Wave Nature of Sound  Discuss sound as a wave in terms of reflection, refraction, diffraction, interference.  Demonstrate interference of sound.  Discuss the factors affecting acoustics.  Discuss noise pollution and noise reduction.  Discuss the speed of sound in different media.  Demonstrate that sound requires a medium. 1

Wave Nature of Sound  Sound travels as a longitudinal wave.  Thus it exhibits the properties of waves.  Excludes polarisation which only affects transverse waves. 2

Reflection  Sound reflects off surfaces.  We usually call these reflections “echoes”.  This is used by bats and dolphins for navigation (echo- location). 3 By Petteri Aimonen – public domain

Refraction  Sound travels at different speeds in different media and in gases with different temperatures.  Hence it refracts when changing media or passing through air with varying temperature. 4 Temperature ( o C)Speed (m/s)

Diffraction  Explains why we can hear around corners.  Hearing through doorways is particularly easy, since the door gap is around the width as the wavelength of common sounds. 5

To Demonstrate Interference of Sound 1.Attach two loudspeakers to a signal generator and turn on the signal generator. 2.Walk in a straight line in front of the loudspeakers, keeping the same distance from them. 3.The loudness regularly increases and decreases. 4.This is because the sound waves interfered, which proves sound is a wave. 6

Acoustics  Acoustics is the science of sound.  “Acousticians” and “acoustical engineers” take reflection, interference etc. into account when designing spaces and technology. 7

Acoustics  Acoustic engineers design speaker placement at concerts to avoid overlaps and interference.  Soft surfaces absorb sound, so walls in indoor concert venues and cinemas have cloth or carpet over them to prevent reflection. 8

Acoustics  Recording studios and labs use wedges to absorb sound to avoid reflection.  This is more effective than just using soft walls but is also expensive to design. 9 link

Acoustics  I just love acoustics :45

Noise Pollution  The modern world is very noisy.  In cities, traffic, industry, and construction often produce loud consistent noise.  In rural areas, barns and coops can produce lots of noise.  Noise pollution has been shown to affect the health and behaviour of people and animals.  It can cause hearing-related issues or even cardiovascular problems in humans.  Ecologies may be upset by noise, hindering predators and affecting food chains. 11

Noise Reduction  Traffic noise is reduced by adding obstacles to the sides of the road. In cities, this is usually trees.  Modern motorways and national roads are built into valleys to reflect the sounds upwards, into forests to absorb the sounds, or with barriers to reflect the sounds between the barriers until they are fully absorbed.  Industrial parks are often built away from residential areas to allow sound to attenuate. 12

Noise Reduction 13  Noise can be reduced using destructive interference.  Noise-cancelling headphones have a microphone that picks up exterior sound.  This sound inverted and played along with music/podcast/whatever to destructively interfere with the exterior noise. link

Speed of Sound  Sound is produced by all vibrating objects (likewise, all sound is produced by vibrating objects).  These vibrations get transferred by contact with nearby molecules, which transfer the vibrations to further molecules etc. 14

Speed of Sound  The speed at which waves travel through these molecules depends on the elasticity of the medium – how quickly they snap back to position after being moved.  Generally, sound travels quickly in solids, slower in liquids, and even slower in gases.  Temperature also affects the speed of sound. 15 MaterialSpeed of Sound (m s -1 ) Air330 Water1500 Copper3400 Steel4800

Vocal Folds  Vocal folds (aka vocal cords) are a pair of tissues in the larynx that control the frequency of our voices by vibrating.  They work together with the mouth to form sounds recognised as speech.  Their length and thickness determine the range of available frequencies. 16 By I, Welleschi – CC-BY-SA 3.0

To Demonstrate that Sound Requires a Medium to travel. 1.Set a bell to ring and place it inside a bell jar. 2.Pump the air out of the bell jar. 3.Note that the bell gets quieter as the air is removed and is inaudible once the jar is evacuated. 4.Pump air back into the bell jar. 5.Note that the bell gets louder as the air is pumped in. 6.Thus, sound requires a medium to travel. 17