Communication Through Waves

Competencies:

• Perform an experimental verification to validate how sound waves can be used for communicating the information to compare the effectiveness of communication through different waves.

• Interpret the ways of encoding and transmitting information using digital signals and analogue signals integrating scientific and technical information to claim that digital signals are more reliable and effective than analog signals.

Scope:

Communication over short and long distances, communication through sound waves, analogue and digital signals

Objectives:

1. Design a device that is used to communicate over short and long distances using electromagnetic waves.

2. Perform an experimental verification to show that sound waves obey the laws of reflection of light.

3. Interpret the ways of encoding and transmitting information using digital signals and analogue signals.

Notes:

These days the messages are sent in the form of electrical signals through high frequency carrier waves like radio waves and microwaves.

• As electromagnetic waves travel, they interact with different objects and media through which they travel. In the process they get reflected, refracted and diffracted. The refraction of waves usually make the signals weak due to absorption.

Communication over short distances

• Wireless communication over short distances from few centimetres to some metres can be done using infrared waves and microwaves. Some of the examples of short range wireless communications are Bluetooth, infrared, Wi-Fi, etc. This type of communication is also used in the network of electronic devices called Internet of Things (IoT) (Phones and tablets, Fax, computers, etc)

• Bluetooth is a type of wireless communication used to transfer data or video or images using microwaves. The devices should be generally within 10 metres from each other.

• Wi-Fi (wireless fidelity) also uses microwaves. The range is generally up to 100 metres.

• Infrared waves are used to transmit information from remote controls. IR devices must have clear line of sight.

Communication over long distances

• Radio waves are used to broadcast radio and television programs and for communication over long distances.

• The radio waves easily diffract around the hills and buildings allowing it to reach all places. Diffraction is the phenomenon of bending and spreading of waves passing through gaps, corners or edges of obstacles.

• Radio waves are reflected from the charged layer of the atmosphere called ionosphere and hence, the reflected signals can cover larger area.

• Microwaves are used in mobile phones, televisions and radar. They have shorter wavelength compared to radio waves and hence they cannot be diffracted around buildings or hills. So they are not suitable for broadcasting programs. When phone calls are made, microwaves from mobile phones reach the nearest transmitter tower. Then the information is relayed through a network of transmitter tower to reach the information to the receiving phone.

• Radio waves get refracted as they travel through different layers of atmosphere and the signal becomes weak after travelling through long distances. But microwaves are not refracted by the different layers of the atmosphere and hence they can be used in satellite communications.

• Signal and information are weakened as they travel long distances. In case of wired communication, optical fibres are the preferred medium for the transmission of information.

• Infrared waves can be used to transmit multiple phone calls, Internet signal and cable television signal through optical fibre. The light signal gets transferred inside the fibre due to series of total internal reflection. They carry the signal in the form of light or invisible infrared waves. They also carry more information compared to copper cables, it has better bandwidth, Longer transmission distances and stronger security.

Communication through sound waves

An echo is the sound you hear when you make a noise and the sound wave reflects off a distant object.

Besides the novelty of hearing your words repeated, echoes can be used to estimate the distance of an object, its size, shape and velocity, as well as the velocity of sound itself. Special effects can be created with echoes reflecting off certain types of surfaces.

Since sound travels at approximately 1000 feet per second (or about 300 meters per second) and if the wall was 50 feet (or 15 meters away), the sound would return in 0.1 second. This can be seen from the relationship:

d = v*t or t = d/v

where

• d = the distance the sound wave traveled back and forth,

• v = velocity of sound, and

• t = the time it takes the sound to go back and forth.

t = 30 m / 300 m/s = 0.1 sec.

(Note that the distance was doubled to show the back and forth motion of the sound.)

Echoes can be used to tell how far away an object is, how fast the object is moving, and even its shape.

By knowing the speed of sound and measuring the time it takes to hear the echo, you can calculate the distance of the object.

Example: 1

What should be the minimum distance between the source and reflector in water so that echo is heard distinctly? (The speed of sound in water = 1400m/s)

Solution

Velocity = 2D/Time

1400 = 2 x D/ 0.1

D = 1400 x 0.1/ 2 = 70 m

Example: 2

A man standing 25 m away from a wall produces a sound and receives the reflected sound. (a) Calculate the time after which he receives the reflected sound if the speed of sound in air is 350m/s-1. (b) Will the man be able to hear a distinct echo? Explain the answer.

Solution

(a)Velocity = 2D/Time

Time = 2 x 25 / 350 =0.143 seconds

(b)Yes, because the reflected sound reaches the man 0.1 second after the original sound is heard and the original sound persists only for 0.1 second.

Bats use echoes to find moths, while flying around at night.

Analogue and digital signals

The information carried by the electromagnetic waves like radio waves, microwaves, infrared waves and visible light may be pictures, video, sound or texts. These information are transmitted in the form of signal. The signal may be of two types- analogue signal and digital signal.

An analogue signal changes its frequency and amplitude continuously just like that of the sound being transmitted.

The signal is added to an electromagnetic wave called carrier wave so that it can be transmitted. This process of imposing input signals into a carrier waves is called modulation.

The carrier wave is removed when the signal is received. When the signal is received, it is demodulated back to the original signal. A carrier wave is a pure wave of constant frequency which does not carry information but helps in transmitting information.

The two most common types of modulation of signals are amplitude modulation (AM) and frequency modulation (FM).

A digital signal has just two values 0 and 1(binary digits) to represent information. The information is converted into codes formed by combination of 0s and 1s. These codes are transmitted as combination of low and high voltages through the carrier wave and decoded back to original signal. A digital signal is represented by a square wave.

The original digital signal can be easily extracted from the distorted signal by removing the noise using digital processing or filter. The digital signal maintains the quality of the information over long distances. More information can be made to fit in a smaller space in digital signals compared to analogue signals. Therefore, digital signals can carry more information per second than analogue signals.

Safety measures against harmful communication waves

• Disable Wireless Functions: To reduce your exposure to EMFs, turn on the Wi-Fi function only when necessary.

• Replace Wireless With Wired Devices: Some devices — such as your wireless keyboard, headset, and mouse — function only when they are transmitting a wireless signal. Replace these devices with a wired version.

• Keep EMF Sources at a Distance: If you have a Wi-Fi router in your home, locate it in a place that is some distance away from where people spend the most time.

• Use Your Smartphone Safely: Keep the device as far away from your body, especially when you are speaking on it.

• Prioritize Sleeping Areas: Remove all unnecessary wireless devices from the sleeping area or deactivate their Wi-Fi feature.

Exercise Questions:

Q1. Karma is a student studying in a boarding school. He wants to call his parents using warden’s mobile phone since he is in need of some money to buy stationaries. Which of the following electromagnetic wave is used in mobile phones to transfer information?

A. Radio waves.

B. Microwaves.

C. Infrared radiation.

D. Gamma rays.

Q2. Although radio wave has the longest wavelength, microwave is used to send signals to satellite this is because radio wave

A. cannot penetrate the ionosphere.

B. cannot be detected by satellite.

C. cannot carry the signal.

D. signal gets attenuated easily.

Q3. How do bats find moths at night? Choose correct answer.

• They have very keen vision

• They listen for chirps from the moths

• They process echoes from the moths

• Bats can see at night.

Q4. If sound travels at 1000 meter/second, why does it take 2 seconds to hear the echo of a wall 1000 meter away?

• Echoes travel at 1/2 the speed of sound

• It takes sound 1 second to get there and 1 second to get back

• It only takes 1 second to hear the echo

• Some of the sound is absorbed by the wall

Q5. School caretaker who is a regular listener to Kuzoo FM radio programmes experienced signal issues recently. He complained and engineers at the studio tried amplifying the signal. Engineers observed that each time the signal was amplified, the unwanted noise was also getting amplified, weakening the original radio broadcast.

i. What kind of radio signal school caretaker is listening to?

ii. Why is the signal in the radio not very clear?

Q6. Radio wave is used for broadcasting radio and television programmes owing to its longest wavelength and high ability to diffract. How would Radio and television communication be if the microwave is used instead of radio waves?

Q7. Why do you prefer digital signals for signal transmission?

Q8. What is meant by an echo? What is the condition necessary for an echo to be heard distinctly?

Q9. What is amplitude modulation and frequency modulation ?