Sound is a Form of Energy
Sound is Produced by Vibrations
Vibrating objects in solids, liquids and gases produce sound. The sound energy is transmitted through matter from the vibrating object.
That sound is a form of energy can be demonstrated by showing that
1. sound can cause movement,
2. sound can be converted to other forms of energy,
3. other forms of energy can be converted to sound.
Our ears can detect vibrations of frequencies from 20 per second to 20,000 /s i.e. 20 Hertz to 20,000 Hz.
Transmission of Sound. Sound does not travel through a vacuum. A medium (solid, liquid, gas) is needed for the transmission of sound Sound travels just over 4 times faster in water than in air. Sound travels faster and further in solids - 10 times faster in the ground than in air, 15 times faster through concrete & 18 times faster thru steel.
Sound is transmitted by waves The wave appears as a series of pressure changes – compressions and rarefactions. The source of the sound is a vibrating object that causes the particles in the medium to vibrate to and fro in line with the direction of travel of the sound. When the vibrating object moves ‘out’ it compresses the particles together and this disturbance is then transmitted from particle to particle. A pulse of compression is transmitted through the medium. Then the object moves ‘back’ giving more space and so the particles spread out reducing the pressure – this results in a rarefaction so a pulse of rarefaction follows the pulse of compression.
Show that the Transmission of Sound Requires a Medium (show that sound cannot travel through a vacuum)
Set up an electric bell in a belljar that is connected to a vacuum pump. Turn on the bell. The vibrating ringing electric bell is heard. Now switch on the vacuum pump. The sound gradually weakens and eventually cannot be heard. Switch off the pump and allow air back into the jar – the bell is heard again. Light does not need a medium for transmission – all the contents of the jar remained clearly visible during and after the removal of air.
Speed of Sound
In air at 0°C sound travels at 330 metres per second. At 20°C its speed is 344 ms-1 .
Light travels almost a million times faster than sound. Many everyday events at a distance seem to confuse us regarding what we see and hear. The confusion is explained by the difference in speed between light and sound – we see the event quite some time before we hear it. There is a time lag between seeing and hearing the same event.
Everyday Examples of Light Travelling Faster Than Sound - ‘seeing before hearing’
1. Distant golfer teeing off or distant goalkeeper kicking out. `2 Fireworks at a distance.
3 Lightning and thunder. 4 What appears to be a ‘false start’ to a race at an athletics meeting.
How far away is the lightning? the time between the ‘flash & the bang’ and divide by 3 to get the distance in km
Reflection of Sound Echos
The bouncing back of sound waves off a surface is called reflection of sound. Sound reflects very well from hard, solid flat surfaces. A sound sensation lasts in the brain for approximately one tenth of a second.
In one tenth of a second the sound travels about 35 metres at room temperature in air.
Therefore to hear a sound a second time the reflecting surface must be about 18 metres away from you – 18 m there and 18 metres back so total distance travelled is 36 m. Reflection of sound can cause of echoes.
‘Echo location’ is the use of reflected sound to estimate the distance to reflecting objects. If the echo occurs 3 seconds after the initial sound then the distance to the reflecting surface e.g. a wall, is 455m.
Calculation Sound detected 3 seconds later.
Speed of sound in air: 330 m/s.
Distance Travelled By Sound: 300 m/s x 3 s = 990 m
Distance to the reflecting surface is half the distance travelled by the sound. 455 m.
Some Everyday Applications of Sound
1. Communication: we use speech to communicate with each other. 2. Dog whistle.
3. Echo location by bats and dolphins. 4. Echo location by boats to find fish and measure depth of water.
5. Ultra-sound scanning - ‘seeing’ developing babies in the womb and internal organs.
6. Ultra-sound to break up kidney stones and gallstones.
Detection of Sound in the Ear
The ear is the sense organ that detects sound waves emitted by vibrating objects. Sound is a series of pressure waves reaching the ear rapidly one after the other. It is in the liquid-filled inner ear that the vibrations are converted to nerve impulses. The inner ear can detect different wave frequencies from 20 to 20,000 per second.
The inner ear also detects differences in wave energy. These impulses are carried along the auditory nerve to the hearing centre of the brain. These impulses are interpreted by the brain producing a sound sensation. Different frequencies give different pitch sensations. Different energy levels give different loudness sensations.
Our ears evolved in a hunter-gatherer environment. Our ears are adapted to what could be considered quiet sounds. Our ears can detect pressure changes as low as one billionth of atmospheric pressure. A sound level that causes damage to the ear is a million million times more intense than the quietest sound we can detect. Sound intensity is measured in decibels, dB. The decibel scale starts at 0 dB and this is the sound that is just detectable for normal ears. Each 10-decibel increase really is an increase in sound intensity of 10. Therefore 10 dB is ten times the energy than 0 dB but 20 dB is a hundred times and 60 dB is a million times more energy. But our hearing sensation is not accurate - each 10 dB rise does not give a sound sensation that is 10 times louder but it felt as only twice as loud.
Exposure To Loud Sounds Can Damage Your Hearing
The sound level of normal conversation is 60 dB. Teachers produce a sound level of about 80 dB in the classroom. Long exposure (>8 hours) each day to sound of 85 dB is considered too great and poses a threat to your hearing. 120 dB will cause pain and will cause damage to the ears within minutes – indoor rock concerts often reach this level. Excessive loudness at particular frequencies leads to permanent deafness to those frequencies. At 160 dB the eardrums will burst.
Modern Life is very noisy in comparison to the environment our species evolved. Over time our hearing can deteriorate if precautions are not taken. By law, workers must be given ear protection if the noise level is over 90 dB. If someone can hear the sound from your earphones you are wearing then the sound level is too high – long exposure to this level will lead to ear damage.