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Background information for Sound

Below you will find some commonly asked questions about sound. You may also want to explore the other topics relating to our school program "Sound Connexions".

What is sound?

Sound is a vibration or wave of air molecules caused by the motion of an object. The wave is a compression wave where the density of the molecules is higher. This wave travels through the air at a speed dependent on the temperature. A sound wave contains energy, which in turn means it can make things move. However, if the wave strikes something solid, the wave will bounce back -- an echo. Sound energy can be changed into other forms of energy, e.g. electrical energy, and vice versa; this is one of its properties that allow us to communicate by telephone.

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What is sound vibration?

Sound is produced when an object moves or vibrates. Without movement there could be no sound. When an object moves or vibrates, the air molecules around the object also vibrate. Vibrating objects (as long as they are not in a vacuum) produce sound.

Sound travels at different rates in different media. In dry air, sound travels at 331.6 m/sec at 0 C.

Each molecule moves back and forwards only a tiny distance, but it is enough to cause the air particles to bump into each other. This creates areas where there are many molecules pushed close together -- compression; and areas where molecules are spread far apart --rarefactions. These compressions and rarefactions move outwards away from the sound source in circles. A sound wave is created when a series of these pressure changes/waves move through the air.

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What is the frequency of sound waves?

When we draw a sound wave, the wave peaks and valleys are close together or far apart. Sound waves vibrate at different rates or "frequencies" as they move through the air. Frequency is measured in cycles per second, or Hertz, after the German physicist who experimented with sound in the 19th century. The faster an object vibrates, i.e. the higher the frequency, then the higher the pitch of the sound. For example, a tuning fork for A above middle C will vibrate 440 times per second and has a frequency of 440 Hertz.

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What is the wavelength of a sound wave?

When a wave is created, the distance between one compression and the next compression is called the wavelength. The faster the sound waves pass a given point, the shorter the wavelength and the higher the frequency. Sounds of all frequencies travel at the same rate in the same medium. (Sound in dry air at 0 C travels at the rate of 1200 kilometres per hour, or 331.6 m/sec; in a solid medium the sound waves travel faster.)

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What is the amplitude of sound waves?

The vibrations can also "squeeze" the air molecules together very hard or very gently. This squeezing is called "amplitude" and is represented on the top half of the diagram below. The bottom half of the diagram is a representation of the pressure of the air during a sound wave. The horizontal line represents normal air pressure.

The more we push an object to make it vibrate, the larger the vibrations and the louder the sound, or the greater the amplitude. Sound waves with the same frequency can have different amplitudes.

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Can sound energy be changed into other forms of energy?

Since sound is a form of energy, it can be changed from one form to another. Other forms of energy can be transformed into sound. Sound energy can be changed into electrical energy. Sound waves that are changed into electricity can be seen on an oscilloscope.

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What is an oscilloscope?

An oscilloscope is a machine that provides a visual record of the voltages of an electrical signal. Connecting a microphone output to an oscilloscope allows us to see the characteristics of a sound wave on a screen.

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How can we use the conversion of sound energy?

Technology enables us to "connect" with other people who are not in the room with us. The telephone converts sound energy into electrical energy, and back into sound energy again. It was invented in 1876 by Alexander Graham Bell, a teacher of the deaf.

Bell's Telephone

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How does a telephone work?

In its simplest form, a telephone set consists of a transmitter (mouthpiece), a receiver (ear-piece) and a dial or push-button mechanism. Parts of the telephone include the jack, electronics box, dial, hand-set and bell.

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What is the telephone electronics box?

A telephone has to do many things. It handles all the signals; it notifies the central office to make a bell ring at the other end; it sends and receives electrical signals, and it "decodes" the electrical signals that it receives. All this is done inside the telephone electronics box. The electronics are made up of different electrical circuits, which include capacitors, resistors and inductors.

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Why is there a telephone bell?

Many modern telephones have a buzzer instead of a bell. But the purpose is the same. When a signal comes into your telephone over the wires, the bell rings to tell you that someone has called your number.

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How does a telephone dial work?

Dials are being replaced by keypads, but there are still certain exchanges (i.e. linked groups of telephones) that cannot handle some styles of telephones with keypads. When you pick up the handset, an electric current flows through the line from your local exchange (generally indicated by the first three digits of your telephone number). The flow of current acts as a signal to the exchange equipment to "handle" a new call. The dial tone indicates that the exchange is ready to accept your call. As you release the dial after each digit of a phone number, the current is turned off and on from one to 10 times, depending on the digit. For example, for the number 3, the electric current is interrupted three times. This "coded" signal goes to a nearby telephone switching office. Here it is used to make the correct connection to the desired telephone line.

Today's keypads have 12 keys. Each of these 12 keys produces a different electrical signal when it is pressed. These sounds are a combination of two of seven possible tones. They are converted into electrical signals that travel along the line to the office. At the exchange, the signals are recognised as the numerals 0, 1, 2, etc.

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How does the telephone handset work?

The handset contains both the ear-piece that you hold to your ear, and the mouthpiece, which is the part into which you speak. The mouthpiece is like the ear of the telephone. It hears what you say and converts these sounds into electricity. The ear-piece is like the mouth of the telephone -- it converts the electrical signals into sounds that you hear. In a telephone, sound waves produced by a person's voice exert pressure on a thin metal plate called a diaphragm, which vibrates and presses on carbon granules in the mouthpiece. When the carbon granules are pressed, the flow of the electrical current from the telephone exchange changes as it passes along the wire. The changes in the current form an electrical "pattern" of the voice.

At the receiving end, the electrical signals flow through a coil in the ear-piece of the telephone. The coil is wrapped around a soft iron core and forms an electromagnet. When the current passes through the coil, it creates a magnetic field which causes a diaphragm in the ear-piece to vibrate in harmony with the incoming signals. In this way, the receiver duplicates the sound waves created by the person on the other phone.

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