread apart. One complete wave consists of one compression and one rarefaction. The distance from one compression to the next is called the wavelength, represented by the Greek letter lambda, λ. It is measured in metres.

Let us learn some important terms that describe sound waves.

Amplitude is the maximum displacement of a particle from its rest position. Think of it as how far the vibrating object moves from its central position. Amplitude is measured in metres.

Time period, denoted by T, is the time taken to complete one full vibration. It is measured in seconds.

Frequency, represented by f or n, is the number of complete vibrations produced in one second. The unit of frequency is hertz, abbreviated as Hz.

These quantities are related by a simple formula: frequency equals one divided by time period.

Or, f = 1/T, where f is frequency in Hz and T is time period in seconds.

The speed of a wave can be calculated using another important relationship. Speed equals frequency multiplied by wavelength.

Or, v = fλ, where v is speed in m/s, f is frequency in Hz, and λ is wavelength in metres. Frequency equals one divided by time period, so we can also write speed equals wavelength divided by time period.

Not all sounds are audible to human ears. The normal human ear can detect sounds with frequencies ranging from 20 Hz to 20,000 Hz. This range is called the audible range.

Sounds with frequencies below 20 Hz are called infrasonic or subsonic sounds. We cannot hear them. For example, a pendulum that takes two seconds for one complete swing vibrates at only 0.5 Hz — well below our hearing threshold.

Sounds with frequencies above 20,000 Hz are called ultrasonic sounds. These are also inaudible to humans, but many animals can hear them. Dogs can hear sounds up to 50,000 Hz, while bats can detect frequencies as high as 100,000 Hz.

Bats use ultrasound in a remarkable way. As they fly at night, they emit ultrasonic squeaks. These sound waves bounce off obstacles and return as echoes. By listening to these returning echoes, bats can navigate perfectly without using their eyes — a process called echolocation.

Now let us explore what makes sounds different from each other. Sound has three main characteristics: loudness, pitch, and quality.

Loudness depends on amplitude. The greater the amplitude of vibration, the louder the sound. When you strike a drum gently, the membrane vibrates with small amplitude, producing soft sound. Strike it hard, and the large amplitude creates a loud boom. Loudness also depends on the area of the vibrating body — a large drum produces louder sound than a small one.

Pitch depends on frequency. A sound with high frequency has high pitch and sounds shrill. A sound with low frequency has low pitch and sounds flat or grave. This is why a girl's voice typically sounds shriller than a boy's — her vocal cords vibrate at a higher frequency.

Musicians change pitch in various ways. In a flute, covering different holes changes the length of the vibrating air column — shorter columns produce higher pitches. In stringed instruments, pitch changes by altering where you pluck the string or by adjusting the tension. Tighter, thinner strings vibrate faster and produce higher notes.

Quality, also called timbre, is what allows you to distinguish between two sounds of the same loudness and pitch. You can recognize your friend's voice on the phone, or tell a violin from a flute, because each produces a unique waveform with different combinations of frequencies.

Like light, sound can be reflected from surfaces. When sound strikes a hard, smooth surface, it bounces back according to the laws of reflection. The angle at which sound hits the surface equals the angle at which it reflects, and all three — the incoming sound, the reflected sound, and the normal line — lie in the same plane.

When you hear a distinct reflected sound after the original, it is called an echo. To hear an echo clearly, the reflecting surface must be at a minimum distance of 16.5 m from the source of sound. This is because the original sound persists in the human ear for about 0.1 s. If the reflected sound arrives sooner, it merges with the original and we cannot distinguish them.

Since sound travels at about 330 m/s in air, it covers 33 m in 0.1 s. The sound must travel to the surface and back, so the one-way distance is half of that — 16.5 m.

Sound can also be absorbed. Soft, porous materials like curtains, carpets, wood, and thermocol absorb sound rather than reflecting it. This is why theatre walls are lined with special materials — to prevent echoes and unwanted noise.

A sound-proof box uses multiple layers of absorbing materials. The walls might have wooden strips, the ceiling could be covered with plaster of Paris over thermocol sheets, and the floor would have thick carpets. Doors and windows are sealed with thick curtains and rubber stripping to block sound from entering or escaping.

Let us quickly calculate the speed of sound using a practical example. Imagine you see lightning flash during a thunderstorm, and you hear the thunder 2.5 s later. If the speed of sound is 330 m/s, how far away was the lightning?

Using the formula: distance equals speed multiplied by time. Distance equals 330 m/s multiplied by 2.5 s, which equals 825 m. Thus, the lightning struck 825 m away. This is why we see lightning before we hear thunder — light travels much faster than sound, taking negligible time to reach us, while sound takes measurable time.

Let us recap the key points from today's lesson.

First, sound is a form of energy produced by vibrating bodies. No vibration means no sound.

Second, sound needs a medium to travel — it cannot pass through vacuum. It travels through solids, liquids, and gases, fastest in solids and slowest in gases.

Third, sound travels as longitudinal waves, with particles vibrating in the direction of wave motion, creating compressions and rarefactions.

Fourth, the human ear hears sounds between 20 Hz and 20,000 Hz. Sounds outside this range are inaudible to us, though many animals can hear them.

Fifth, loudness depends on amplitude, pitch depends on frequency, and quality depends on waveform. Sixth, sound reflects from hard surfaces to create echoes, and gets absorbed by soft materials — principles used in designing concert halls and sound-proof rooms.

Sound is all around us, from the gentle rustling of leaves to the powerful roar of thunder. Understanding how it works helps us appreciate the physics behind music, communication, and even the design of the spaces we live in. Keep observing the sounds in your world, and remember — every sound begins with a vibration.

Thank you for listening, and see you in the next lesson!