Sound

Aim: To learn about the physics of sound In this topic we will learn that: Sound is form of energy Sound is produced by vibrations Sound needs a medium Echoes are reflected sound How the ear detects sound Loud sounds can damage hearing Speed of sound is less than that of light Explain the lag between hearing and seeing an event

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The Facts Sound … 1. Is a form of energy produced & transmitted by vibrating matter 2. Travels in waves 3. Travels more quickly through solids than liquids or gases 4. Sound needs a medium to travel.

Activities To try 1)Tie a metal object, e.g. a spatula, to a piece of string and hold the other end of the string against your ear or a microphone connected to speakers. Now tip the metal object with another metal; the transmission of sound along the string is impressive. 1)Put the earphone of your music player to a sound level meter at the volume you normally listen to music at and see if you are listening to music too loud that could damage your hearing.

Showing Sound is a form of Energy A signal generator is connected to a loudspeaker. A balloon is held near the speaker. When the signal generator is switched ón the balloon vibrates with the sound. This shows sound is a form of energy.

How Sound Travels Sound is caused by making objects vibrate e.g Plucking a guitar string. Your vocal chords vibrating. Tin whistle making air vibrate Drum beating the drum skin vibrates. What is a vacuum? Can sound travel in vacuum? Why/Why not?

Vibration -Back and forth movement of molecules of matter -For example,

Sound – The basics We hear things when they vibrate. If something vibrates with a high frequency (vibrates very ______) we say it has a _____ pitch. If something vibrates with a low frequency (vibrates ______) we say it has a ____ pitch. The lowest frequency I could hear was… Words – slowly, low, high, quickly

Showing Sound cannot travel in a Vacuum A bell is set ringing as setup in the bell jar ón the right. The air is removed from the bell jar using a pump. Eventually the bell cannot be heard but you can see the bell is still working. Sound cannot travel in a vacuum and needs a medium to travel in. Why can sound travel faster in liquids and solids than gases?

Drawing sounds… This sound wave has a _____ frequency: This sound wave has a ___ _frequency:

Drawing sounds… This sound wave has a _____ amplitude (loud): This sound wave has a _____ amplitude (quiet):

Sound Waves -Alternating areas of high & low pressure in the air -ALL sound is carried through matter as sound waves -Sound waves move out in ALL directions from a vibrating object

Wavelength & Frequency -Wavelength is the distance between one part of a wave and the same part of the next wave -Frequency is the number of waves moving past a point in one second

Pitch A measure of how high or low a sound is Pitch depends on the frequency of a sound wave For example, - Low pitch - Low frequency - Longer wavelength - High pitch - High frequency - Shorter wavelength

Sound and Instruments -Instruments can be played at different pitches by changing lengths of different parts. -For example, -Another way to make different pitches is to change the thickness of the material that vibrates. -For example, A trombone’s mute absorbs some of the sound waves produced, thus producing a softer note when played.

Speed of Sound The speed of sound in air is 340 m/s, fast for short distances. Over longer distances a time lag results. We often see the fireworks before hearing the explosion. The speed of sound depends on a)The density of the material, the greater the density the greater the speed. b)The atmospheric pressure of the air. c)The temperature. The speed of light is much greater nearly a million times faster.

Speed of Sound Evidence that light travels faster than sound is seen with thunder and lightning. Often we see the lightning before we hear the thunder. The distance the storm is away can be calculated my measuring the time lag between the flash of lightling and the thunder.Problem A flash of lightning is seen and 6 seconds later the clap of thunder heard. How far away is the lightning if the speed of sound is 340 m/s?

Speed of Sound Answer distance = speed x time = 340 x 6 = 2040 m

Reflection and Refraction of Sound Like light, sound can be… Reflected – sound reflections are called Echoes. Refracted – bend around corners and obstacles. This is why you might sound strange if you try talking underwater

Sonar -An instrument that uses reflected sound waves to find underwater objects -For example, Animals use sonar or echo location to find their prey; these sounds have such a high pitch or frequency that the human ear cannot hear Humans use sonar to locate or map objects

Echos (good and bad) Echos can be a nuisance in halls, cinemas and theatres as they can cause silent spots, muffled noise etc. This can be stopped by having soft padded walls and thick carpets. Ships can use echos to find the depth of the sea. Some animals use echos to locate their prey. e.g bats.

Calculating the seabed depth If a sound is sent down to the seabed by a ship and the echo picked up 1.2 seconds later, what is the depth of the water if the speed of sound in water is 1500 m/s. Speed =distance (distance travelled by the echo) time Distance = speed x time = 1500 x 1.2 Distance = 1800 m The distance travelled by the echo Is twice the distance to the seabed so Depth of the water is 1800 ÷ 2 Depth of the water = 900 m

The Ear

Sound is carried to our ears through vibrating air molecules. The outer ear collects the sound and directs to the eardrum. The eardrum vibrates and transmits the sound to the middle ear and in turn to the inner ear. Nerves in the inner bring the sound as electrical signals to the brain. Middle Ear

Hearing problems Our hearing range can be damaged by several things: Too much ear wax! (1)Damage to the auditory nerve (2)Illness or infections (3)Old age (4)Listening to music too loud

Noise levels Our ears are sensitive, they can hear soft or quiet sounds as well as loud sounds. Loud sounds carry alot of energy and if loud enough can break the eardrum (painful). It the nerves in our ears are damaged by loud sounds we cannot recover from them. We must therefore be careful not to expose ourselves to sounds that are too loud protect our ears. sound levels are measured using the decibel scale.

Sound level comparisons

Hearing Precautions Our ears need to be protected to prevent hearing loss. The following are some ways we can do this. Heed warning signs Wear ear muffs where necessary Move away from high-level sound sources Double-glazed windows reduce noise from outside High walls/trees reduce noise from busy roads. Remember if you are in a place where you have to shout to be heard by a person arm's length away from you it is too noisy.

Review 1)What causes sound? 2)Why does sound need a medium? 3)Which is faster light or sound? 4)What causes echoes? 5)How can echoes be usful? 6)How do our ears detect sound? 7)When must ear protection be worn? 8)How can we prove sound is a form of energy? 9)Why does sound travel faster in solids than liquids or gases? 10)Why do auditoriums, theatres and cinemas have soft walls?