Sound

What is Sound

Sound is a form of energy that gives us the ability to hear. It is really important as it is used by us humans to communicate. Sound is of different types. Some pleasant like the chirping of birds or music, some unpleasant such as the crying of a child and the blaring of the horn, some loud, some soft, some bass, and some shrill.

How is Sound Produced

Sound is Produced due to vibrations. Vibrations are the to and fro movement / oscillations of an object around a fixed center point. Let’s understand this with a few activities. 

1.      Aim – To Show that Vibrations Produce Sound
Materials Required – A stick / Rod, Metal Plate

Process – Take a metal plate and a strike it with a stick / rod. Immediately after this, touch the metal plate gently. You will feel some vibration. Next, repeat the process but instead of touching the plate, put in near your ear. Now, you can hear a slight humming sound.

Observation – The Vibrations Observed in Test one create the Sound seen in Test two.

2.      Aim – To show that Vibrations cause Sounds
Materials Required – Tuning Fork, Stick / Rod
Process – Take a Tuning Fork and use a Rod / Stick to hit it. You can see it oscillate / vibrate and produce sound at the same time.

Observation – The Vibrations observer produce sound. Once it stops vibrating, the sound stops as well.

3.      Aim – To Show that Vibrations Cause Sounds

Materials Required – A Plastic Scale / Ruler, Table / Surface

Process – Take the Plastic scale / ruler and put it on the table such that half of it is hanging and half of it is on the table. Hold the half that is on the table and hit the other half. You will see the scale vibrate and a humming sound being produced.

Observation – The vibrating of the scale is creating sound

4.      Aim – To Show that Vibrations Create Water Ripples

Materials Required – Metal Container / Bowl, Water

Process – Take a metal container and fill ½ of it with water. Then hit the metal container on the side and see it gently vibrate.

Observation – You can see that after hitting it on the side, the water has gentle ripples starting to form.

We can also see this effect when we touch a speaker when its playing music.

How Sound Travels

We now know that sound is produced due to vibrations. But how does it travel? It doesn’t directly travel into our ears. Instead, energy is transferred from one vibrating molecule to another. A vibrating molecule causes the molecules near it to vibrate which cause the molecules around them to vibrate and this continues. This happens in the form of waves. So, We can say that sound travels in waves. These Waves are known as sound waves. This can happen in two ways. Compressions and Rarefactions. In Compressions, two molecules are bought closer together while in rarefactions, they are pushed away from each other. Now Let’s return to the previous example of a music speaker. The Speaker creates a two and fourth vibration and transfers energy to molecules around it. It keeps on spreading through compressions and rarefactions in a 360 degree radius around the speaker.

Sound also needs a medium to travel through. This is because sound requires other molecules to pass on the energy to. An Example of a medium can be solid, liquid, and / or gas. Solids such as wood, metal etc, Liquids such as water, and gases such as air all allow sound to travel through them. Since sound requires molecules to travel, it can not travel in a vacuum. Since the moon doesn’t have an atmosphere, this is why astronauts can not talk to each other. Let’s do an activity to test both of these concepts.

Aim – To Show that sound can travel though solids, liquids, and gases.

Materials Required – A Friend, A Table, A Bell, and a Big Container Full of Water.

Process – Start by laying your head on the table ensuring your ear is touching the table and the other ear is covered / closed. Ask your friend to tap the table and you will be able to hear the sound of the table being hit therefore proving that sound can travel through solids.

Take the Large Container Full of water and put your ear in it. Ensure that the other ear is covered, and that water does not go in either ear. Take a bell and ring it under the water. You will be able to hear the bell ring. This Proves that sound can travel through liquids.

Talk to your friend in general. Since you can hear them, this proves that Sound can travel through gases.

Observation – Sound Can Travel Through Different Mediums.

Aim – To show that sound can not travel through a vacuum.

Materials Required – A Closed Jar, An Alarm Clock, a Vacuum Sealer.

Process – Take the Alarm Clock and set it to ring in 10 minutes. Put it into the jar and use the vacuum sealer to extract all the air from the jar and seal it. Wait for the timer to ring and you will not be able to hear it. This is because sound requires molecules to travel which are not present in vacuums.

Conclusion – Sound Can not travel through vacuums.

Speed Of Sound

Sound travels at different speeds in different mediums. This depends on whether the molecules are packed closely together or the speed at which the sound is made. The Closer the Molecules are packed together, the faster the sound travels and vice versa. Sound travels the slowest through air traveling at about 340 m / s. It is followed by liquids at 1498 m / s. Which is topped by solids. In steel, sound travels roughly at a speed of 5780 m / s. This is also one of the reasons in which we can see lightning before we hear the thunder. Light travels almost instantly ( 3,00,000 km / s ) causing it to reach our eyes faster than the sound can reach out ears. Animals Such as dolphins and whales use sound to communicate across hundreds of meters. Human Beings have developed aircrafts that can go faster than the speed of sound. The speed of sound is also known as Mach 1. We have been able to develop aircrafts that are faster than their own sound. These aircrafts are known as supersonic aircrafts. They go at around the seed of 1236 km / hr. An example of this is the Concorde.

Aim – Making of a cup based telephone.

Materials Required – Two Paper Cups, a Thick String, and a Sharp Needle.

Process – Poke a hole at the center of the base of both cups and tie the string to it. Speech into one cup and have a friend listen into the other.

Observation – Your Friend Will Be able to hear you speak as sound is travelling through the string. The Thicker the string, more the sound since there are more molecules.

Characteristics of Sound

Sound waves can be classified based on the pitch, loudness, and quality of sound.

·         Pitch – Pitch is determined by frequency of the Sound Wave. We know that for sound to be made, an object needs to vibrate. The Frequency is the number of times an object vibrates per unit time ( usually a second ). Its SI unit is Hertz (Hz ). Frequencies below 20 Hz are called infrasonic and frequencies above 20,000 Hz are called Ultrasonic. Thanks to moderns technologies, us humans use these ultrasonic waves in medical investigation and autonomous navigation. Different organisms communicate and hear at a different frequency. For example:

o   Humans – Humans Can hear frequencies ranging from 20 Hz – 20,000 Hz. Our voice boxes are also capable of producing sound from 60 Hz – 13,000 Hz.

o   Dogs – Dogs can Hear From  50 Hz – 45,000 Hz.

o   Cats – Cats Can Hear from 45 Hz – 85,000 Hz.

o   Bats – Bats Can Hear up to 1,20,000 Hz and usually produce a sound of around / above 70,000 Hz.

o   Dolphins – Dolphins Can Hear up to 2,00,000 Hz.

o   Whales – Whales create sound waves that travel up to 800 km in distance.

o   Elephants – Elephants Can Hear up to 5 Hz – 10,000 Hz.

Frequency determines pitch. Higher the frequency, higher the pitch. Pitch is something used to determine the shrill – ness of a sound. The Higher the Pitch, the shriller the sound is. The lower the pitch, the heavier, coarse, and bass the sound is. For example, a birds chirp or a whistle is of higher frequency therefore having a High Pitch and a shriller sound. While a drum or a roar of a lion is of lower frequency and therefore having a Lower Pitch and a Deeper Sound.

·         Loudness – Loudness is determined by Amplitude of the Sound Wave. Amplitude is the maximum distance a vibrating object has reached from its mean / center point. The Higher the Amplitude, the louder the sound. The Lower the amplitude, the quieter the sound. Loudness of objects and scenes are measured in Decibel ( dB ). It is the SI Unit of Loudness. Here are some Decibel values of common scenes.

Graphical Representation of amplitude and frequency. Since sound is a form of mechanical energy released in the form of waves, it can be represented graphically using curved lines. The center horizontal line is the mean / resting point of the object. The deviances from this point are shown by curved lines. The closer they are together, the more the frequency while the further they are from the center line the louder the sound. This applies vice versa as well.

·         Quality – Quality is determined by the Source of the Sound Wave. The quality of a sound is also known as timber. It depends on the source of the sound. Two objects that have the same frequency, and same loudness but may sound different if they have different timber. This is how we can distinguish between two people in a chorus or a violin and a sitar.

How we Hear Sound

Our Ears are a sensory organ responsible for giving us the ability to hear. They consist of three parts. The outer ear, the middle ear, and the inner ear.

·         The Outer Ear – The Outer Ear is the only part of the ear we can physically touch. The rest of the ear is present deep inside the skull. The outer ear consists of the Pinna and the ear tube. The Pinna is a round object that collects and transfers the sound waves into the ear tube. The ear tube is hollow cylindrical passageway which transfers these sound waves to the ear drum. The ear drum is a thin stretched membrane that mimics the vibrations of the sound waves. It separates the outer ear from the middle ear.

·         The Middle Ear – The Middle Ear consists of thee delicate bones. The Hamer, the anvil, and the stirrup. These bones vibrate along with the ear drum and send the information to the inner ear.

·         The Inner Ear – The inner ear gathers these vibrations through a spiral shaped cochlea. The inner ear consists of a liquid like substance containing many small hairs which process this information. It then sends the information into the brain via an auditory nerve.

How we Produce Sound

Us Humans produce sound using our voice box or larynx. The voice box is located at the upper end of the wind pipe below the epiglottis. The sound produced depends on two vocal chords present inside the voice box. These two chords are placed in such a way that there is a slight gap between them. When the lungs pushes air through the wind pipe, it sends it through this gap therefore producing sound. If they are tight and small, a higher pitch sound is produced. If they are long and loose a bass sound is produced. The larynx of a woman is around 15mm in size. Their vocal chords are short, giving them a high pitched voice. The larynx of a male is around 20 mm in size. Their vocal chords are long, giving them a deep voice. Children have very small vocal chords which is why they sound different from men and women.

How Animals Produce Sound

Most mammals such as cows, sheep, dogs, cats, etc produce sound using their voice box. However, some animals use other means to produce sound. For example, birds have a ring cartilage present at the start of their wind pipe. This cartilage is known as a syrinx. Some birds have two parts to this syrinx. These birds can produce sounds of two notes. Some insects such as bees, mosquitos, etc produce their buzzing sound by rapidly flapping their wings. Crickets make their chirping sounds by rubbing their wings together. Grass hoppers by rubbing their hind legs with their wings. Snakes hiss by forcing air out of their mouth.

How Musical Instruments Produce Sound

Musical instruments use various techniques to produce sounds of different pitch, frequency, and volume. They can broadly be classified into three types. String Instruments, Wind Instruments, and / or percussion / membrane instruments.

·         String Instruments – String Instruments are instruments that have strings. Ex – Guitar, Violin, Sitar, etc. Sound is produced by striking these strings. The pitch and loudness depends on the length, thickness, and tautness of the string. The thicker the string, the lesser the pitch, the shorter / tauter the string, the higher the pitch. There instruments have knobs in the top to control these ranges.

·         Wind Instruments – Wind Instruments are instruments that produce sound via the vibration of a wind column. The frequency is manipulated by increasing and / or decreasing the length of this wind column. Ex – Flute, Saxophone, etc. When the musician blows into this instrument, the air column inside starts to vibrate. In a flute, for example, by closing or opening the holes present on the body known as bores, the length of the air column changes therefore changing the pitch.

·         Percussion Instruments / Membrane Instruments – Percussion instruments are instruments with a membrane that needs to be hit that produces sound. Ex – Drums, Tabla, etc. They usually have a hollow cylindrical body with an adjustable membrane on top which is used to manipulate the sound by changing its tautness.

Reflection and Absorption of Sound

When speaking in an empty room, you must have heard your voice repeating when you say something. This is called an echo. Sound can either be reflected or absorbed. This happens the same way a ball bounces back when hitting a wall. An echo is the reflection of a sound wave. Echo’s can also be heard when in mountain ranges or even in valleys. Sound gets reflected from surfaces whether they are far or near. But why don’t we hear them? We hear an echo only if it takes 1/10^th of a second to reflect and come back. So, taking 340 m/s as the speed of sound in air. We get this math equation:

This is the total distance sound that can cover in air during this 1/10^th of a second. To find out the maximum distance an object can be for an echo, we can divide this number by 2 therefore giving us 17m.

Echo’s are used in various fields. For example, we use ultrasonic echo’s in medical image processing to obtain an image of the organs or for a ships SONAR to detect the depth of the ocean and object below them. They are also used by bats to determine the location of nearby objects.

Sound is both absorbed and reflected when it hits an object. However, some materials are better at absorbing them than others. Usually, soft, porous materials are good at absorbing sound. For example, cotton, curtains, clothes, etc. However, less porous materials such as concrete, rock, etc reflect sound waves more.

Noise Pollution

If a sound is produced with regular frequencies and is pleasing to hear, it is called music. Most musical instruments follow this concept and maintain regular frequencies. If a sound has irregular frequencies and is unpleasing to hear, it is called noise. Noise, if exposed over a long period of time, is called noise pollution and is harmful to health. Noise Pollution at a particular place is determined by:

Noise pollution is caused due to:

·         Telephones, bells, ringing in offices.

·         Machinery clattering.

·         Automobiles honking and tires screeching.

·         Multiple Conversations talking of each other.

·         Road Work Going on noisily.

Noise Pollution can be harmful to us human as well. It may cause:

·         Deafness.

·         If listening to sounds over 50 dB – Partial Hearing Loss.

·         Irritation.

·         Lack of Sleep.

·         Reduced Productivity of Work.

·         Chronic Mental Illnesses.

·         Annoyance.

To Control Noise Pollution, We can:

·         Plant trees around noisy industries to absorb sound from noisy industries.

·         Position industries away from residential areas.

·         Reduce the volume at which home radios and TV’s are being played at.

·         Ban Heavy Load Construction Vehicles from Entering Residential Areas.

·         Stop the Use of loud speakers in public places for events.

·         Avoid honking car horns if not necessary.